JP2004342295A - Diffraction element and optical head device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To obtain an optical head device which is high in optical utilization efficiency by obtaining and using a diffraction element of which the diffraction efficiency is optimized for both wavelengths of a CD and a DVD in spite of one diffraction element. <P>SOLUTION: This diffraction grating 1 is provided with a cyclic concave-convex section formed on one transparent substrate 3. The convex section is provided with an optical multilayer film disposed in parallel with a transparent substrate surface. Moreover, the concave section is filled with a filling material 2 consisting of a transparent material which is an inorganic substance or contains an inorganic substance as the principal component. The diffraction element is obtained, in which the top face of the transparent material is flush with the top face of the convex section and is smooth, and the element is disposed in an optical path between a light source and an objective lens of the optical head device. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a diffraction element and an optical head device, and more particularly to an optical head device that records and reproduces information on an optical recording medium.

  In order to record and reproduce information using CD, DVD, and next-generation DVD, which are optical discs with different standards, using the same optical head device, compatible optical head devices using light waves of a plurality of wavelengths have been proposed. That is, it is a compatible optical head device including a plurality of semiconductor lasers that emit light waves in the wavelength band of 780 nm for CD, light waves in the 660 nm band for DVD, and light waves in the 405 nm band for next-generation DVD.

JP 10-59746 A

  The diffractive element used in the compatible optical head device is used in various applications such as a diffractive element for generating three beams for tracking, and a holographic element for guiding return light reflected from an optical disk as an optical recording medium to a photodetector. .

  In order to reduce the number of components, reduce the size, and reduce the weight of optical head devices that use light waves with multiple wavelengths, there is a great need for a diffraction element that has a single diffraction element with optimized diffraction efficiency for each wavelength. . However, in the conventional diffraction element, the wavelength dependence of the diffraction efficiency is almost determined only by the material and depth of the grating, and it is difficult to obtain a desired diffraction efficiency at each wavelength.

  The present invention has been made to solve the above-described problems, and is a diffraction grating including periodic uneven portions formed on at least one transparent substrate, and the convex portions are parallel to the transparent substrate surface. And the concave portion is filled with an inorganic substance or a transparent material mainly composed of an inorganic substance, and the upper surface of the transparent material coincides with the upper surface of the convex portion or is flat. The diffraction element is characterized in that the upper surface of the transparent material is flat.

  Further, the present invention provides the above-described diffraction element in which the diffraction element is further laminated with a multilayer substrate made of an organic thin film having birefringence or an inorganic material having birefringence.

  Further, the present invention provides the above diffraction element, wherein the transparent substrate is made of an inorganic substrate having birefringence.

  The optical multilayer film provides the above-described diffraction element configured such that when the wavelength of a light wave incident on the diffraction element deviates from a predetermined wavelength, the diffraction efficiency increases as the wavelength becomes longer than the predetermined wavelength.

Detection a light source for emitting light waves of a wavelength lambda ₁ and wavelength lambda _2, an objective lens for focusing the light waves having a wavelength lambda ₁ and wavelength lambda ₂ to the optical recording medium, the optical wave reflected by the condenser has been optical recording medium In the optical head device that records and reproduces information on the optical recording medium, the above-described optical path is shared between the light wave of wavelength λ ₁ and wavelength λ ₂ between the light source and the objective lens. a light source for emitting light waves of the optical head apparatus wavelengths lambda ₁ and wavelength lambda _2, characterized in that the diffraction element is installed, an objective lens for focusing the light waves having a wavelength lambda ₁ and wavelength lambda ₂ to the optical recording medium A light detector that detects a light wave that has been collected and reflected by the optical recording medium, and in an optical head device that records and reproduces information on the optical recording medium, a wavelength λ ₁ between the light source and the objective lens it and shared light wave of wavelength λ ₂ is In the optical path, to provide an optical head device, characterized in that said diffraction element is installed.

  Since the diffraction element of the present invention has a diffraction grating composed of an optical multilayer film on a transparent substrate, the wavelength dependence of diffraction efficiency can be easily controlled by designing the optical multilayer film. Further, the diffraction element of the present invention comprises a diffraction grating composed of an optical multilayer film, and an inorganic material filled in the concave portion of the diffraction grating or a transparent filler mainly composed of an inorganic material, and further, the diffraction grating and the transparent material Since the surface on which the filler is formed is flat, it is possible to obtain a good three beam with little fluctuation in temperature of diffraction efficiency and less noise.

  Further, by mounting the diffraction element of the present invention on an optical head device, an optical head device with a high degree of design freedom and high light utilization efficiency can be obtained.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

“First Embodiment”
The present invention is a diffractive element having a periodic concavo-convex portion formed on a single transparent substrate, and the convex portion is composed of an optical multilayer film arranged in parallel to the transparent substrate surface. The concave portion is a diffraction element filled with an inorganic material or a transparent material mainly composed of an inorganic material. That is, the concave portion is filled with a transparent material, and the height of the upper surface of the concave portion and the upper surface of the transparent material coincide with each other, and the surface is flat.
Alternatively, the recess may be filled with a transparent material, and the upper surface of the recess may be covered with the transparent material. Also in this case, the surface of the transparent material is flat.

Furthermore, the diffraction element provided with said periodic uneven | corrugated | grooved part may be formed in both the surfaces of one transparent substrate.
Furthermore, the above-described periodic uneven portion may be sandwiched between two transparent substrates. Also in this case, the upper surface of the concave portion may coincide with the upper surface of the filled transparent material, and the upper surface of the concave portion may be covered with the transparent material.

  As used herein, flat means the product Δnd of the regular undulation peak-to-valley d remaining on the surface as a result of filling the transparent material and the difference Δn between the refractive index of the transparent material and the refractive index of air, It means that it is 1/15 or less of the wavelength of visible light to transmit. Under this condition, diffraction due to undulation can be suppressed to 1% or less.

  FIG. 1 is a conceptual cross-sectional view of a diffraction element according to a first embodiment of the present invention. The diffraction element includes a diffraction grating 1 formed of an optical multilayer film formed on a transparent substrate 3 and a concave portion of the diffraction grating 1. It consists of the filler 2 which is a transparent material with which it filled.

  As the transparent substrate 3, an optically isotropic transparent parallel substrate such as glass or quartz glass can be used.

  As the diffraction grating 1, an optical multilayer film formed by depositing a dielectric, which is an optical multilayer film such as quartz or titanium oxide, on the transparent substrate 3 by vapor deposition or sputtering may be used. It is also possible to use a film obtained by forming a lift-off film on the transparent substrate 3 and patterning, and then depositing a dielectric such as quartz or titanium oxide by vapor deposition or sputtering, and removing the lift-off film. Good. In FIG. 1, for the sake of simplicity, the diffraction grating 1 is depicted as a five-layer optical multilayer film composed of two kinds of materials, but the material, type and number of layers of the film are designed so as to obtain a desired diffraction efficiency. do it.

  The transparent material as the filler 2 may be an inorganic material such as quartz glass or titanium oxide, or a material added with an organic material such as an acrylic resin or an epoxy resin containing the inorganic material as a main component. In the diffractive element according to the first embodiment, as described above, the transparent material filler 2 may be filled only in the concave portions of the diffraction grating 1 so that the upper portions coincide with each other, as shown in FIG. You may fill so that not only the recessed part of the diffraction grating 1 but a convex part may be covered. Reference numerals 2 and 3 in FIG. 2 denote a filler and a transparent substrate, respectively, as in FIG. In the following figures, reference numerals 1, 2 and 3 all mean the same element.

At this time, the difference between the temperature dependence of the refractive index of the optical multilayer film material forming the diffraction grating 1 and the temperature dependence of the refractive index of the transparent material forming the filler 2 is 100 × 10 ⁻⁶ / ° C. or less. Thus, it is preferable to select each material so that the temperature dependence of the diffraction efficiency in the diffraction element according to the first embodiment of the present invention can be reduced.

  As a means for filling the concave portion of the diffraction grating 1 with the filler 2, the transparent material described above may be formed and filled by CVD (chemical vapor deposition), or a solution of the transparent material described above may be filled with the diffraction grating 1. The film may be dropped and filled by spin coating.

  At this time, the transparent filler 2 is not only filled in the concave portions of the diffraction grating 1 but also deposited on the convex portions, and irregularities are generated on the surface of the transparent filler 2, thereby forming the transparent filler 2. A diffraction grating is formed (see FIG. 7). However, by optimizing the film formation conditions, the height of the irregularities on the surface of the transparent filler 2 can be reduced to 1/20 or less of the height of the irregularities of the diffraction grating 1.

In order to further flatten the surface of the transparent filler 2, polishing may be performed until the unevenness due to the transparent filler 2 is eliminated (see FIG. 2), or polishing may be performed until it matches the upper surface of the diffraction grating 1. Good (see FIG. 1).
Further, as a flattening means, a transparent substrate 6 having a flat surface may be laminated using an adhesive (not shown) having the same refractive index as that of the transparent filler 2 as shown in FIG. .

  As described above, the diffraction element according to the first embodiment of the present invention can increase the degree of freedom of design related to the wavelength dependence of diffraction efficiency by using a diffraction grating composed of an optical multilayer film formed on a transparent substrate. .

Furthermore, the temperature dependency of the diffraction efficiency can be reduced by filling the concave portion of the diffraction grating made of the optical multilayer film with a transparent filler whose refractive index temperature dependency is almost the same as that of the optical multilayer film material.
Further, by flattening the surface of the transparent filler, unnecessary diffracted light or scattered light that may be caused by small irregularities on the surface can be eliminated.

“Second Embodiment”
It is preferable that the diffractive element is a diffractive element in which a laminated substrate made of an organic thin film having birefringence or an inorganic material having birefringence is further laminated. The organic thin film or the inorganic substrate having birefringence may be disposed on the transparent substrate side or on the diffraction grating side.

  4 and 5 are conceptual cross-sectional views of the diffraction element according to the second embodiment of the present invention. The diffraction element includes a diffraction grating 1 formed of an optical multilayer film formed on a transparent substrate 3 and the diffraction grating 1. The transparent material filler 2 filled in the concave portion of the diffraction grating 1 and the organic thin film or inorganic substrate 4 having birefringence, and the laminated substrate 4 made of the organic thin film or inorganic material having birefringence, It may be laminated on the surface of the transparent filler 2 (see FIG. 4), or may be laminated on the surface of the transparent substrate 3 (see FIG. 5).

  As the diffraction grating 1, the transparent substrate 3, and the transparent filler 2, those similar to the diffraction element of the first embodiment of the present invention can be used.

As the laminated substrate 4 of the organic thin film or inorganic material having birefringence, a thin film in which birefringence is induced by uniaxial stretching of polycarbonate, or a thin crystal plate made of quartz can be used.
Further, an optically isotropic transparent substrate may be laminated on the organic thin film or inorganic material laminated substrate 4 having birefringence in order to protect the laminated substrate 4 or improve flatness (see FIG. Not shown).

  As described above, the diffraction element according to the second embodiment of the present invention realizes the light wave deflection control by the diffraction grating and the light wave polarization control by the laminated substrate of the organic thin film or the inorganic material having birefringence with one element. it can.

“Third embodiment”
The transparent substrate is preferably a diffractive element made of an inorganic substrate having birefringence.
FIG. 6 is a conceptual cross-sectional view of a diffraction element according to a third embodiment of the present invention. The diffraction element includes a diffraction grating 1 composed of an optical multilayer film formed on an inorganic substrate 5 having birefringence, It consists of a transparent filler 2 filled in the recesses of the diffraction grating 1.

  The diffraction grating 1 and the transparent filler 2 can be the same as the diffraction element of the first embodiment of the present invention. As the inorganic substrate 5 having birefringence, a single plate such as quartz or lithium niobate can be used. Further, an optically isotropic transparent substrate may be laminated on the diffraction grating 1 and the transparent filler 2 (not shown) in order to improve surface flatness.

  As described above, the diffraction element according to the third embodiment of the present invention can realize light wave deflection control by the diffraction grating and light wave polarization control by the inorganic substrate having birefringence with a single element.

“Fourth Embodiment”
FIG. 8 is an optical layout diagram showing an optical head device according to a fourth embodiment of the present invention. For example, a case where the diffraction element 31 of the present invention is mounted will be described. The optical head device of the fourth embodiment is a CD / DVD compatible optical head device for recording / reproducing information on DVD 37 and CD 38 which are optical recording media, and has a wavelength λ ₁ for DVD and a wavelength λ ₂ for CD. The diffraction element 31 of the present invention is provided between the two-wavelength light emitting semiconductor laser 32 that emits a light wave and the beam splitter 33.

  The diffraction element 31 will be described with reference to FIG. The diffractive element 31 includes a diffraction grating 11 made of an optical multilayer film and a first diffractive portion made of a transparent filler 14 filled in a concave portion of the diffraction grating 11 formed on the upper surface of the transparent substrate 13. Are formed of a diffraction grating 12 made of an optical multilayer film and a second diffraction portion made of a transparent filler 15 filled in the recesses of the second diffraction grating 12.

The first diffractive portion is designed to diffract the light wave of wavelength λ ₁ for DVD by about 6% and transmit the light wave of wavelength λ ₂ for CD almost 100%, while in the second diffractive portion, The optical wave of wavelength λ ₁ for DVD is diffracted almost 100%, and the optical wave of wavelength λ ₂ for CD is designed to transmit about 6%. Thereby, the diffractive element 32 can efficiently divide the light wave of each wavelength into three beams.

  The light waves of each wavelength from the two-wavelength light oscillation semiconductor laser 32 divided into three beams by the diffractive element 31 are transmitted through the beam splitter 33 and the collimator lens 34, and are applied to the DVD 37 or CD 38, which is an optical recording medium, by the objective lens 36. Focused. The light wave reflected by the DVD 37 or CD 38 returns in the order of the objective lens 36 and the collimating lens 34, is reflected by the beam splitter 33, and is detected by the photodetector 35. Three beams of light waves of each wavelength obtained by the diffraction element 31 can be used for tracking.

"Example 1"
This example is a specific example of the diffraction element of the first embodiment shown in FIG. An optical multilayer film made of quartz (SiO ₂ ) and titanium oxide (TiO ₂ ) is formed on the surface of the transparent substrate 3 made of glass by vacuum deposition, and the rectangular diffraction grating 1 is formed by photolithography and etching techniques. Formed.

  Next, as a transparent filler 2 in the concave portion of the diffraction grating 1, polysilazane is mixed with acrylic resin and titanium oxide, and the refractive index is adjusted to 1.51 with respect to a light wave having a wavelength of 589 nm and cured. did. Further, in order to flatten the slight unevenness remaining on the surface of the transparent filler 2 (see FIG. 7), the unevenness was polished by a precision polishing machine, and an antireflection film was applied to the polished surface (not shown). In addition, an antireflection film was applied to the surface of the transparent substrate 3 opposite to the surface on which the diffraction grating 1 was formed (not shown).

  Here, the diffraction efficiency of the diffractive element of the present embodiment is such that, in a light wave having a wavelength of 660 nm, the diffraction efficiency of 0th order light (transmittance of transmitted light) is 84%, the diffraction efficiency of + 1st order light is 6%, and the wavelength is 780 nm. The optical multilayer film constituting the diffraction grating 1 is designed (see Table 1) so that the diffraction efficiency (transmittance of transmitted light) of 0th order light is almost 100% in the light wave, and is made of quartz and titanium oxide. A 29-layer film was formed.

When the diffracted light with respect to the light wave having a wavelength of 660 nm of the diffractive element manufactured as described above was examined, the surface of the transparent filler 2 was flattened, so that three beams can be efficiently obtained without diffracted light and scattered light becoming noise. I was able to.
Furthermore, when the temperature dependence from room temperature to 100 ° C. was examined with respect to the ratio between the 0th-order diffraction efficiency and the + 1st-order diffraction efficiency, the ratio (0th / + 1st order) was between 14 ± 2 and the diffraction efficiency Thus, a diffraction element having a sufficiently small temperature fluctuation in practical use could be obtained.

  Next, the diffractive element produced in the above-described embodiment was mounted at the position of the diffractive element 31 of the optical head device shown in FIG. 8 and the tracking characteristics of the DVD 37 were examined. The information recording / reproducing characteristics were also good.

"Example 2"
This example is a specific example of the diffraction element of the first embodiment shown in FIG. Nine layers shown in Table 2 made of quartz (SiO ₂ ) and titanium oxide (TiO ₂ ) by vacuum deposition on the upper surface of the transparent substrate 3 made of glass with an antireflection film on the lower surface of the diagram. An optical multilayer film having a structure is formed. Next, the optical multilayer film is processed into a rectangular diffraction grating 1 by photolithography and etching techniques.

  Next, as a transparent filler 2 in the concave portion of the diffraction grating 1, polysilazane is mixed with acrylic resin and titanium oxide, and the refractive index is adjusted to 1.56 with respect to a light wave having a wavelength of 589 nm, and then cured. To do. Furthermore, a transparent substrate 6 made of glass having an antireflection film on the upper surface in the drawing is bonded onto the diffraction grating 1 with an adhesive.

Here, the diffraction efficiency of the diffraction element according to the present embodiment is as follows. The diffraction efficiency of 0th-order light (transmittance of transmitted light) is 87%, the diffraction efficiency of + 1st-order light is 5%, and the wavelength is 780 nm. In the light wave, the diffraction efficiency of 0th-order light (transmittance of transmitted light) is 87%, and the diffraction efficiency of + 1st-order light is 5%.
That is, according to the diffraction element of the present embodiment, the diffraction efficiencies for light waves having different wavelengths of 660 nm and 780 nm can be matched.

  For example, in a CD / DVD compatible optical head device, a tracking method for obtaining a tracking signal for CD and DVD with a single diffraction element has been proposed. When the diffraction element of this embodiment is used as the tracking signal diffraction element, a tracking signal having the same diffraction efficiency is recorded in both recording and reproduction of a CD using a light wave having a wavelength of 780 nm and a DVD using a light wave having a wavelength of 660 nm. Can be obtained.

"Example 3"
This example is one of the specific examples of the diffraction element of the first embodiment shown in FIG. Table 3 consisting of quartz (SiO ₂ ) and tantalum pentoxide (Ta ₂ O ₅ ) is formed on the upper surface of the transparent substrate 3 made of glass with an antireflection film on the lower surface of the diagram by vacuum deposition. An optical multilayer film having a 41-layer structure is formed. Next, the optical multilayer film is processed into a rectangular diffraction grating 1 by photolithography and etching techniques. Next, as a transparent filler 2 in the concave portion of the diffraction grating 1, the temperature dependence of the refractive index is 100 × 10 ⁻⁶ / ° C. or less. For example, polysilazane is mixed with an acrylic resin and titanium oxide, and a light wave having a wavelength of 660 nm is obtained. In contrast, a material adjusted to have a refractive index of 1.53 is filled and cured. Further, a glass transparent substrate 6 having an antireflection film on the upper surface in the figure is bonded onto the diffraction grating 1 with an adhesive.

  The diffraction efficiency of the diffractive element of this example is as follows: in the light wave with a wavelength of 660 nm, the diffraction efficiency of 0th order light (transmittance of transmitted light) is 87%, the diffraction efficiency of + 1st order light is 5%, and the light wave with a wavelength of 780 nm. The diffraction efficiency of 0th-order light (transmittance of transmitted light) is 99%.

In particular, the optical multilayer film is designed such that the diffraction efficiency with respect to light waves in the vicinity of a wavelength of 660 nm increases as the wavelength increases. That is, the diffraction efficiency is designed to increase as the wavelength becomes longer than 660 nm and decrease as the wavelength becomes shorter than 660 nm. Further, when the temperature of the diffraction element of the present invention rises, the material of the transparent filler 2 is selected so that the diffraction efficiency with respect to the light wave near the wavelength of 660 nm becomes small.
For example, when the temperature of the system including the diffractive element of this embodiment and the semiconductor laser that is the light source rises (or falls), the fluctuation of the diffraction efficiency due to the change in the oscillation wavelength accompanying the temperature rise (or fall) of the semiconductor laser, Variations in diffraction efficiency due to the material constituting the diffraction element of the present embodiment can be offset. That is, in the system including the diffraction element and the light source of the present embodiment, the temperature variation of the diffraction efficiency can be effectively suppressed to substantially zero.

  For example, in a CD / DVD compatible optical head device, when the laser beam is split, the diffraction element of this embodiment is used, so that the temperature of the system fluctuates due to the usage environment of the device and the heat generated during driving. Since the laser beam can be divided with substantially constant efficiency, the laser beam can be operated stably.

In addition, according to the method described in the present Example, as long as the transparent filler 2 is a material having a temperature dependency of the refractive index of 100 × 10 ⁻⁶ / ° C. or less, the above-described effects can be obtained regardless of whether the material is inorganic or organic. Can be obtained.

  Since the diffraction element of the present invention has an optical multilayer film structure, it can be used when design freedom is required for the wavelength dependence of diffraction efficiency.

Sectional drawing which shows an example of the cross section of the diffraction element of the 1st embodiment of this invention. Sectional drawing which shows the other example of the cross section of the diffraction element of the 1st embodiment of this invention. Sectional drawing which shows another example of the cross section of the diffraction element of the 1st embodiment of this invention. Sectional drawing which shows an example of the cross section of the diffraction element of the 2nd embodiment of this invention. Sectional drawing which shows the other example of the cross section of the diffraction element of the 2nd embodiment of this invention. Sectional drawing which shows an example of the diffraction element of the 3rd embodiment of this invention. Sectional drawing of this element in the middle of preparation of the diffraction element of this invention. The conceptual diagram which shows the optical arrangement | positioning of the optical head apparatus of the 4th embodiment of this invention. Sectional drawing which shows an example of the diffraction element of this invention mounted in the optical head apparatus of the 4th embodiment of this invention.

Explanation of symbols

1, 11, 12: Diffraction gratings 2, 6, 14, 15: Filler 3: Transparent substrate 4: Laminated substrate 5: Inorganic substrate 13: Transparent substrate 31: Diffraction element 32: Two-wavelength light oscillation semiconductor laser

Claims (5)

  A diffraction grating having a periodic concavo-convex portion formed on at least one transparent substrate, wherein the convex portion is composed of an optical multilayer film arranged in parallel to the transparent substrate surface, and the concave portion is made of an inorganic substance or an inorganic substance. A diffractive element, which is filled with a transparent material as a main component, and whose upper surface is flat so as to coincide with the upper surface of the convex portion, or is flat so as to cover the upper surface of the convex portion.   The diffractive element according to claim 1, further comprising a laminated substrate made of an organic thin film having birefringence or an inorganic material having birefringence laminated on the diffractive element.   The diffraction element according to claim 1, wherein the transparent substrate is made of an inorganic substrate having birefringence.   4. The optical multilayer film according to claim 1, wherein when the wavelength of the light wave incident on the diffraction element deviates from a predetermined wavelength, the diffraction efficiency increases as the wavelength becomes longer than the predetermined wavelength. The diffraction element as described. Detection a light source for emitting light waves of a wavelength lambda ₁ and wavelength lambda _2, an objective lens for focusing the light waves having a wavelength lambda ₁ and wavelength lambda ₂ to the optical recording medium, the optical wave reflected by the condenser has been optical recording medium In an optical head device that records and reproduces information on an optical recording medium, and in a light path shared by light waves of wavelength λ ₁ and wavelength λ ₂ between the light source and the objective lens. 5. An optical head device, wherein the diffraction element according to any one of 1 to 4 is installed.

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