JP2004133074A - Diffraction grating and optical pickup - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a diffraction grating capable of realizing high diffraction efficiency by reducing the grating pitches, and to provide an optical pickup having high detection efficiency and capable of recording/reproducing at high speed by using the diffraction grating. <P>SOLUTION: On the surface of a transparent material such as SiO<SB>2</SB>of the diffraction grating, rectangular grooves (recesses) are formed by etching or the like. Of the diffraction grating, the pitch Λ between protrusions (gratings) is 8 μm, the depth T of the groove is 1 μm, the incident light side is the air whose refractive index n<SB>1</SB>is 1, and the transmission light side is SiO<SB>2</SB>whose refractive index n<SB>2</SB>is 1.45. Monochromatic light whose wavelength λ is 0.66 μm is made incident from the incident light side to measure the 0th-order diffracted light and the ±1st order diffracted light of the transmission light side. When the width of the grating on the SiO<SB>2</SB>side is represented by D and the pitch Λ is made constant, the measured diffraction efficiency of the ±1st order diffracted light with respect to the variations of D/Λ reaches 26.4 % when the pitch Λ is 8μm (the broken line in the figure) and the D/Λ is close to 0.5. Further, when the pitch Λ is 1.5 μm (solid line) and D/Λ is 0.5, the diffraction efficiency is 20.9%. That means that, when D/Λ is made smaller, that is, when the width of the SiO<SB>2</SB>side D is made smaller, the diffraction efficiency is improved. <P>COPYRIGHT: (C)2004,JPO

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an optical information processing apparatus for recording or reproducing information on or from an optical recording medium of a plurality of standards having different use wavelengths such as a CD (Compact Disk), a DVD (Digital Video Disk, Digital Versatile Disk), and a Blu-Ray Disc. The present invention relates to a diffraction grating and an optical pickup used.
[0002]
[Prior art]
2. Description of the Related Art In a conventional information processing apparatus for recording or reproducing information on or from a recording medium, one of methods for recording and reproducing the information is a method of recording or reproducing on a recording medium using light. As a typical example of such an optical recording / reproducing method, a DVD-standard disc (optical recording medium) is required in order to record image information compressed in MPEG2 format on one side of an optical recording medium having a diameter of 12 cm for 2 hours or more. Has been commercialized. According to the DVD standard, the storage capacity of the disk is 4.7 GB on one side, the track density is 0.74 μm / track, and the linear density is 0.267 μm / bit. Hereinafter, a disc based on the DVD standard is simply referred to as a DVD.
[0003]
Reproduction of information recorded on an optical recording medium such as a DVD is performed using an optical head. In this optical head, a light beam emitted from an LD (semiconductor laser) is focused by an objective lens on a pit row on a track of an optical recording medium. Further, the light beam reflected by the optical recording medium is condensed on a photodetector by a condensing lens, and a reproduction signal is obtained. The reproduction signal from the photodetector is input to a reproduction signal processing system, and the data is decoded. In the case of DVD, the wavelength of the LD in the optical head is 650 nm, and the numerical aperture (NA) of the objective lens is 0.6.
[0004]
Furthermore, there is a Blu-Ray @ Disc, which is a new standard that is a high-density DVD standard. This is the next generation that can record and reproduce up to 27 GB of video data repeatedly on one side of one layer of a phase-change optical recording medium having a diameter of 12 cm, the same as CDs and DVDs, using a 405 nm wavelength blue-violet laser. Is a standard for large-capacity optical recording media.
[0005]
In the Blu-Ray @ Disc, the beam spot is miniaturized by using a short-wavelength blue-violet laser and setting the numerical aperture (NA) of the objective lens for condensing the light beam to 0.85. In addition, the adoption of an optical recording medium structure with a light transmission protective layer thickness of 0.1 mm corresponding to the increase in the aperture of the lens reduces aberrations due to the inclination of the optical recording medium, further reduces reading errors and improves recording density. I am planning. As a result, the recording track pitch of the optical recording medium is reduced to about half of that of DVD, that is, 0.32 μm, and high-density recording of a maximum of 27 GB is realized on one surface of the optical recording medium.
[0006]
FIG. 9 shows a schematic configuration diagram of a pickup that performs recording and reproduction on an optical recording medium (DVD medium). A pickup 1 of an optical recording medium (DVD medium) generally uses a polarization optical system. That is, a PBS (Polarizing Beam Splitter) 4 is arranged in the optical path from the LD 2 of the light source to the objective lens 6 to transmit light of the same polarization plane as the linearly polarized light of the LD 2, and 1 / The light is converted into circularly polarized light by the four-wavelength plate 5, condensed by the objective lens 6, and irradiated on the recording layer below the substrate of the optical recording medium 8.
[0007]
Further, the reflected light from the reflection surface of the optical recording medium 8 becomes circularly polarized light having a rotation opposite to that of the incident light, and when transmitted through the quarter-wave plate 5, becomes linearly polarized light having a polarization plane perpendicular to the polarization plane of the LD 2, The light is reflected by the PBS 4 and guided to the PD (photodetector) 10 via the condenser lens 9. If the wave plate (1/4 wave plate) 5 has perfect circular polarization, the transmitted light of the PBS 4, that is, the return light of the LD 1 becomes "0", and the medium reflected light is completely detected by the PD 10.
[0008]
As an optical pickup used in an optical disk drive device, which is an optical information processing device, various types of optical pickups having an optical system that splits reflected light from an optical recording medium by a diffraction grating and receives the branched light by a photodetector have been proposed. There is known a diffraction grating using a polarizing diffraction grating. For example, an optical head device and a method of manufacturing the same disclosed in Patent Document 1 include an optical head device that records and reproduces information by irradiating an optical recording medium with a light beam from a light source through a diffraction grating. It is described that a diffraction grating having optical anisotropy formed by an anisotropic polymer is used.
[0009]
On the other hand, a so-called super combo drive that can record and reproduce a CD and record and reproduce a DVD with one optical disk drive device has been put to practical use. In the optical pickup used in the CD / DVD combined optical disk drive, a semiconductor laser for a CD having a wavelength of 790 nm and a semiconductor laser for a DVD having a wavelength of 650 nm are arranged in a separated state. In this optical pickup, light emitted from a semiconductor laser (650 nm wavelength band and 790 nm wavelength band) is synthesized on the same optical axis by a wavelength synthesizing prism, passes through a beam splitter, is converted into parallel light by a collimating lens, and Light enters the lens. The light beam transmitted through the objective lens and condensed on the information recording surface of the optical recording medium is reflected on the information recording surface, and the reflected light (hereinafter, referred to as signal light) has the same optical path as the original outward path. Goes backwards.
[0010]
That is, the signal light is again converted into parallel light by the objective lens, and is condensed on the light receiving surface of the photodetector via the collimator lens and the beam splitter. Then, the light is converted into an electric signal by the photodetector.
[0011]
In addition, as a semiconductor laser that emits light of two wavelengths, for example, a monolithic two-wavelength semiconductor laser in which a semiconductor laser of a 790 nm wavelength band and a semiconductor laser of a 650 nm wavelength band are formed in one chip, or a semiconductor laser for each wavelength band A two-wavelength semiconductor laser comprising a plurality of chips in which laser chips are arranged so that the distance between light emitting points is about 100 to 300 μm has also been proposed. If these two-wavelength semiconductor lasers are used, the number of parts is reduced, and the size and cost can be reduced as compared with the conventional optical pickup in which the two semiconductor lasers are configured as separate units as described above.
[0012]
FIG. 10 shows a schematic configuration of a semiconductor laser unit for two wavelengths in which laser chips of respective wavelength bands are arranged close to each other. The LDs (semiconductor lasers) 2a and 2b in each wavelength band of CD and DVD are arranged so that the distance between the light emitting points is about 100 to 300 μm, and the PD (photodetector) 10 is arranged in the same package. Diffraction gratings 11a and 11b for guiding CD and DVD for guiding the signal light passing through the return path to PD 10 are arranged in front of LDs 2a and 2b.
[0013]
When such LDs 2a and 2b for CD and DVD, PD10, and diffraction gratings 11a and 11b for CD and DVD are arranged integrally, the beam diameters of the light for CD and the light for DVD in the return path are overlapped. Therefore, it is necessary to provide a wavelength selectivity that does not diffract the light when the DVD light transmits the CD light and the CD light transmits the DVD light.
[0014]
Further, the diffraction gratings 11a and 11b must be arranged at positions close to the respective LDs 2a and 2b. In order to guide light from these close positions to the PD 10, the diffraction angle is increased to 15 to 20 degrees. There must be. Further, in order to increase the diffraction angle, it is necessary to narrow the grating pitch of the diffraction grating, and the pitch must be around 2 μm from the diffraction angle.
[0015]
In the case of a diffraction grating having wavelength selectivity, if the wavelength selection condition and the wavelength are determined, the diffraction efficiency of light having a wavelength on the diffracted light side is uniquely determined, and the diffraction efficiency cannot be set arbitrarily. On the other hand, in the two-wavelength diffraction grating and the optical head device of Patent Document 2, the maximum diffraction efficiency can be obtained when the ratio of the width of the convex portion of the grating to the period is 0.5 with respect to the diffraction grating having a wide pitch. It is described that, by setting this ratio to a value other than 0.5 and reducing the diffraction efficiency, the diffraction efficiency is arbitrarily set in a region equal to or less than the maximum value of the diffraction efficiency.
[0016]
[Patent Document 1]
JP-A-9-50642
[Patent Document 2]
JP 2001-281432 A
[0017]
[Problems to be solved by the invention]
However, the method described in Patent Document 2 is applied only to a wide-pitch diffraction grating, and cannot set the diffraction efficiency to a value equal to or greater than the maximum value. Further, the diffraction efficiency in the narrow pitch region theoretically converges to around 30%, and it is difficult to obtain higher diffraction efficiency.
[0018]
As a technique for increasing the diffraction efficiency of the wide pitch grating, there is a method of blazing the grating shape, and the diffraction efficiency can be improved by changing the ratio of the efficiency of the diffracted light in the + direction and the-direction to one side. This method is effective for wide-pitch gratings, but it is difficult to create blazed shapes for narrow-pitch gratings because the pitch is small with respect to the depth of the groove, and forms a highly efficient diffraction grating. It is difficult to do.
[0019]
Currently, a polarizing optical system is mainly used in a recording DVD, and a polarizing diffraction grating having a return-path diffraction efficiency of about 32% is used. If the speed is further increased in the future, the amount of PD detection light will be insufficient, and it will be difficult to realize a high-speed recording drive. When the diffraction efficiency of the polarizing diffraction grating increases from the current 32%, the amount of PD detection increases, and a DVD recording drive capable of high-speed recording can be realized.
[0020]
Further, in an optical pickup used for an optical disk drive device using a light source wavelength of 400 nm such as a Blu-Ray @ Disc, a more efficient diffraction grating is required. That is, the transmittance of an optical material such as a lens is 95% or more in the wavelength range of 660 nm, but decreases to 90% in the wavelength range of 400 nm. Further, a large number of elements for correcting various aberrations such as spherical aberration and coma aberration are included, and a decrease in the amount of light generated when the light passes through each optical component is a serious problem. In addition, there is a problem that the photoelectric conversion efficiency of the photodetector decreases due to a decrease in quantum efficiency due to the shortening of the wavelength, and the amount of PD detection light decreases sharply. In consideration of the above-described decrease in the amount of detected light, an optical pickup used in an optical disc drive device using a light source wavelength of 400 nm needs a diffraction efficiency of 60% or more, and it is necessary to configure an optical pickup using a narrow pitch diffraction grating. Had the problem of being difficult.
[0021]
The present invention is directed to solving the above-described problem of the related art, and realizes a diffraction grating capable of increasing diffraction efficiency in a diffraction grating in which a grating pitch is reduced. It is an object of the present invention to provide an optical pickup that solves the problems of the related art using the same.
[0022]
[Means for Solving the Problems]
In order to achieve this object, the diffraction grating according to the present invention in the diffraction grating and the optical pickup according to the present invention is a diffraction grating having a cross-sectional shape having a periodic uneven shape. With respect to the width D of the member having a large refractive index forming the portion and the pitch 形成 between the convex portions, the following condition “D / Λ ≦ 0.4” should be satisfied. In a diffraction grating having a pitch of 凸, a depth of a concave portion of the diffraction grating is T, an average refractive index of the diffraction grating is n, a wavelength of a light source is λ, and a thickness shape of the diffraction grating is defined. When the Q value is Q = 2πλT / nΛ²The refractive index of a member having a large refractive index that forms the convex portion of the diffraction grating is represented by n₂, The refractive index of a member having a small refractive index forming the concave portion of the diffraction grating is represented by n₁And the refractive index difference Δn = n₂-N₁By satisfying the following condition “D / Λ = １ ／ · exp (−3ΔnQ / 2)” in the pitch 間 の between the projections and the width D of the projections when A diffraction grating that satisfies the condition of the pitch の between the projections of the diffraction grating and the ratio D / Λ of the width D of the projections can provide a large diffraction efficiency in the 0th-order diffracted light and ± 1st-order diffracted light.
[0023]
The diffraction gratings according to the third and fourth aspects are the diffraction gratings according to the first and second aspects, wherein the pitch between the convex portions of the diffraction grating is Λ, the depth of the concave portion of the diffraction grating is T, and the average refraction of the diffraction grating. Assuming that the ratio is n and the wavelength of the light source is λ, Q = 2πλT / nΛ²Satisfy the following condition “Q> 1”, and further satisfy D / Λ ≦ 0.4 in the width D of the protrusions of the diffraction grating and the pitch 間 の between the protrusions. By using a photomask having a mask pattern in which the light-to-dark width ratio is changed from 1: 1, a mask in which the light-to-dark width ratio is changed so as to obtain a desired D / Λ value. By forming a diffraction grating using a photomask having a pattern, a large diffraction efficiency can be obtained for the 0th-order diffracted light and ± 1st-order diffracted light.
[0024]
According to a fifth aspect of the present invention, in the diffraction grating having the uneven shape according to the first to fourth aspects, the convex portion is formed of a birefringent medium, and the concave portion is filled with an isotropic medium. By using an organic stretched film for the birefringent medium of the part, or using liquid crystal for the birefringent medium of the convex part, the diffraction efficiency can be arbitrarily varied depending on the angle of the polarization plane of the incident light, and transmitted in the outward path. Can be diffracted on the return path.
[0025]
The diffraction grating according to claim 8 is the diffraction grating according to claims 1 to 7, wherein the diffraction grating transmits one wavelength λ1 using the wavelength λ1 and the wavelength λ2 as the wavelength of the light source, By diffracting the other wavelength λ2, the diffraction grating can be integrated with a two-wavelength light source, a photodetector, and the like to be integrally formed.
[0026]
The optical pickup according to the ninth and tenth aspects employs the diffraction grating according to any one of the first to eighth aspects, takes in light emitted from a light source, and condenses the light on an optical recording medium by a condenser lens. An optical pickup that performs recording or reproduction by using a diffraction grating disposed in an optical path between a light source and an optical recording medium, and a photodetector that receives light reflected from the optical recording medium by being split by the diffraction grating. In addition, by integrating the light source, photodetector, and diffraction grating, high detection efficiency can be obtained, high-speed recording / reproduction can be performed, the assembly time of the optical pickup is reduced, and the optical system adjustment is also made. Easy to do.
[0027]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
[0028]
FIG. 1 is a partial cross-sectional view showing a schematic configuration of a diffraction grating having an uneven shape according to Embodiment 1 of the present invention. For example, this diffraction grating includes SiO₂A groove (recess) as shown in FIG. 1 is formed on the surface of a transparent member such as by etching. The groove is rectangular, and the pitch Λ between the convex portions (grating) of the diffraction grating is 8 μm, and the groove depth T is 1 μm. The incident light side is air and the refractive index n₁= 1, transmitted light side is SiO₂And the refractive index n₂= 1.45. Monochromatic light having a wavelength λ = 0.66 μm is incident from the incident light side, and the 0th-order diffracted light and ± 1st-order diffracted light on the transmission side are measured. The higher refractive index side of the grating, ie, SiO₂Let D be the width of the side, and consider D / Λ.
[0029]
FIG. 2 shows the diffraction efficiency of the ± 1st-order diffracted light with respect to the change in D / と き when the pitch Λ is fixed. When the pitch の of the dashed line shown in FIG. 2 is 8 μm, D / ２６ becomes a maximum of 26.4% when it is slightly smaller than 0.5, and the diffraction efficiency decreases as D / Λ increases. Similarly, as D / Λ decreases, the diffraction efficiency decreases. Further, when the grating pitch １．５ is 1.5 μm, as shown by the solid line in FIG. 2, when D / Λ is 0.5, the diffraction efficiency is 20.9%, but does not reach the maximum. When Λ is reduced, that is, when SiO₂Becomes smaller, the diffraction efficiency increases. When D / Λ is close to 0.2, the diffraction efficiency is 46.0%, and greatly exceeds the maximum diffraction efficiency of 26.4% when the pitch Λ is 8 μm. Thus, by adjusting D / Ｄ, the diffraction efficiency can be made larger than when D / Λ is 0.5.
[0030]
From the above, the maximum value of the diffraction efficiency appears in a region where D / Λ is small, particularly in a region of 0.4 or less. Therefore, by making D / Λ smaller than 0.4, the ± 1st-order diffraction efficiency is increased. Can be.
[0031]
In a diffraction grating having such a cross-sectional shape having a periodic uneven shape, the pitch between the convex portions is Λ, the depth of the concave portion is T, the average refractive index of the diffraction grating is n, the wavelength of the light source is λ, and the Q value is Q = 2πλT / nΛ²And the refractive index of a member having a large refractive index forming the convex portion of the diffraction grating is n₂, The refractive index of a member having a small refractive index forming the concave portion of the diffraction grating is represented by n₁And the refractive index difference Δn = n₂-N₁Where D / Λ is the ratio of the width D of the member (convex portion) having a large refractive index to the pitch 間 の between the convex portions, the value D / Λ at which the ± 1st-order diffraction efficiency has the maximum value. / Λ_maxCan be expressed as (Equation 1) from various experimental results and calculations.
[0032]
(Equation 1)
D / Λ_max= 1 / 2exp (-3ΔnQ / 2)
Here, when the pitch Λ = 8 μm, D / Λ calculated from (Equation 1)_maxIs 0.48, and D / Λ when the pitch Λ = 1.5 μm._maxIs 0.18, which is consistent with the result of FIG. Therefore, by making the D / Λ of the diffraction grating close to the D / 計算 calculated using (Equation 1), a diffraction grating having ± 1st-order diffraction efficiency larger than the conventional diffraction grating having a D / Λ of 0.5 can be obtained. realizable.
[0033]
The Q value of the diffraction grating is defined as a guide for the thin grating and the thick grating. When the wavelength of the light source is λ, the depth of the concave portion of the diffraction grating is T, the average refractive index of the diffraction grating is n, and the pitch of the grating (between the convex portions) is Λ, the Q value of the diffraction grating is Given in 2).
[0034]
(Equation 2)
Q = 2πλT / nΛ²
Here, there is a guideline of a thin planar grating when Q <1, a thick volume grating when Q> 10, and an intermediate region grating from a flat type to a volume type when Q <10. In the example of FIG. 2, when the pitch Λ = 8 μm, Q = 0.05, which is a thin planar grating. Further, when the pitch １．５ = 1.5 μm, Q = 1.5, and the lattice becomes an intermediate region. In the case of a thin planar grating with a pitch Λ = 8 μm, the effect of improving the ± 1st-order diffraction efficiency by adjusting D / 現 れ does not appear from FIG. The effect of improving the ± first-order diffraction efficiency by adjusting Λ is remarkable.
[0035]
As described above, in the region from the intermediate region to the thick volume grating, that is, in the region where Q> 1, the effect of improving the diffraction efficiency by adjusting D / Λ is remarkable. Therefore, by adjusting D / の to 0.4 or less in the diffraction grating in the region of Q> 1, the ± 1st-order diffraction efficiency can be increased.
[0036]
Next, a method of forming the diffraction grating according to the second embodiment of the present invention will be described with reference to FIGS. First, as shown in FIG. 3A, a photoresist layer is formed on a transparent substrate by spin coating or the like. When the photoresist layer is exposed to a lattice pattern and developed, a diffraction grating pattern is formed on the transparent substrate shown in FIG. Here, the ratio of the line and space (bright and dark) of the grating pattern at the time of exposure is shifted from 1: 1, and the D / Λ of the completed diffraction grating is adjusted and formed to have a desired value. This is a significant feature of the diffraction grating according to the second embodiment.
[0037]
A metal layer of aluminum, chromium, or the like is formed on the photoresist lattice pattern formed as described above by vacuum evaporation, sputtering, or the like (see FIG. 3C). The photoresist pattern and the metal layer thereon are removed by elution with a solvent or removal by decomposition in oxygen plasma. As shown in FIG. 3D, the remaining metal grid pattern serves as a mask for the subsequent dry etching.
[0038]
Next, the metal grid pattern formed on the transparent substrate is placed in a dry etching apparatus such as ion beam etching, reactive ion etching, or plasma etching, and is etched using the metal pattern as a mask (FIG. e)). After etching to a desired depth, the metal mask is removed with an acid, whereby a diffraction grating having a desired value of D / Λ shown in FIG. 3F can be formed.
[0039]
According to the third embodiment, a desired D mask is formed by using a photomask having a mask pattern in which the ratio of the light and dark widths of the grating pattern is changed from 1: 1 with almost no change in the conventional diffraction grating forming process. A diffraction grating having a value of / Λ can be formed, and a large diffraction efficiency can be obtained with respect to the 0th-order diffracted light and ± 1st-order diffracted light.
[0040]
FIG. 4 is a diagram showing a schematic configuration of an LD / PD unit serving as a CD / DVD common optical path according to Embodiment 3 of the present invention. The distance between the CD LD 2a and the DVD LD 2b shown in FIG. 4 is set close to 100 μm to 300 μm. The light in the 780 nm wavelength band emitted from the CD LD 2a has a spread angle θ._CDTransmit through the diffraction grating (HOE) 11b for DVD and the HOE 11a for CD. The light transmitted through the HOE 11a for CD and the HOE 11b for DVD transmits through the CL (collimating lens) 3, the wavelength plate (1/4 wavelength plate) 5, and the OL (objective lens) 6, and irradiates the information recording surface of the optical recording medium 8. Is done.
[0041]
The signal light reflected from the information recording surface of the optical recording medium 8 passes through the OL 6 and CL 3 again, and passes through the HOE 11 a for CD and the HOE 11 b for DVD. At this time, the CD HOE 11a diffracts the CD light and transmits the DVD light. Conversely, the DVD HOE 11b transmits the CD light and diffracts the DVD light. The diffracted light enters the PD (photodetector) 10 for both CD and DVD light, and detects the amount of reflected light from the optical recording medium 8.
[0042]
In such a CD / DVD common optical system, the CD optical system is a non-polarizing optical system, and the DVD is a polarizing optical system. Therefore, the HOE 11a for CD is a non-polarization diffraction grating, and the zero-order diffraction efficiency with respect to DVD light must be 95% or more in multiple paths. Also, in the case of CD light, in the case of a CD writing system pickup, the zero-order diffraction efficiency needs to be 80% or more, and the first-order diffraction efficiency needs to be 8% or more.
[0043]
Based on these conditions, a non-polarized CD HOE was designed. The material is BK7 and the refractive index is n₂= 1.51, the refractive index difference Δn = 0.51, the average refractive index of the grating portion n = 1.26, and the shape is a rectangular lattice having a pitch Λ = 2.4 μm and D / Λ = 0.23. FIG. 5 shows the 0th-order diffraction efficiency of DVD light (wavelength λ = 0.66 μm), the 0th-order diffraction efficiency of 1st-order diffraction efficiency CD light (wavelength λ = 0.78 μm) with respect to the groove depth T of the rectangular grating having the above shape, 1 shows first-order diffraction efficiency. In the case of the wavelength selective CD HOE, since the DVD light is transmitted and the CD light is partially diffracted, the groove depth is about 1.5 μm, which is the second maximum value of the DVD 0th-order diffracted light in FIG. When the groove depth T was 1.55 μm, a DVD 0th-order diffraction efficiency of 95%, a CD 0th-order diffraction efficiency of 81%, and a CD first-order diffraction efficiency of 9.1% were obtained.
[0044]
Generally, in the design of a diffraction grating, the diffraction efficiency is simply obtained by algebraic calculation based on the scalar diffraction theory. According to the scalar diffraction theory, the zero-order diffraction efficiency η at the wavelength λ₀And ± 1st order diffraction efficiency η₁Are (Equation 3) and (Equation 4), respectively.
[0045]
(Equation 3)
η₀= (2D / Λ-1)²Sin²θ + cos²θ
[0046]
(Equation 4)
η₁= 4 (Sin (πD / Λ) / π)²Sin²θ
However, the phase change θ is (Equation 5).
[0047]
(Equation 5)
θ = π / λ · ΔnT
When the DVD 0th-order diffraction efficiency with respect to the groove depth T of the diffraction grating under the above conditions is obtained using this equation, the curve shown in FIG. 6 is obtained. However, in FIG. 6, the calculation was performed with D / Λ = 0.5. In the scalar calculation of (Equation 3), the diffraction efficiency becomes a regular cosine function with respect to the groove depth T as shown in FIG. 6, and reaches the maximum value of the second peak when the groove depth T is 1.3 μm.
[0048]
However, the actual diffraction grating does not have the curve shown by the thick line in FIG. 6, but the curve shown by the broken line in FIG. This curve is a value calculated by the RCWA (Exact Coupled Wave Analysis) method, and matches the measured data. The cause of the reduction of the 0th-order diffracted light at the second peak is the generation of even-order diffracted light that does not occur in the scalar calculation, and the efficiency of the 0th-order diffracted light is reduced by the even-order diffracted light. The calculation of the curve shown by the broken line in FIG. 6 was performed at D / Λ = 0.5, but when D / Λ was adjusted to D / Λ = 0.23, the calculation of the curve shown by the thin line in FIG. As described above, the diffraction efficiency of the second peak of the DVD 0th-order diffracted light increases. The maximum value is obtained when the groove depth T is 1.45 μm, which is larger in each step than the diffraction efficiency when D / Λ = 0.5, and is a value close to a scalar calculation. Thus, by adjusting D / Λ to a value smaller than 0.4, the diffraction efficiency of the diffraction grating can be significantly increased.
[0049]
Next, a method for producing the diffraction grating according to the fourth embodiment of the present invention as Example 1 will be described with reference to FIGS. The diffraction grating of the first embodiment is a polarizing diffraction grating. First, a birefringent medium is formed on a transparent substrate such as optical glass. In this method, a film of a birefringent medium is attached to a substrate, the birefringent medium is applied by spin coating or the like, or formed by a physical film forming method such as vacuum evaporation or sputtering. Further, a photoresist layer is formed on the birefringent medium by spin coating or the like (FIG. 7A).
[0050]
After exposing and developing the grid pattern on the photoresist layer, a diffraction grating pattern is formed on the transparent substrate (FIG. 7B). Here, the ratio of the line and space of the grating pattern at the time of exposure is shifted from 1: 1 so that D / Λ of the completed diffraction grating is adjusted to a desired value. This is the same as in Embodiment 2 described above. A metal layer of aluminum, chromium, or the like is formed on the lattice pattern of the photoresist thus formed by vacuum deposition, sputtering, or the like (FIG. 7C), and the photoresist pattern and the metal layer thereover are acetone. The photoresist pattern and the metal layer thereon are removed by elution with an organic solvent, such as, or by decomposition in oxygen plasma (FIG. 7D). The remaining metal grid pattern becomes a mask for the subsequent dry etching.
[0051]
Next, a birefringent medium having a metal lattice pattern formed on a transparent substrate is placed in a dry etching apparatus such as ion beam etching, reactive ion etching, or plasma etching, and the birefringent medium is etched using the metal pattern as a mask. (FIG. 7 (e)). After etching to a desired depth, the metal mask is removed with an acid, whereby a rectangular grid having an uneven shape is formed in the birefringent medium (FIG. 7F).
[0052]
Further, the uneven portion of the birefringent medium is filled with an isotropic medium, and a transparent substrate is placed thereon (FIG. 7 (g)). At this time, it is desirable that the refractive index of the isotropic medium is equal to either the ordinary ray refractive index or the extraordinary ray refractive index of the birefringent medium in order to increase the transmittance and the diffraction efficiency. Further, the uppermost transparent substrate is not always necessary, and may not be necessary as long as the flatness of the isotropic medium can be ensured.
[0053]
In the first embodiment, as a method of forming a metal mask for dry etching, a method using a so-called lift-off method has been described. However, the present invention is not limited to this method, and a metal layer is formed directly on a birefringent medium. A photoresist is applied on the layer, and a grid pattern is exposed and developed to form a grid pattern of the photoresist, and the metal layer is etched using the grid pattern as an etching mask. Thereafter, the photoresist can be removed to form the state shown in FIG. By the above method, a polarizing diffraction grating having a desired value of D / Λ and high diffraction efficiency can be formed.
The birefringent medium may be any transparent substance exhibiting birefringence, and an organic film or an organic stretched film can be used. Examples of the material of the organic stretched film include organic materials such as polyester, polyimide, polyethylene, polycarbonate, polyvinyl alcohol, polymethyl methacrylate, polystyrene, polysulfone, polyethersulfone, and polyethylene terephthalate. Can be used. Particularly, a stretched film of PET (polyethylene terephthalate) or PC (polycarbonate) has a large refractive index difference Δn of 0.1 between the stretching direction and the perpendicular direction, and is optimal for the first embodiment.
[0054]
Further, as a method of forming the diffraction grating in Example 2 of Embodiment 4, a liquid crystal can be used instead of the birefringent medium. Hereinafter, a method of forming a liquid crystal as Example 1 of Embodiment 4 will be described. First, a transparent substrate is subjected to an alignment treatment, a photocurable liquid crystal is uniformly applied thereon, and a lattice pattern is exposed according to the alignment direction. Here, the ratio of the line and space of the grating pattern at the time of exposure is shifted from 1: 1 so that D / Λ of the completed diffraction grating is adjusted to a desired value. This is a major feature of this patent. When the uncured portion is removed after the curing, the liquid crystal becomes uneven. By filling the concave portion with an isotropic substance and bonding a transparent substrate to the surface of the isotropic substance, a polarizing diffraction grating can be formed.
[0055]
Further, as a method of forming a diffraction grating in Example 3 of Embodiment 4, there is a method of forming a liquid crystal polarizing diffraction grating as described below. The two transparent substrates that have been subjected to the alignment treatment are placed facing each other with the alignment direction aligned with the alignment surface inside. The gap between the two substrates is kept constant by a gap material that keeps the gap constant. A photocurable liquid crystal is injected into the gap, and a lattice pattern is exposed to form a diffraction grating of a liquid crystal cured portion and an uncured portion. In this case, similarly, the ratio of the line and space of the grating pattern at the time of exposure is shifted from 1: 1 so as to adjust D / の of the completed diffraction grating to a desired value. Thereafter, an electric field is externally applied to the liquid crystal, and the entire surface is exposed with the electric field applied. The uncured portion of the liquid crystal is cured in the state of electric field alignment, and a polarizing diffraction grating made of the liquid crystal can be formed.
[0056]
The method of forming the polarizing diffraction grating includes not only the formation method described in each of the above-described embodiments but also various methods. The polarizing diffraction grating thus formed can have a refractive index difference Δn between the direction of the liquid crystal and the direction perpendicular to the liquid crystal of 0.2 at the maximum, and is optimal for the present invention.
[0057]
The operation of the polarizing diffraction grating prepared as described above will be described. The polarizing diffraction grating has a structure in which grooves of a diffraction grating formed in a birefringent medium are filled with an isotropic medium. The refractive index of the isotropic medium is selected to be substantially the same as the ordinary light refractive index or the extraordinary light refractive index of the birefringent medium. Here, it is assumed that the refractive index of the isotropic medium is the same as the ordinary refractive index of the birefringent medium.
[0058]
Consider a case where linearly polarized light is incident on such a diffraction grating. First, when light having a polarization plane according to the ordinary ray refractive index is incident, the refractive index of the birefringent medium of the grating portion is the ordinary ray refractive index, and the refractive index of the isotropic medium is also equal to the ordinary ray refractive index. The grating cannot be recognized for light having a polarization plane. Therefore, the incident light is transmitted as it is and does not function as a diffraction grating. Conversely, when light having a polarization plane according to the extraordinary ray refractive index is incident, the index of refraction of the birefringent medium in the grating portion is the extraordinary ray index, and the index of refraction of the isotropic medium is equal to the ordinary ray index. Light having this plane of polarization recognizes the grating and diffracts the light. Thus, the diffraction efficiency can be changed depending on the direction of deflection of the incident light.
[0059]
A DVD HOE 11b using a polarizing diffraction grating in an optical pickup according to a fifth embodiment of the present invention will be described with reference to FIG. As described with reference to FIG. 9, the DVD uses a polarization optical system in the case of a writing optical system in which the illumination efficiency must be increased due to the limitation of recording power. The light emitted from the DVD LD 2b enters the DVD HOE 11b. At this time, since the DVD HOE 11b is a polarizing diffraction grating, the polarization plane is adjusted so that the transmitted light is maximized with respect to the outward light. That is, the HOE 11b for DVD functions as a transparent plate for the light on the outward path.
[0060]
Thereafter, the light transmitted through the wave plate (1/4 wavelength plate) 5 and reflected from the optical recording medium 8 is transmitted through the wave plate 5 again, becomes linearly polarized light having a polarization plane rotated by 90 °, and is transmitted to the DVD HOE 11b. Light enters from the opposite side. Since the polarization plane is rotated by 90 °, the DVD HOE 11b functions as a diffraction grating for the return light, diffracts the light, and guides the signal light to the PD 10. At this time, the DVD light is transmitted through the CD HOE 11a both in the forward path and the return path. However, as described in the third embodiment, the CD HOE is transparent to the DVD light, so that there is no problem.
[0061]
FIG. 4 shows the LD and PD units in the optical path common to the CD and the DVD as described above, and the distance between the LD 2a for the CD and the LD 2b for the DVD is set close to 100 μm to 300 μm. The light in the 660 nm wavelength band emitted from the LD 2b for DVD has a spread angle θ._DVDTransmit through the HOE 11b for DVD and the HOE 11a for CD. The light that has passed through the CD / DVD HOE passes through a CL (collimating lens) 3, a wave plate 5, and an OL (objective lens) 6, and irradiates the information recording surface of the optical recording medium 8. The signal light reflected from the information recording surface again passes through OL6 and CL3, and passes through HOE 11a for CD and HOE 11b for DVD. At this time, the CD HOE 11a diffracts the CD light and transmits the DVD light. Conversely, the DVD HOE 11b transmits the CD light and diffracts the DVD light. The diffracted light enters the PD (photodetector) 10 for both CD and DVD light, and detects the amount of reflected light from the optical recording medium 8.
[0062]
In such a CD / DVD common optical system, the CD light is a non-polarization optical system, and the DVD light is a polarization optical system. Therefore, the HOE 11b for DVD is a polarizing diffraction grating, and the zero-order diffraction efficiency for CD light in the return path needs to be 95% or more. Also, in the case of DVD light, in the case of a DVD writing system pickup, the primary diffraction efficiency is required to be 32% or more.
[0063]
Based on these conditions, a polarizing HOE for DVD was designed. The material is a liquid crystal, a refractive index difference Δn = 0.2, the shape is a rectangular lattice having a pitch Λ = 2.0 μm, and D / Λ = 0.26. FIG. 8 shows the first-order diffraction efficiency of DVD light (wavelength λ = 0.66 μm) and the zero-order diffraction efficiency of CD light (wavelength λ = 0.78 μm) with respect to the groove depth T of the rectangular grating having the above-described shape. In the case of the DVD HOE, the CD light is transmitted and the DVD light is diffracted, so that the groove depth T is about 5.8 μm in FIG. When the groove depth T was 5.8 μm, a DVD first-order diffraction efficiency of 38% and a CD zero-order diffraction efficiency of 96% were obtained.
[0064]
Here, the incident angle of the CD 0th-order diffracted light will be described with reference to FIG. In the fifth embodiment, the HOE 11b for DVD is disposed on the light source side, and the HOE 11a for CD is disposed on the OL 6 side. When the CD / DVD HOEs 11a and 11b are arranged in this manner, when the CD light passes through the DVD HOE 11b on the return path, the CD light has already been diffracted by the CD HOE 11a, and the DVD HOE 11b has the CD HOE 11a. At an incident angle with a diffraction angle of. The pitch の of the HOE 11a for CD is 2.4 μm, and the CD light diffraction angle of the HOE 11a for CD is 19 ° from this pitch Λ. The 0th-order light diffraction efficiency of the CD of the DVD HOE 11b must consider the incident angle 19 ° of the CD light. The 0th-order light diffraction efficiency of the CD calculated previously and the 0th-order light diffraction efficiency of the CD in FIG. 8 are values in consideration of the CD incident angle.
[0065]
By using a diffraction grating in which D / Λ is adjusted to 0.4 or less as in the above-described embodiments for an optical pickup, high detection efficiency can be obtained, and an optical pickup capable of high-speed recording / reproduction can be realized. .
[0066]
In addition, as shown in FIG. 4 of the third embodiment, by integrating a light source having a plurality of wavelengths, a photodetector, and a diffraction grating for a plurality of wavelengths, when assembling an optical pickup, a plurality of wavelengths can be obtained. Since the light source, the photodetector, and the diffraction grating for a plurality of wavelengths are integrated, the assembling time is reduced, and the optical system can be easily adjusted.
[0067]
【The invention's effect】
As described above, according to the present invention, the diffraction grating having the pitch Ｄ between the convex portions and the D / の of the width D of the convex portion of the diffraction grating having the concave-convex shape satisfies the respective conditions. A diffraction grating is formed using a photomask having a mask pattern in which the ratio of the light and dark widths is changed so as to obtain a value of / Λ, to obtain a large diffraction efficiency in the 0th-order diffracted light and ± 1st-order diffracted light. By using a polarizing diffraction grating in which the convex portion is formed of a birefringent medium and the concave portion is filled with an isotropic medium, the diffraction efficiency can be arbitrarily changed depending on the angle of the plane of polarization of the incident light, allowing transmission on the forward path and diffraction on the return path. In addition, by transmitting one of the two wavelengths of the wavelength λ1 and the wavelength λ2 of the light source and diffracting the other, the two-wavelength light source, the photodetector, and the diffraction grating for a plurality of wavelengths can be integrated.
[0068]
In addition, by using the above-described diffraction grating for the optical pickup, high detection efficiency can be obtained, high-speed recording and reproduction can be performed, the assembly time of the optical pickup can be reduced, and the optical system adjustment can be easily performed. It has the effect of being able to do it.
[Brief description of the drawings]
FIG. 1 is a partial cross-sectional view showing a schematic configuration of a diffraction grating having an uneven shape according to a first embodiment of the present invention.
FIG. 2 is a diagram showing the diffraction efficiency of ± 1st-order diffracted light with respect to a change in D / Λ when the pitch Λ is fixed.
FIG. 3 is a diagram for explaining a method of forming a diffraction grating according to a second embodiment of the present invention.
FIG. 4 is a diagram showing a schematic configuration of an LD / PD unit serving as a CD / DVD common optical path according to a third embodiment of the present invention;
FIG. 5 is a diagram showing DVD 0th-order diffraction efficiency, CD0th-order diffraction efficiency, and 1st-order diffraction efficiency with respect to the groove depth T of the diffraction grating in the third embodiment.
FIG. 6 is a diagram showing a curve showing a DVD 0th-order diffraction efficiency with respect to a groove depth T of the diffraction grating in the third embodiment.
FIG. 7 is a view for explaining a method of forming a polarizing diffraction grating according to a fourth embodiment of the present invention.
FIG. 8 is a diagram showing first-order diffraction efficiency of DVD and zero-order diffraction efficiency of CD with respect to groove depth T of the diffraction grating in the third embodiment.
FIG. 9 is a configuration diagram schematically showing a pickup that performs recording and reproduction on an optical recording medium.
FIG. 10 is a diagram showing a schematic configuration of a semiconductor laser unit for two wavelengths in which laser chips of respective wavelength bands are arranged close to each other.
[Explanation of symbols]
1. Optical pickup
2 LD (semiconductor laser)
2a @ CD LD
2b @ DVD LD
3 CL (collimating lens)
4 PBS (Polarizing beam splitter)
5-wavelength plate (1/4 wavelength plate)
6 OL (Objective lens)
7mm condenser lens
8 Optical recording medium
10 PD (photodetector)
11a @ HOE for CD (diffraction grating)
11b @ HOE for DVD (diffraction grating)

Claims (10)

In a diffraction grating having a periodic uneven shape in cross section, a width D of a member having a large refractive index forming a convex portion of the uneven shape of the diffraction grating and a pitch 間 の between the convex portions satisfy the following condition D / Λ ≦ 0.4
A diffraction grating characterized by satisfying the following. In a diffraction grating having a cross-sectional shape having a periodic uneven shape, the pitch between the convex portions of the diffraction grating is Λ, the depth of the concave portion of the diffraction grating is T, the average refractive index of the diffraction grating is n, and the wavelength of the light source is the lambda, the Q value that defines the thickness profile of the diffraction grating as ^{Q = 2πλT / nΛ 2, n} 2 the refractive index of a large member of the refractive index for forming the convex portion of the diffraction _grating, the recess of said diffraction grating n ₁ the refractive index of the small member refractive index to form _a, when the refractive index difference was Δn = n ₂ -n _1, in the width D of the convex portion and the pitch Λ between the convex portions, the following Condition D / Λ = １ ／ · exp (−3ΔnQ / 2)
A diffraction grating characterized by satisfying the following. The pitch between the convex portions of the diffraction grating lambda, the depth of the concave portion of the diffraction grating T, the average refractive index of the diffraction grating n, as the wavelength of the light source lambda, Q given by Q = 2πλT / nΛ ² The value is the following condition Q> 1
3. The diffraction grating according to claim 1, wherein: A photomask having a mask pattern in which the ratio of the light and dark widths is changed from 1: 1 so as to satisfy D / Λ ≦ 0.4 in the width D of the protrusions of the diffraction grating and the pitch 間 の between the protrusions. The diffraction grating according to any one of claims 1 to 3, wherein the diffraction grating is formed using the diffraction grating. The diffraction grating according to any one of claims 1 to 4, wherein in the diffraction grating having the uneven shape, a convex portion is formed of a birefringent medium, and a concave portion is filled with an isotropic medium. 6. The diffraction grating according to claim 5, wherein an organic stretched film is used as the birefringent medium of the projection. 6. The diffraction grating according to claim 5, wherein a liquid crystal is used as the birefringent medium of the projection. The diffraction grating according to any one of claims 1 to 7, wherein one wavelength λ1 is transmitted and the other wavelength λ2 is diffracted by using a wavelength λ1 and a wavelength λ2 as the wavelength of the light source. . An optical pickup that uses the diffraction grating according to any one of claims 1 to 8 to capture light emitted from a light source, collect light on an optical recording medium by a condenser lens, and perform recording or reproduction.
A diffraction grating disposed in an optical path between the light source and the optical recording medium; and a photodetector configured to receive reflected light from the optical recording medium by branching the diffraction grating. Optical pickup. The optical pickup according to claim 9, wherein the light source, the photodetector, and the diffraction grating are integrated.

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