JP2002341125A - Diffraction element and optical head device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain an diffraction element for generating a plurality of beams which have approximately the same diffraction efficiency of ±1st and ±2nd diffracted light beams, and further to obtain an optical head device with which a high speed processing is available. SOLUTION: Such an optical diffraction element is manufactured that the element is provided with a diffraction grating 11 having a grating pitch P and formed on a light-transmissive substrate 12, P is divided into four regions, the ratio of the regions to P is W1 , W2 , W3 , and W4 , the phase between transmitting light at regions W1 and W2 is ϕ1 , the phase between transmitting light at regions W2 and W4 is ϕ2 (ϕ1 ≠ϕ2 ), and W1 , W2 , and W3 are so specified that a specifying relation A(m) including W1 , W2 , and W3 becomes 3.6<A(±2)/A(±1)<4.4 for m=±1 and ±2, and the element is mounted on the device.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a diffraction element and an optical head device, and more particularly to a diffraction element for dividing incident light into a plurality of beams, and an optical head device used for recording and reproducing information on an optical recording medium.

[0002]

2. Description of the Related Art Optical recording media such as CDs, DVDs and magneto-optical disks (hereinafter collectively referred to as "optical disks").
An optical head device that records and reproduces information on an information recording surface is used. In this optical head device, various tracking methods are used to focus light on tracks on the information recording surface following the rotation of the optical disk.

In the tracking method, a three-beam method is generally used as a signal (information) detection method for an information recording surface of a read-only optical disk, and a differential push-pull method is generally used as a signal detection method for a recording optical disk. . In either method, light emitted from a light source such as a semiconductor laser is diffracted by a diffraction grating, and three beams of 0-order diffracted light and ± 1st-order diffracted light are used.

[0004] Of the three beams, the zero-order diffracted light is used as a main beam for reproducing or recording a signal on a track. Further, the ± 1st-order diffracted light is arranged as two sub-beams in a straight line before and after the main beam, and these three beams obliquely cross the track line, and the interval between the two sub-beams is set to the size of the track pitch. 1/4 to 1 /
Shift by about two. Each reflected light of these three beams by the information recording surface is guided to the light receiving element and converted into an electric signal.

FIG. 8 is a sectional view of a conventional diffraction grating used for generating such three beams. A diffraction grating 61 having a periodic uneven shape is formed on the surface of a glass substrate 62. This diffraction grating 61 acts as a phase modulation type diffraction grating to generate diffracted light. In order to maximize the sum of the 0th-order diffracted light and the ± 1st-order diffracted light among the diffracted lights, a two-divided structure in which the ratio of the width of the concave part to the convex part of the diffraction grating is 1: 1 is generally used. .

[0006]

In a conventional optical head device equipped with a diffraction grating for generating three beams, when recording and reproducing information at high speed, the tracking stability is deteriorated, and errors occur during recording and reproduction. There is a problem that is likely to occur.

[0007]

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems, and comprises a phase modulation type diffraction grating having a grating pitch P, wherein the grating pitch P has a first region. , the second region, third region and fourth region are arranged in succession in the pitch direction, and the phase of the light transmitted through the phase and third regions of the light transmitted through the first region is equally phi _1, the The phase of the light transmitted through the two regions and the phase of the light transmitted through the fourth region are equal to φ ₂ (φ ₂ ≠ φ ₁ ), and the ratio of the width of each region to P is W ₁ , W ₂ , W ₃ , W ₄ and the diffraction order is m (m is a positive or negative integer),
Provided is a diffraction element that satisfies the expressions (1) and (2).

[0008]

(Equation 2)

A phase modulation type having a grating pitch P
A diffraction grating is provided, and the grating pitch P has a first area, a second area,
Region, the third region and the fourth region are continuous in the pitch direction.
And the phase of light passing through the first region and the phase of light passing through the third region.
The phase of the light passing through is equal to φ ₁Through the second area
Phase of light passing through the fourth region is equal to that of light passing through the fourth region.
φ_Two(Φ_Two≠ φ₁) And the width P of each region with respect to P
W ratio₁, W_Two, W_Three, W_FourThen W₁, W_Two, W_Three,
W_FourMinimum value W of_minIs between the incident light wavelength λ and λ /
P ≦ W_minProvide a diffraction element satisfying a relationship of ≦ 0.15
You.

A light source, an objective lens for condensing light emitted from the light source on an optical recording medium, and a photodetector for detecting light condensed and reflected by the optical recording medium are provided. An optical head device provided with the diffraction element, wherein the diffraction element is provided in an optical path between a light source and an optical recording medium or in an optical path between a photodetector and the optical recording medium. .

[0011]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS [First Embodiment] In this embodiment shown in FIG. 1, a diffraction grating 11 is formed by processing the surface of a light-transmitting substrate 12 into an uneven shape, and a diffraction element for generating a plurality of beams. 1
It is set to 0. FIG. 2 is a partially enlarged view of the diffraction grating 11, which has a periodic structure with a grating pitch P.
The diffraction grating has a linear grating that generates at least ± first-order and ± second-order diffracted lights with respect to incident light.

The grating pitch P of the diffraction element is spatially continuous.
And continuous widths in the pitch direction are a₁,
a_Two, A_Three, A_FourFirst, second, third and fourth regions
Divided into areas. Their cross-sectional structure has a refractive index n, high
Width d with d_TwoAnd width a_FourRectangular convex part and width a₁and
Width a_ThreeAnd a rectangular recess in the direction perpendicular to the paper surface.
The irregularities are elongated. That is, a₁+ A_Two+ A_Three+ A_Four
= P and width a₁, A_Two, A _Three, A_FourIs the ratio of P to
W₁, W_Two, W_Three, W_FourAnd the diffraction grating 11 is refracted.
Rate n_a(In FIG. 1, air).

The phase of the transmitted light in the area of width a ₁ and the phase of the transmitted light in the area of width a ₃ are equal to φ _1, and the phase of the transmitted light in the area of width a ₂ and the transmission of the light in the area of width a _4. The phase of light is equal to φ ₂
(Φ ₂ ≠ φ ₁ ). A phase modulation type diffraction grating having a phase difference φ = φ ₂ −φ ₁ is configured. At this time, assuming that the wavelength of the incident light is λ, the phase difference φ is φ = 2π (n−
n _a ) d / λ. 0 generated by the diffraction element
The diffraction efficiencies of the order diffracted light and the m-th order diffracted light (m is a positive or negative integer) can be approximately described by equations (3) and (4), respectively.

[0014]

(Equation 3)

Here, A (m) is described by the above equation (1).

Therefore, in the configuration of the diffraction grating 11 shown in FIG. 2, the diffraction efficiency I (m) of the diffracted light other than the zero-order diffracted light is obtained.
Is a part that depends on the phase difference φ ｛2 sin (φ / 2) /
π｝ ² is common, and this part does not depend on the diffraction order m, but depends on W ₁ , W ₂ , W ₃ , and W _4. A (m) / m ²
Only depends on the diffraction order m. Therefore, when the ratio of the two diffraction efficiencies is taken, the portion dependent on φ is eliminated, and when changing m, the change of I (m) contributes only to the portion of A (m) / m ² . That is, in order to realize a diffraction element for generating multiple beams in which the diffraction efficiencies of the m-th order diffracted light are substantially equal, W ₁ , W ₂ , W ₃ , W ₃ are set so that A (m) / m ² becomes almost equal. _What is necessary is just to specify the ratio of ₄ .

Specifically, a diffraction element in which the diffraction efficiency ratios of the ± 1st-order diffracted light and the ± 2nd-order diffracted light are almost equal is 3.6 <A (±
2) W ₁ , W ₂ , W ₃ , satisfying /A(±1)<4.4
It may be configured to have a ratio of W _4. Preferably, 3.9 <A (± 2) / A (± 1) <4.1.
Most preferably, A (± 2) / A (± 1) = 4,
In this case, the diffraction efficiency ratio between the ± 1st-order diffracted light and the ± 2nd-order diffracted light becomes equal.

With this configuration, a diffraction element having diffraction efficiency ratios of ± 1st-order diffracted light and ± 2nd-order diffracted light that are substantially equal is obtained. Irrespective of the phase difference φ, that is, the height d of the rectangular convex portion, the diffraction efficiencies of the ± 1st-order and ± 2nd-order diffracted lights become almost equal, and by adjusting the height d, the 0th-order diffracted light is ± 1st-order and ± 2nd-order The efficiency ratio with folding light can be adjusted arbitrarily.

Further, a diffraction element in which all three types of diffraction efficiency ratios of ± 1st-order diffracted light, ± 2nd-order diffracted light, and ± 3rd-order diffracted light are almost equal, furthermore has A (± 3) / A (± 1) of approximately 9. _{W 1, W 2, W 3} , it may be the ratio of W ₄ meet. As an example in which all three diffraction efficiency ratios are almost equal, W ₁ : W ₂ :
W ₃ : W ₄ may be set to 0.23: 0.13: 0.21: 0.43.

When all of the 0th-order diffracted light, ± 1st-order diffracted light and ± 2nd-order diffracted light are used as signal light, and the diffraction efficiency ratios of the ± 1st-order diffracted light and ± 2nd-order diffracted light are not always equal, the signal light amount (0th-order diffracted light, In order not to lower (the sum of ± 1st-order diffracted light and ± 2nd-order diffracted light), it is preferable to reduce the diffraction efficiency of ± 3rd or higher order diffracted light.

In the above, the width a_TwoAnd width a_FourRectangle
Although only the convex portion has the height d, the width a ₁And width a_ThreeRectangle
Fill the concave part with an optically isotropic medium and set it at the same height as the convex part
d may be used. Also in this case, the width a₁And width a_ThreeArea of
Phase of the transmitted light is equal to φ_Two(Φ_Two≠ φ₁).

[Second Embodiment] The diffraction element of this embodiment is
The lattice pitch P is divided into a _first region, a second region, a third region, and a fourth region having widths a ₁ , a ₂ , a ₃ , and a ₄ , respectively.
_{1, a 2, a 3,} a ratio W ₁ for P of _4, W _2, W _3,
It has a linear grating of W ₄ and has a phase modulation type diffraction grating. The phase of the transmitted light in the area of the width a ₁ and the phase of the transmitted light in the area of the width a ₃ are equal to φ _1, and the phase of the transmitted light in the area of the width a ₂ and the phase of the width a ₄ are equal. The phase of the transmitted light in the region is equal to φ ₂ (φ ₂ ≠ φ ₁ ), and W ₁ , W ₂ ,
Assuming that the minimum value of W ₃ and W ₄ is W _min , the condition of λ / P ≦ W _min ≦ 0.15 is satisfied with the wavelength λ of the light incident on the diffraction element.

With this configuration, it is possible to suppress the generation of diffracted light beams of orders other than the desired order and to stably generate a plurality of highly efficient beams. Refractive index n, a rectangular convex portion having a width at the height d a ₂ and a width a _4, when the diffractive element having a rectangular concave portion having a width a ₁ and a width a _3, ± the influence of the wall surface of the rectangular protrusion
Since the third-order or higher order diffracted light increases and the signal light quantity decreases,
The effect of the limitation of λ / P ≦ W is further enhanced, which is preferable.

In the diffraction element described in the above two embodiments,
Where the wavelength λ₁Or wavelength λ_Two(Λ ₁≠ λ_TwoTwo waves
When long linearly polarized light is incident, the phase difference φ
Long λ ₁Is approximately an integral multiple of 2π
By processing the height of the convex portion to d, the wavelength λ₁Entering
Functions as a light-transmitting substrate that does not generate diffracted light for incident light
I do. On the other hand, the wavelength λ_TwoHas a phase difference φ of 2
Diffraction element of the above two embodiments because it is a non-integer multiple of π
Functions as a diffraction grating.

For example, the height d of the rectangular projection is λ ₁ / (n−
If processed into n _a ), the phase difference φ becomes 2π with respect to the incident light of wavelength λ ₁ , and no diffracted light is generated. On the other hand, the phase difference φ becomes 2π (λ ₁ / λ ₂ ) with respect to the incident light having the wavelength λ ₂ and becomes 2π.
Therefore, diffracted light is generated.

These diffractive elements can be manufactured by, for example, known fine processing of the surface of a glass substrate, fine processing of a resin layer formed on the glass substrate, and molding of a transparent resin material. Further, FIG. 1 illustrates the case where the uneven phase modulation type diffraction grating 11 is formed at the interface between the translucent substrate 12 and the air, but as shown in FIG.
The concave portions of the diffraction grating 21 are filled with a translucent filler 23 which is an optically isotropic medium having _a refractive index n _a having a different refractive index from that of the convex portions, and the translucent substrate 22 and the translucent substrate 24 are used. The diffraction element 20 may be configured to have a sandwiched configuration.

[Third Embodiment] FIG. 4 is a schematic cross-sectional view for explaining the operation principle of an example of a polarizing diffractive element 30 according to a third embodiment of the present invention. Diffractive element 30, the ordinary refractive index n _o, uneven birefringent material layer optical axes are aligned in a predetermined direction parallel to the surface of the transparent substrate 32 with extraordinary refractive index _{n e (n e ≠ n o} ) The diffraction grating 31 is processed into a shape, and the concave portion of the birefringent material is filled with a light-transmitting filler 33 having an isotropic refractive index n _s , a light-transmitting substrate 34 is overlapped, and a light-transmitting substrate 32 is formed. And a light-transmitting substrate 34.

[0028] When the refractive index n _s is substantially equal to the ordinary refractive index n _o of the birefringent material layer, if the incident light of the first linear polarized light and the polarization of the ordinary refractive index with respect to the birefringent material, the irregularities Since the diffraction grating and the translucent filler correspond to the same refractive index layer, no diffracted light is generated (FIG. 4A). On the other hand, the second linearly polarized light having a polarization direction orthogonal to the first linearly polarized light corresponds to the polarized light having the extraordinary refractive index of the birefringent material. When the phase difference phi and the wavelength of the incident light _{λ, φ = 2π (n e} -n s) d / λ becomes diffracted light is generated (Figure 4 (b)).

Here, the grating pitch P of the concave-convex shaped diffraction grating 31 is sequentially divided at a ratio of W ₁ , W ₂ , W ₃ , and W ₄ with respect to P. By setting the same ratio as the diffraction element described in the second embodiment,
A diffractive element 30 for generating a plurality of polarized light beams is obtained, which generates a desired diffracted light only for incident polarized light having an extraordinary refractive index and does not generate diffracted light for incident light having an ordinary refractive index. .

Further, by introducing a switching means for rotating the polarization direction of the incident light by 90 ° on the light incident side of the diffraction element 30, the output light of the diffraction element 30 can be switched between a single beam and a plurality of beams. As the switching means, a method of switching in and out of a half-wave plate, a method of electrically turning on and off a function of a half-wave plate by using a birefringent material such as a liquid crystal or an electro-optic crystal, and the like. Is mentioned.

As the light-transmitting substrate of the diffraction element, for example, a substrate such as glass, plastic, or optical crystal can be used. Stretched polycarbonate film,
Birefringent optical crystals and the like can be used, and further, as the translucent filler for the concave portion, for example, acrylic, epoxy,
A urethane-based adhesive or the like can be used.

Fourth Embodiment FIG. 5 shows a side view of an example in which the diffraction element of the present invention described in the above three embodiments is mounted on an optical head device. Light emitted from the laser light source 2 passes through the diffraction element 1, is reflected by the beam splitter 3, is collimated by the collimator lens 4, and is condensed on the information recording surface of the optical disc 6 by the objective lens 5. The light reflected on the information recording surface again passes through the objective lens 5 and the collimating lens 6, passes through the beam splitter 3, and is condensed on the light receiving surface of the photodetector 7.

The 0 transmitted through the diffraction element 1 without being diffracted
The second order diffracted light is focused on the track on the information recording surface, and the diffracted ± 1st order diffracted light and ± 2nd order diffracted light are
After being condensed in the vicinity of tracks adjacent to each other across the track where the focal point of the next diffracted light is located and reflected by the information recording surface,
The light is focused on the light receiving surface of the photodetector 7.

Here, the 0th-order diffracted light generated by the diffractive element of the present invention is used as a main beam, for example, for reproducing a signal on a track or recording a signal.
The next-order diffracted light is used as a sub-beam for tracking signal detection by, for example, a differential push-pull method. As a result, the tracking accuracy is improved, and in a recording optical disk such as a CD-R or a CD-RW, the optical disk can be rotated at a high speed to increase the recording speed.

As another application, a total of seven diffracted lights, ie, the 0th-order diffracted light, ± 1st-order diffracted light and ± 2nd-order diffracted light generated by the diffractive element of the present invention, and ± 3rd-order diffracted light depending on the use situation, By condensing light on a plurality of tracks adjacent to each other on the recording surface and condensing the light reflected on the information recording surface on the light receiving surface of the photodetector 7, information on the plurality of tracks can be read in parallel. . As a result, high-speed reproduction is realized.

When a two-wavelength laser is used as a laser light source, the use of the diffractive element of the present invention results in a wavelength-selective diffractive element that generates a plurality of beams for only one wavelength light. Does not cause a reduction in diffraction efficiency and does not generate stray light. Therefore, stable signal detection can be performed for DVD and CD optical disks having different wavelengths.

In the diffraction element of the present invention, a phase plate for changing the polarization state of transmitted light may be integrated. By using, for example, a quarter-wave plate for converting incident linearly polarized light into circularly polarized light as the phase plate, stable recording and reproduction of the optical disk can be performed in the optical head device. This is because the linearly-polarized light emitted from the laser beam passes through the quarter-wave plate, becomes circularly polarized light, and is condensed on the information recording surface, and the return light reflected by the information recording surface again passes through the quarter-wave plate. The transmitted light becomes linearly polarized light having a polarization plane orthogonal to the linearly polarized light of the laser oscillation light, and becomes return light to the laser emission point. Therefore, interference between the laser oscillation light and the return light is suppressed, and oscillation output fluctuation is reduced. This is because it can be reduced.

When a birefringent optical crystal such as quartz is used as the phase plate, a birefringent optical crystal can be used as the light-transmitting substrate constituting the diffraction element. Further, as the phase plate, an organic thin film having a phase difference generating function such as a birefringent film obtained by stretching polycarbonate or a polymer liquid crystal layer having a uniform orientation may be used.

In the case of a phase plate made of polycarbonate, as shown in FIG.
A configuration in which the wavefront aberration is held between 2 and 44 is preferable because the wavefront aberration of the transmitted light is not deteriorated. When a polymer liquid crystal is used as the phase plate, the liquid crystal monomer before being polymerized without using the adhesive 46 is subjected to an alignment treatment on the inner surface, and the light transmitting substrate 42 having a peripheral portion sealed. After injecting the mixture between the layers 44, the mixture is polymerized and cured to form a polymer liquid crystal layer. Reference numeral 41 denotes a diffraction grating formed on a light-transmitting substrate 42.

The use of a birefringent material of an organic thin film is preferable because a phase difference variation due to a difference in the incident angle of incident light is small and a uniform and uniform phase difference can be generated as compared with a quartz phase plate.

[0041]

Example 1 First, a diffraction element of this example will be described with reference to FIG. After the SiO ₂ film having a refractive index n = 1.45 on a glass substrate was formed so that a thickness 330nm is light-transmissive substrate 42, periodically removing the SiO ₂ film by photolithography and etching techniques The diffraction grating 41 was formed into an uneven shape so as to form an interface with air.

The grating pitch P of the diffraction grating 41 is set to 30 μm.
And the grid pitch P is set to the width a₁, A_Two, A _Three, A_FourDivided in the order of
And a rectangular protrusion having a height d = 297 nm is converted to a width a._Two= 9.6μ
m and width a_Four= 10.2 μm and the width of the rectangular recess₁=
1.5 μm and width a_Three= 8.7 μm. Sand
That is, the ratio W of each width to the lattice pitch P₁: W_Two: W_Three:
W_Four= 0.05: 0.32: 0.29: 0.34
At this time, from equation (1), A (2) / A (1) is
4. Also, the ratio W₁And the ratio W_ThreeLight passing through the area
Have the same phase and the ratio W_TwoAnd the ratio W_FourThrough the area
The phases of the passing light are the same.

When a laser beam having a wavelength λ of 785 nm is incident on the diffraction grating 41, about 72% is transmitted as the 0th-order diffracted light and about 5.2% of ± 1st and ± 2nd-order diffracted lights are generated. An element was obtained. Further, a quarter-wave plate made of polycarbonate was used as a phase plate 45 for converting incident linearly polarized light into circularly polarized light, and the quarter-wave plate was held between two upper and lower glass substrates with an adhesive 46 and fixed. Also,
The minimum value W _min = W ₁ of the ratios W ₁ , W ₂ , W ₃ , W ₄ is 0.05
Λ / P = 0.02
6, the relational expression of λ / P <W ₁ <0.15 holds.

The diffraction element 40 thus obtained is
The optical head device shown in FIG. 5 on which a high-output semiconductor laser having a wavelength of 785 nm as the laser light source 2 was mounted was disposed on the light emission side of the semiconductor laser and used as a diffraction element for generating a plurality of beams for detecting a tracking error signal. . As a result, C
In recording optical discs such as DR and CD-RW,
Tracking accuracy was improved by the differential push-pull method using five beams, and high-speed recording and reproduction could be performed. Further, as shown in FIG. 6, since the 1/4 wavelength plate was integrated, there was no change in the laser oscillation output, and stable recording and reproduction of the optical disk were realized.

In the optical head device shown in FIG. 5, a laser light source having a wavelength of 790 nm for CD and a wavelength of 650 nm for DVD are used.
By combining the two wavelengths with a dichroic mirror using the above laser light source, an optical head device capable of recording and reproducing data even on a DVD optical disk can be configured. In this case, by separately arranging the diffraction element of the present invention between the laser light source for DVD and the dichroic mirror, an optical head device capable of performing high-speed recording and reproduction on an optical disk for DVD is obtained.

In the diffraction element of the present invention, A (m) / m
² (where m ≠ 0), if the ratio of the widths of the lattice pitches W ₁ : W ₂ : W ₃ : W ₄ is maintained so as to be substantially equal, the phase difference φ, that is, SiO ₂ processed into an uneven shape. Irrespective of the thickness of the film, the diffraction efficiency of the m-th order diffracted light is almost equal.
By adjusting the thickness of the SiO ₂ film, the efficiency ratio between the 0th-order diffracted light and the mth-order diffracted light (m ≠ 0) can be arbitrarily adjusted.

The zero-order diffracted light, ± 1 in 5 beam generating diffraction element using diffracted light and ± 2-order diffracted light as the signal light, by changing the thickness of the SiO ₂ film 0 efficiency ratio of the diffracted light and the m-order diffracted light (m = ± 1, ± 2) are 1, 2, 3, and 4 in Table 1. The total efficiency indicates the sum of the 0th order diffracted light, ± 1st order diffracted light and ± 2nd order diffracted light.

[0048]

[Table 1]

Example 2 First, the diffraction element of this example will be described with reference to FIG. A liquid crystal monomer solution is applied to a glass substrate, which is a translucent substrate 52 on which an alignment film subjected to an alignment treatment is formed, so that the liquid crystal monomer liquid has a constant thickness, and is irradiated with ultraviolet rays to be polymerized and solidified, thereby obtaining an ordinary light refractive index. n _o = 1.55, layer thickness in the extraordinary refractive index n _e = 1.60 was formed a polymer liquid crystal layer having a birefringence of 2.97Myuemu.

Next, the polymer liquid crystal layer is processed into a concave / convex diffraction grating 51 by photolithography and etching techniques, and a translucent filler 53 having an isotropic refractive index n _s = 1.55 is used. The concave portion of the polymer liquid crystal layer was filled, and a quarter-wave plate as a polycarbonate phase plate 55 and a glass substrate as a translucent substrate 54 were bonded using an adhesive 56 to form a diffraction element 50. Other configurations such as the lattice pitch P and the division structure of the lattice pitch P were the same as those in Example 1.

This diffraction element 50 has a wavelength λ ₁ of 655 nm.
When the DVD laser beam is incident as a polarization of the ordinary refractive index, the refractive index n _s of the ordinary refractive index n _o and the transparent filler of the polymer liquid crystal layer are equal, the diffraction element 50 is a diffraction grating of a phase modulation type And does not generate diffracted light. On the other hand, the wavelength λ when the ₂ CD laser beam 785nm is incident as a polarization of the extraordinary refractive index of the polymer liquid crystal layer extraordinary refractive index n _e
And the refractive index n _s of the optical filler are not equal, the diffraction element 50 functions as a diffraction grating of the phase difference _{φ = 2π (n e -n s} ) d / λ 2, diffracted light is generated.

At this time, only for the laser light for CD,
About 72% is transmitted as the 0th-order diffracted light, and about 5.2% ± 1.
A wavelength-selective diffraction element for generating a plurality of beams, in which second- and second-order diffracted lights are generated, was obtained.

In the optical head device shown in FIG. 5, a two-wavelength laser that emits light having a wavelength of 655 nm and a wavelength of 785 nm is used as the laser light source 2, so that a wavelength selection for generating a plurality of beams only for the light having a wavelength of 785 nm. Acted as a diffractive element. Therefore, high-speed recording was realized by using a plurality of beams for a CD recording optical disk, and a conventional reproduction signal detection method using a single beam was applicable to a DVD reproduction optical disk.

[0054]

As described above, by using the diffraction element of the present invention, it is possible to realize a diffraction element for generating a plurality of beams in which the diffraction efficiency ratio of ± 1st-order diffraction light and ± 2nd-order diffraction light other than the 0th-order diffraction light is adjusted. . Such a diffraction element is referred to as CD-R, CD-
In an optical head device for recording such as RW, by using as a diffraction element for generating a plurality of beams for tracking control using a differential push-pull method, even when the recording speed is increased by increasing the rotation speed of the optical disk, Accurate tracking control can be performed, and high-speed recording can be realized.

Further, by using such a diffractive element as a diffractive element for generating a plurality of beams for reading signals of parallel tracks in a reproducing optical head device, it is possible to simultaneously read a plurality of track recording signals with high precision. Therefore, high-speed reproduction can be realized.

In an optical head device equipped with a two-wavelength laser light source including a semiconductor laser having a wavelength of 790 nm for a CD and a semiconductor laser having a wavelength of 650 nm for a DVD, light beams having different wavelengths from the two-wavelength laser are incident on a diffraction element. Since the incident light of one wavelength is transmitted without being diffracted, and the incident light of the other wavelength acts as a diffraction grating for generating a plurality of beams, it becomes a wavelength-selective diffraction element having excellent light use efficiency. Therefore, DVD and CD signal detection can be performed quickly and stably. Further, by adopting an element configuration in which the quarter-wave plate is integrated, fluctuations in laser oscillation output in the optical head device are reduced, and stable recording and reproduction of the optical disk are realized.

[Brief description of the drawings]

FIG. 1 is a conceptual side view showing an example of a diffraction element of the present invention.

FIG. 2 is a partially enlarged view of the diffraction grating of FIG.

FIG. 3 is a conceptual side view showing another example of the diffraction element of the present invention.

FIGS. 4A and 4B are side views for explaining the operation principle of the diffraction element of the present invention, in which (a) polarized light having an ordinary refractive index is incident, and (b) polarized light having an extraordinary refractive index is incident.

FIG. 5 is a side view of the optical head device of the present invention.

FIG. 6 is a side view showing an example in which a phase plate is integrated with the diffraction element of the present invention.

FIG. 7 is a side view showing another example in which a phase plate is integrated with the diffraction element of the present invention.

FIG. 8 is a side view of a conventional diffraction grating for generating three beams.

[Explanation of symbols]

1, 10, 20, 30, 40, 50: Diffraction element 11, 21, 31, 41, 51, 61: Diffraction grating 12, 22, 24, 32, 34, 42, 44, 52, 5
4, 62: translucent substrate 23, 33, 53: translucent filler 46, 56: adhesive 45, 55: phase plate 2: laser light source 3: beam splitter 4: collimating lens 5: objective lens 6: optical disk 7: Photodetector

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Ryuichiro Goto 1-8 Machiikedai, Koriyama-shi, Fukushima Pref. DA16 DA33 5D119 AA28 BA01 EC41 JA22 JA24 LB05

Claims (4)

[Claims] 1. A phase modulation type diffraction grating having a grating pitch P, wherein a first area, a second area,
The third region and fourth region are arranged in succession in the pitch direction, and the phase of the light transmitted through the phase and third regions of the light transmitted through the first region is equally phi _1, passes through the second region The phase of the light is equal to the phase of the light transmitted through the fourth region, φ
₂ (φ ₂ ≠ φ ₁ ), and the ratio of the width of each region to P is W ₁ , W ₂ , W ₃ , W ₄ , and the diffraction order is m (m is a positive or negative integer). A diffraction element satisfying the expressions (1) and (2). (Equation 1) 2. A phase modulation type diffraction grating having a grating pitch P, wherein the grating pitch P includes a first region, a second region,
The third region and fourth region are arranged in succession in the pitch direction, and the phase of the light transmitted through the phase and third regions of the light transmitted through the first region is equally phi _1, passes through the second region The phase of the light is equal to the phase of the light transmitted through the fourth region, φ
₂ (φ ₂ ≠ φ ₁ ) and the ratio of the width of each region to P is W ₁ , W ₂ , W ₃ , W ₄ , W ₁ , W ₂ , W ₃ , W ₄
Lambda / P between the wavelength lambda minimum value W _min of the incident light of the
A diffraction element that satisfies the relationship ≦ W _min ≦ 0.15. 3. An incident light having a first linear polarization direction is transmitted without exhibiting a diffractive function, and an incident light having a second linear polarization direction perpendicular to the first linear polarization direction is transmitted. 3. A diffraction element according to claim 1, wherein the diffraction element has a diffraction function. 4. A light source, an objective lens for condensing light emitted from the light source on an optical recording medium, and a photodetector for detecting light condensed and reflected by the optical recording medium. An optical head device provided with the diffraction element according to claim 1, 2, or 3 in an optical path between a light source and an optical recording medium or in an optical path between a photodetector and an optical recording medium. Optical head device.