JP5082792B2 - Optical head device - Google Patents

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 disk.

  A diffraction grating or a hologram element that diffracts light is usually manufactured by providing a concave-convex grating with a periodic cross-sectional shape on the surface of a glass or plastic substrate. In such a diffraction element, the diffraction efficiency depending on the wavelength is almost determined only by the depth of the grating. For this reason, there has been a problem that the diffraction efficiency cannot be freely controlled for light of a plurality of wavelengths.

  Further, in an optical head device for reproducing and recording information on an optical disk, the wavelengths of laser light used for diversification of optical disks are 790 nm (CD system), 650 nm (DVD system), and 405 nm (next-generation DVD system). Wavelengths of In order to reproduce the information on these optical disks with the same optical head device, it is necessary to mount semiconductor lasers having a plurality of wavelengths.

  On the other hand, the diffraction element used in the optical head device is used for various purposes such as a diffraction element for generating three beams for tracking and a hologram element for changing the optical path in order to guide the return light reflected by the optical disk to the photodetector. .

  In such an optical head device used for light of a plurality of wavelengths, there are many needs for optimizing the diffraction efficiency for each wavelength. However, as described above, in a normal diffraction element, the wavelength dependence of the diffraction efficiency is almost determined only by the grating depth, and it is difficult to obtain a desired diffraction efficiency for each wavelength.

  An object of the present invention is to provide a diffractive element that solves the above-mentioned drawbacks of the prior art and can control the wavelength dependence of diffraction efficiency. Another object of the present invention is to provide an optical head device that solves the above-mentioned drawbacks of the prior art by using the diffractive element of the present invention in an optical head device using a plurality of wavelengths and widens the design freedom.

The present invention includes a light source, an objective lens for condensing the light emitted from the light source onto an optical recording medium, and a photodetector for detecting the emitted light that is collected and reflected by the optical recording medium. In the optical head device, a diffraction element is disposed in an optical path between the light source and the objective lens, and the diffraction element is provided with a first diffraction grating and a second diffraction grating overlapped with each other, At least one of the first diffraction grating and the second diffraction grating has a periodic uneven portion on the transparent substrate, and the convex portion of the uneven portion has a refractive index parallel to the transparent substrate surface. An optical multilayer film in which two different kinds of materials are alternately arranged is provided, the concave portion is filled with a filler made of solid or liquid, and the first diffraction grating is perpendicularly incident on the transparent substrate surface. wavelength of the light emitted from the light source 6 50 with diffracts against the light of m, the wavelength is hardly diffracted to light of 790 nm, the second diffraction grating with wavelength is diffracted to light of 790 nm, the light of wavelength 650nm In contrast, an optical head device that hardly diffracts is provided.

Further, only the first diffraction grating, to provide an optical head device of the said Ru comprising an optical multilayered film and the filler. The first diffraction grating provides the above-described optical head device in which the first-order diffraction efficiency is about 6% with respect to light having a wavelength of 650 nm.

In addition, the above- mentioned optical head device is provided in which the optical multilayer film includes alternately stacked SiO ₂ films and TiO ₂ films .

  The diffractive element of the present invention is formed by forming an uneven diffraction grating using an optical multilayer film such as an oxide periodically stacked on a transparent substrate. This diffractive element has almost constant wavelength dependency of diffraction efficiency. For example, the diffraction efficiency can be controlled.

  A conceptual cross-sectional view of an example of the diffraction element of the present invention is shown in FIG. In FIG. 1, reference numeral 1 denotes an optical multilayer film, which comprises a plurality of layers formed on a transparent substrate 3 in parallel with the substrate surface, and constitutes a convex portion. In FIG. 1, for simplicity, the film has five layers of two materials, but the material, type, and number of layers of the film may be designed so as to obtain a desired diffraction efficiency. 2 is a filler having a refractive index different from that of the transparent substrate, and may be filled between the convex portions by the optical multilayer film 1 on the transparent substrate 3, that is, filled with the concave portions. (See FIG. 2. The meaning of the reference numerals is the same as in FIG. 1.)

The filler is made of solid or liquid, for example, metal materials such as Au and Al, oxide and fluoride materials such as SiO ₂ , ZrO ₂ and MgF ₂ , inorganic materials such as H ₂ O, acrylic resins, epoxy resins, etc. Organic materials, and birefringent materials such as liquid crystals can be used. As the transparent substrate, a substrate such as glass or plastic can be used.

As the material of the optical multilayer film, inorganic materials such as SiO ₂ , SiON, ZrO ₂ , TiO ₂ , Al ₂ O ₃ , CeO ₂ , MgO, and MgF ₂ can be used. Moreover, an organic material may be sufficient. Examples of the method for forming the multilayer film include a vacuum deposition method and a sputtering method.

  In order to improve the transmitted wavefront aberration, it is preferable to provide a cover 4 having a flat surface made of glass or resin on the filler side of the diffraction element as shown in FIGS.

  FIG. 5 shows an example in which the diffraction element of the present invention is formed on both the front surface and the back surface of the transparent substrate 3. 11 is a 2nd optical multilayer film, 12 is a 2nd filler. Here, it is preferable because the degree of freedom in design can be increased, such as the wavelength dependency of the diffraction efficiency of the diffraction elements formed on both sides, and adjustment of the diffraction direction by changing the grating pitch.

  Moreover, this diffraction element and another optical element may be laminated and integrated. Examples of other optical elements include a wave plate, a normal diffraction grating, a polarizing diffraction grating, an optical element using liquid crystal, a phase control element, and the like. By integrating these optical elements and the diffractive element of the present invention into an integrated diffractive element, a single integrated diffractive element has multiple functions and the number of optical components when incorporated in an optical head device. This is preferable.

  Table 1 shows a first design example of the constituent material of the optical multilayer film in the diffraction element of the present invention.

For example, 13 layers of SiO ₂ films and TiO ₂ films are alternately stacked on a transparent substrate made of glass except for the lowermost layer and the uppermost Al ₂ O ₃ film, and when an Al ₂ O ₃ film is added, a total of 15 After the optical multilayer film is processed into a periodic uneven shape and a grating is produced, the concave portion of the grating is filled with a filler having a refractive index of 1.93 to form a diffraction element ( (See FIG. 1).

An example of the wavelength dependence of the first-order diffraction efficiency in this diffraction element is shown in FIG. FIG. 6 also shows the first-order diffraction efficiency of a conventional surface uneven-type diffraction grating as a comparative example. Thus, it can be seen that the concavo-convex diffraction grating formed by the optical multilayer film of the present invention exhibits a diffraction efficiency close to the theoretical limit of 40% for light with a wavelength of 350 nm to 900 nm. The refractive indexes of the SiO ₂ film, the TiO ₂ film, and the Al ₂ O ₃ film used in Table 1 were 1.47, 2.27, and 1.56, respectively, in order. However, if the refractive index is slightly different from the designed value depending on the film forming conditions, the film thickness is adjusted.

  Table 2 shows a second design example of the constituent material of the optical multilayer film in the diffraction element of the present invention.

As described above, SiO ₂ films and TiO ₂ films are alternately stacked on a transparent substrate made of glass, for example, to form an 29-layer optical multilayer film. The optical multilayer film is processed into a periodic concavo-convex shape to form a lattice. After fabrication, the grating recesses were filled with a filler having a refractive index of 1.51 to form a diffraction element (see FIG. 2).

An example of the wavelength dependence of the first-order diffraction efficiency in this diffraction element is shown in FIG. FIG. 7 also shows the diffraction efficiency of a conventional surface uneven diffraction grating as a comparative example. Thus, it can be seen that when the concave and convex diffraction grating is formed on the optical multilayer film of the present invention, light with a wavelength of 790 nm is transmitted without being diffracted, and light with a wavelength of 650 nm is diffracted by about 6%. Furthermore, it can be seen that there is almost no change in diffraction efficiency over a wide wavelength band from 600 nm to 670 nm. Note that the refractive indexes of the SiO ₂ film and the TiO ₂ film used in Table 2 were 1.48 and 2.32, respectively, in different order. However, similarly to the above, when the refractive index is slightly different from the design depending on the film forming conditions, the film thickness is adjusted.

  The case where the above-described diffraction element is mounted on an optical head device having a light source for DVD (wavelength 650 nm) and CD (wavelength 790 nm) optical disks, for example, will be described. For example, as shown in FIG. 8, the diffraction element 23 of the present invention is disposed between the collimating lens 24 and the light sources 21 and 22.

  This diffraction element will be described with reference to FIG. For example, the first diffractive portion (upper side of the transparent substrate 3) made of the first optical multilayer film 1 and the first filler 2 diffracts light having a wavelength of 650 nm by about 6%, and light having a wavelength of 790 nm is almost 100%. The second diffractive portion (the lower side of the transparent substrate 3) that is designed to transmit light and includes the second multilayer film 11 and the second filler 12 diffracts light having a wavelength of 790 nm by about 6%, and has a wavelength of 650 nm. Is designed to transmit almost 100% of the light. Thereby, the diffraction element 23 can generate | occur | produce three beams of each wavelength efficiently.

  The light from each light source divided into three beams by the diffraction element 23 passes through the collimating lens 24 and the beam splitter 25 and is condensed on the optical recording medium 27 by the objective lens 26. The light reflected by the optical recording medium 27 returns in the order of the objective lens 26 and the beam splitter 25 and is detected by the light detection optical system 28. Three beams of light of each wavelength obtained by the diffraction element 23 can be used for tracking.

  Further, the diffraction element of the present invention can also be used for near infrared light with a wavelength of 950 nm to 1700 nm used for optical communication, and in particular, it can be used as a component for wavelength division multiplex communication using light with a wavelength of 1550 nm band. For example, a beam splitter using a diffraction element having a constant diffraction efficiency in a wide wavelength band, a gain equalizer that adjusts the wavelength dispersion of the diffraction efficiency, and the like can be given.

  This embodiment will be described with reference to FIG. A relief-type first diffraction grating 32 was produced on one surface of a quartz glass substrate 31 which is a transparent substrate by photolithography and etching techniques. The grating pitch was 20 μm, the duty ratio was ½, the depth was about 1.4 μm, light with a wavelength of 650 nm was transmitted without being diffracted, and light with a wavelength of 780 nm was diffracted by about 10%.

  Further, an antireflection film for light having a wavelength of 650 nm and a wavelength of 790 nm was formed on the concavo-convex portion of the relief type diffraction grating by a vacuum deposition method. This relief type diffraction grating hardly diffracts light having a wavelength of 650 nm and has a diffraction efficiency (that is, transmittance) of 0th order light of about 90%, while light having a wavelength of 790 nm is diffracted by about 10% as first order diffracted light. The diffraction efficiency of the next light was about 55%.

  A dielectric multilayer film, which is an optical multilayer film shown in Table 2, was formed on the surface of the quartz glass substrate 31 opposite to the surface on which the relief-type diffraction grating 32 was produced by vacuum deposition. In the same manner as described above, the second diffraction grating having the dielectric multilayer film convex portion 33 was produced by photolithography and etching techniques. The lattice pitch was 20 μm, the duty ratio was 1/2, and the etching was stopped when the quartz glass of the substrate was exposed, so that only the dielectric multilayer film was uneven.

  An antireflection film for light having a wavelength of 650 nm and a wavelength of 790 nm was formed on one surface of the glass substrate by a vacuum deposition method to produce a cover glass 35. The concave portion of the second diffraction grating is filled with an acrylic adhesive 34 having a refractive index of 1.51 as a filler, the cover glass 35 is overlaid thereon, and the cover glass 35 is adhered by the adhesive 34. A diffraction element as shown in FIG.

  When the transmittance and diffraction efficiency of this diffractive element were examined, for light with a wavelength of 650 nm, the diffraction efficiency of the 0th order light was about 70%, the diffraction efficiency of the first order light was about 6%, while the light with a wavelength of 790 nm In contrast, the diffraction efficiency of the zero-order light was about 54%, and the diffraction efficiency of the first-order light was about 9%. And it was possible to obtain three beams efficiently for light of any wavelength.

  This diffractive element was mounted at the position of the diffractive element 23 in the optical head device shown in FIG. When an LD with a wavelength of 650 nm, which is a light source for DVD, is used as the light source 21, and an LD with a wavelength of 790 nm, which is a light source for CD, is used as the light source 22, the light of 790 nm and the light of 650 nm are efficiently divided into three beams. In addition, the light utilization efficiency of the optical head device could be improved. These three beams were used for tracking of the optical head device, and good reproduction characteristics were obtained when reproducing information from the optical disk.

  As described above, the diffractive element of the present invention has a diffractive diffraction grating formed by using an optical multilayer film such as an oxide periodically stacked on a transparent substrate. The diffraction efficiency can be controlled by making the wavelength dependence almost constant.

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

The side sectional view showing an example of the diffraction element of the present invention. Side sectional drawing which shows the other example of the diffraction element of this invention. FIG. 2 is a side sectional view showing a diffraction grating according to the present invention in which a transparent substrate is superimposed on the diffraction element of FIG. 1. FIG. 3 is a side sectional view showing a diffraction grating of the present invention, in which a transparent substrate is superimposed on the diffraction element of FIG. 2. Side sectional drawing which shows another example of the diffraction element of this invention. The graph which shows the wavelength dependence of the primary diffraction efficiency in the diffraction element obtained by the design example of the optical multilayer film shown in Table 1. The graph which shows the wavelength dependence of the primary diffraction efficiency in the diffraction element obtained by the design example of the optical multilayer film shown in Table 2. 1 is a schematic side view showing an example of an optical head device equipped with a diffraction element of the present invention. The schematic side sectional drawing of the diffraction element in an Example.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1, 11: Optical multilayer film 2, 12: Filler 3: Transparent substrate 4: Cover 21, 22: Light source 23: Diffraction element 24: Collimating lens 25: Beam splitter 26: Objective lens 27: Optical recording medium 28: Light detection Optical system 31: Quartz glass substrate 32: Diffraction grating 33: Dielectric multilayer film convex part 34: Adhesive 35: Cover glass

Claims (4)

An optical head device comprising: a light source; an objective lens for condensing the light emitted from the light source onto an optical recording medium; and a photodetector for detecting the emitted light collected and reflected by the optical recording medium. A diffractive element is disposed in the optical path between the light source and the objective lens,
The diffraction element is provided with a first diffraction grating and a second diffraction grating overlapping,
At least one of the first diffraction grating and the second diffraction grating has a periodic uneven part on a transparent substrate, and the convex part of the uneven part has a refractive index parallel to the transparent substrate surface. And an optical multilayer film in which two different materials are alternately arranged, and the concave portion is filled with a solid or liquid filler,
Wherein the first diffraction grating, with the wavelength of the light emitted from the light source incident perpendicularly to the transparent substrate surface is diffracted by pairs of 6 50 nm light, almost diffraction to light of wavelength 790nm Without
The second diffraction grating is an optical head device that diffracts light having a wavelength of 790 nm and hardly diffracts light having a wavelength of 650 nm . Only the first diffraction grating, the optical head device according to claim 1, Ru with the optical multilayer film and the filler. 3. The optical head device according to claim 1, wherein the first diffraction grating has a first-order diffraction efficiency of about 6% with respect to light having a wavelength of 650 nm . The optical multilayer film is composed of alternately stacked SiO. ₂ Film and TiO ₂ The optical head device according to claim 1, further comprising a film.

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