JP2004069977A - Diffraction optical element and optical head unit - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To increase the light use efficiency of a CD/DVD compatible optical head unit and to make the head unit small-sized and lightweight by obtaining a wavelength-selective diffraction optical element which has a small decrease in diffraction efficiency even when its grating pitch becomes fine. <P>SOLUTION: The diffraction optical element is mounted on the optical head unit. This diffraction optical element is equipped with a diffraction grating 12 which has a staircase shape in the cross section and a diffraction grating 13 which has a rectangular shape in the cross section, on one surface of a transparent substrate 11. Then the optical head unit has wavelength selectivity such that one of different-wavelength light beams made incident on the diffraction optical element is diffracted and the light of the other wavelength is transmitted. <P>COPYRIGHT: (C)2004,JPO

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a diffractive optical element and an optical head device, and more particularly to an optical head device that records and reproduces information on an optical recording medium.
[0002]
[Prior art]
In an optical head device that records and reproduces information on an optical recording medium (hereinafter referred to as an optical disk) such as a CD or DVD, light emitted from a semiconductor laser as a light source is collected on the optical disk by a lens and reflected by the optical disk. It becomes the return light. The return light is guided to a light receiving element which is a photodetector using a diffractive optical element or a beam splitter, and information on the optical disk is converted into an electric signal.
[0003]
CD / DVD compatible optical head devices have been commercialized in order to record and reproduce information using CD and DVD, which are optical discs having different standards, with the same optical head device. In an optical head device based on reproduction such as a CD-R / RW in which a medium having a high wavelength dependency with respect to reflection / absorption of light is used as a recording layer of an optical disc, a semiconductor having a wavelength band of 790 nm is used for CD. A laser is used. A semiconductor laser having a wavelength band of 660 nm is used for DVD.
[0004]
[Problems to be solved by the invention]
In a CD / DVD compatible optical head device, a wavelength-selective diffractive optical element that controls diffraction efficiency according to the wavelength of transmitted light is used in order to record and reproduce information on optical discs having different standards. Furthermore, in order to efficiently record and reproduce information on an optical disk, a diffraction optical element approximating a sawtooth shape with a stepped grating instead of a rectangular grating can be used to improve the diffraction efficiency. it can. However, as the CD / DVD compatible optical head device becomes smaller and lighter, the grating pitch of the step-like diffraction grating having wavelength selectivity becomes finer, and the grating has a rectangular cross section depending on the polarization state and incident angle of incident light. There is a problem that it is difficult to obtain high diffraction efficiency as compared with the above.
[0005]
[Means for Solving the Problems]
The present invention has been made to solve the above problems, and is a diffractive optical element including at least one transparent substrate and a diffraction grating formed on at least one surface of the transparent substrate. Wavelength selection that diffracts light of one wavelength out of two different wavelengths incident on the diffractive optical element and transmits light of the other wavelength, with a grating having a stepped section and a rectangular section Provided is a diffractive optical element characterized by having a property.
[0006]
Further, the diffractive optical element has a polarization selectivity that diffracts linearly polarized light that is incident as either one of extraordinary light or ordinary light, and transmits linearly polarized light that is incident as one of the other. I will provide a.
[0007]
Further, the present invention provides the above diffractive optical element in which a wave plate is integrated with the diffractive optical element.
[0008]
Further, two light sources that emit light of different wavelengths, an objective lens that condenses the light of different wavelengths on an optical recording medium, and light that is collected by the objective lens and reflected by the optical recording medium 2 are detected. An optical head device comprising two photodetectors, wherein the optical head device comprises the diffractive optical element in the optical path between the light source and the objective lens. I will provide a.
[0009]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
[0010]
“First Embodiment”
The diffractive optical element according to the first embodiment of the present invention is a diffractive optical element including a diffraction grating formed on one surface of a transparent substrate, and the diffraction grating has a step-like grating and a cross section. And a rectangular grid. And it is a diffractive optical element which has the wavelength selectivity which diffracts the light of one wavelength among the light of two different wavelengths which injects into a diffractive optical element, and permeate | transmits the light of the other wavelength. The diffraction grating may be formed separately on two surfaces of one transparent substrate, or may be formed on one surface or both surfaces of the opposing surfaces of the two transparent substrates.
[0011]
FIG. 1 is a schematic view showing a cross section of the diffractive optical element according to the first embodiment of the present invention. In the diffractive optical element according to the first embodiment, a diffraction grating 12 having a stepped cross section and a diffraction grating 13 having a rectangular cross section are formed on one surface of an optically isotropic transparent substrate 11 such as glass or quartz glass. It is a mixed configuration.
[0012]
When the refractive index of the grating material constituting the diffraction grating 12 and the diffraction grating 13 is n, the height of one step of the stepped diffraction grating 12 is λ ₂ / (n−1), so that the wavelength λ transmitted through the _second light may have a wavelength selectivity of diffracting other wavelengths lambda ₁ light.
[0013]
At this time, the phase difference generated by the height of one step of the diffraction grating 12 with respect to the wavelength λ ₁ is approximated to 2π × (λ ₂ / λ ₁ ) when the refractive index wavelength dispersion of the grating material is ignored. 2π × (λ ₂ / λ ₁ ) and 2π × m (where m is the phase difference δ, where the absolute value of the difference from the natural number is the smallest), stepwise diffraction that maximizes the diffraction efficiency of the wavelength λ ₁ The number of stages N of the grating 12 is when N × δ is closest to 2π.
[0014]
For example, when λ ₁ = 660 nm and λ ₂ = 790 nm, 2π × (λ ₂ / λ ₁ ) is 2π × 1.20, so δ = 2π × 0.2 and N = 5. Actually, taking into account the refractive index wavelength dispersion of the grating material, the number N of stages of the diffraction grating 12 that maximizes the diffraction efficiency of the wavelength λ ₁ is determined.
However, when the grating pitch of the stair-like diffraction grating 12 is fine, the influence of the grating side wall becomes large, and the transmittance for light of wavelength λ ₂ is lowered.
[0015]
On the other hand, the height of the rectangular diffraction grating 13 is also an integral multiple of λ ₂ / (n−1), so that it has wavelength selectivity for transmitting light of wavelength λ ₂ and diffracting light of wavelength λ _1. Can do.
[0016]
In general, the diffraction efficiency of the step-like diffraction grating 12 with a low grating height and negligible influence by the grating sidewalls is larger than the diffraction efficiency of the rectangular diffraction grating 13, but depending on the conditions of the grating pitch, incident polarization, and incident angle. Since the influence due to the grating side wall cannot be ignored, the diffraction efficiency of the stepped diffraction grating 12 may be lower than the diffraction efficiency of the rectangular diffraction grating 13.
[0017]
Therefore, the rectangular diffraction grating 13 is formed in a grating region where the diffraction efficiency of the stepped diffraction grating 12 is smaller than the diffraction efficiency of the rectangular diffraction grating 13, and the stepped diffraction grating 12 is formed in another region. As a result, it is possible to suppress a decrease in the diffraction efficiency of the wavelength λ ₁ and a decrease in the transmittance of the wavelength λ _{2 in} the entire diffractive optical element.
[0018]
By using the diffractive optical element according to the first embodiment of the present invention configured as described above, it is possible to realize a wavelength-selective diffractive element that suppresses a decrease in diffraction efficiency.
[0019]
“Second Embodiment”
The diffractive optical element according to the second embodiment of the present invention has polarization selectivity that diffracts linearly polarized light that is incident as one of extraordinary light and ordinary light, and transmits linearly polarized light that is incident as either of the other. . That is, when diffracting linearly polarized light incident as extraordinary light, the linearly polarized light incident as ordinary light is transmitted without being diffracted. Conversely, when diffracting linearly polarized light incident as ordinary light, the linearly polarized light incident as extraordinary light is transmitted without being diffracted.
[0020]
FIG. 2 is a schematic diagram showing a cross section of the diffractive optical element according to the second embodiment of the present invention. The diffractive optical element according to the second embodiment has a rectangular diffraction grating 22 and a rectangular cross section made of a birefringent material between transparent optical substrates 21 and 27 such as glass and quartz glass. In this configuration, the diffraction grating 23, an isotropic medium 24, a wave plate 25, and an adhesive 26 are sandwiched.
[0021]
The extraordinary light polarized light having two different wavelengths λ ₁ and λ ₂ is incident, and the extraordinary refractive index of the birefringent material as the grating material is n _e , and the refractive index of the isotropic medium 24 is n _s (≠ _ne). ), The height of one step of the stepped diffraction grating 22 is λ ₂ / (n _e −n _s ), so that the extraordinary light polarized with wavelength λ ₂ is transmitted and the extraordinary light with wavelength λ ₁ is transmitted. Wavelength selectivity for diffracting polarized light can be imparted.
[0022]
As in the case of the first embodiment, when the grating pitch of the stepped diffraction grating 22 is fine, the influence of the grating side wall becomes large, and the transmittance for the extraordinary light polarized light having the wavelength λ ₂ is lowered. On the other hand, the height of the rectangle of the diffraction grating 23, by a natural number multiple of _{λ 2 / (n e -n s} ), passes through the extraordinarily polarized light of wavelength lambda _2, the wavelength lambda ₁ of the extraordinarily polarized beam diffraction Wavelength selectivity can be imparted.
[0023]
In general, the diffraction efficiency of the step-like diffraction grating 22 where the grating height is low and the influence of the grating sidewall can be ignored is larger than the diffraction efficiency of the rectangular diffraction grating 23, but depending on the conditions of the grating pitch, incident polarization, and incident angle. Since the influence due to the grating side wall cannot be ignored, the diffraction efficiency of the step-like diffraction grating 22 may be lower than the diffraction efficiency of the rectangular diffraction grating 23.
[0024]
Therefore, the rectangular diffraction grating 23 is formed in a grating region where the diffraction efficiency of the stepped diffraction grating 22 is smaller than the diffraction efficiency of the rectangular diffraction grating 23, and the stepped diffraction grating 22 is formed in another region. Accordingly, it is possible to suppress a decrease in the diffraction efficiency with respect to the extraordinary light polarized light having the wavelength λ ₁ and a decrease in the transmittance with respect to the extraordinary light polarized light having the wavelength λ _{2 in} the entire diffractive optical element.
[0025]
In the wavelength band to be used, the refractive index n _s of the isotropic medium 24 and the ordinary refractive index n _o of the birefringent material serving as the grating material are matched to each other, so that the diffractive optical element according to the second embodiment is used. Is preferable because it can transmit light linearly as ordinary light linearly regardless of the wavelength and has polarization selectivity that functions as a wavelength-selective diffractive optical element with respect to light incident as abnormal light.
[0026]
When the light travels back and forth through the diffractive optical element of the second embodiment, as shown in FIG. 2, when the diffractive optical element is integrated with a quarter wave plate as the wave plate 25, the diffraction grating 22 and the diffraction grating are used. 23, normal light polarized light incident on the forward path is preferable because abnormal light can be incident on the return path.
[0027]
As a result, it has a function of a polarization diffraction grating that transmits the wavelength λ ₁ in the forward path and generates diffracted light in the return path, and has a function of linearly transmitting the wavelength λ ₂ in the forward path and the return path. In particular, even when a waveplate broadband characteristic that functions as a quarter-wave plate in the entire wavelength range to be used, has the function of linearly transmits incident light regardless of the polarization state of the wavelength lambda _2.
[0028]
According to the diffractive optical element of the second embodiment of the present invention configured as described above, it is possible to realize a wavelength-selective diffractive element that has polarization selectivity and suppresses a decrease in diffraction efficiency.
[0029]
“Third embodiment”
FIG. 3 is an optical layout diagram showing the optical head device according to the third embodiment of the present invention, and includes, for example, the diffractive optical element 6 according to the second embodiment. The optical head device of the third embodiment is a CD / DVD compatible optical head device for recording / reproducing information on DVD and CD optical discs 8 and is a semiconductor that emits linearly polarized light of wavelength λ ₁ for DVD as a light source. and a two light source the semiconductor laser 1B for emitting a linearly polarized light having a wavelength lambda ₂ of the laser 1A and CD.
[0030]
The linearly polarized light emitted from the semiconductor laser 1A passes through the beam splitter 4 and passes through the collimating lens 5. On the other hand, the linearly polarized light emitted from the semiconductor laser 1 </ b> B reflects the beam splitter 4 and passes through the collimator lens 5. The linearly polarized light having two wavelengths transmitted through the collimator lens 5 enters the diffractive optical element 6 of the present invention described in the second embodiment as ordinary light.
[0031]
Therefore, the linearly polarized light of the two wavelengths passes through the diffractive optical element 6 straight by the quarter-wave plate laminated on the diffractive optical element 6 and becomes circularly polarized light. Focused on top. Here, it is preferable to use a wavelength-selective beam splitter that totally transmits the wavelength light for DVD and totally reflects the wavelength light for CD, as the beam splitter 4 can efficiently use the light from the semiconductor laser.
[0032]
The two-wavelength circularly polarized light reflected from the optical disk 8 becomes linearly polarized light by the quarter-wave plate and then enters the diffraction grating of the diffractive optical element 6 as extraordinary light. The wavelength light for DVD is diffracted by the diffractive optical element 6, passes through the collimating lens 5 and the beam splitter 4, and is collected on the photodetector 2 </ b> A. On the other hand, the wavelength light for CD passes straight through the diffractive optical element 6, passes through the collimator lens 5, reflects off the beam splitter 4, is diffracted by the hologram element 3, and is collected on the photodetector 2B. Here, it is desirable to use a laser hologram unit in which the semiconductor laser 1B, the photodetector 2B, and the hologram element 3 are integrated as one component because the number of components of the optical head device can be reduced and the size and weight can be reduced.
[0033]
Even if the optical head device according to the third embodiment of the present invention configured as described above is reduced in size and weight, the light emitted from the two semiconductor lasers 1A and 1B can be efficiently used by using the diffractive optical element 6. it can.
[0034]
【Example】
"Example 1"
This example is a specific example of the diffractive optical element of the first embodiment shown in FIG. A diffraction grating 12 having a stepped cross section and a diffraction grating 13 having a rectangular cross section were produced on one surface of a transparent substrate 11 made of glass using photolithography and etching techniques. Further, a diffractive optical element was fabricated so as to have wavelength selectivity for transmitting light having a wavelength of 780 nm and diffracting light having a wavelength of 660 nm.
[0035]
The step-like diffraction grating 12 has a value 0.45 obtained by subtracting the refractive index of air from the refractive index 1.45 of the grating material, and a value of 1.73 μm obtained by dividing the wavelength of 780 nm as one step height. A stepped diffraction grating having a total height of 5.2 μm consisting of three steps was used.
[0036]
On the other hand, the height of the rectangular diffraction grating 13 was set to 3.47 μm, which is twice as high as 1.73 μm. As a result, a diffractive optical element that transmits light having a wavelength of 780 nm and efficiently diffracts light having a wavelength of 660 nm could be produced.
[0037]
Furthermore, depending on the grating pitch, incident polarization, and incident angle, the rectangular diffraction grating 13 is placed in a region where the diffraction efficiency by the step-like diffraction grating 12 is smaller than the diffraction efficiency by the rectangular diffraction grating 13, and other regions. A stepped diffraction grating 12 was formed. The plane pattern of these diffraction grating distributions is shown in FIG. The plane pattern consists of four regions, stepped diffraction grating 12 in the area _A 1, _{A 2} is a diffraction grating 13 of the rectangular region _B 1, _{B 2} are formed.
According to the diffractive optical element of this example configured as described above, a diffractive element having wavelength selectivity and high diffraction efficiency could be produced.
[0038]
"Example 2"
This example is a specific example of the diffractive optical element of the second embodiment shown in FIG. A method for manufacturing the diffractive optical element of this example will be described. A polyimide film is applied to one surface of a glass transparent substrate 21 and subjected to a rubbing alignment treatment to form an alignment film (not shown), and then the liquid crystal monomer applied on the alignment film is photopolymerized and cured to form a polymer. A liquid crystal layer was formed. Refractive index of the polymer liquid crystal, an extraordinary refractive index after photopolymerization curing n _e of about 1.7, the ordinary refractive index n _o was about 1.5. Next, a polymer liquid crystal formed on the transparent substrate 12 was fabricated using a photolithography and etching technique to produce a diffraction grating 22 having a stepped cross section and a diffraction grating 23 having a rectangular cross section. The plane pattern of these diffraction grating distributions was as shown in FIG.
[0039]
Further, an isotropic medium 24 is filled in the gap between the two diffraction gratings. At this time, an adhesive having a refractive index _ns of about 1.5 was used as the isotropic medium 24 to fill the gap of the diffraction grating, and at the same time, the wave plate 25 was bonded.
[0040]
Further, in the diffractive optical element of this example, the height of the stepped diffraction grating 22 and the rectangular diffraction grating 23 is set as follows so as to have wavelength selectivity for transmitting light of wavelength 780 nm and diffracting light of wavelength 660 nm. I decided so. The step-like diffraction grating 22 is obtained by dividing the value 3.9 μm obtained by dividing the wavelength 780 nm by the value 0.2 obtained by subtracting the refractive index of the adhesive 24 from the extraordinary refractive index 1.7 of the grating material. The height is a stepped diffraction grating having a total height of 11.7 μm consisting of three steps.
[0041]
On the other hand, the height of the rectangular diffraction grating 23 was set to 7.8 μm, which is twice the height of 3.9 μm. Further, depending on the grating pitch, incident polarization, and incident angle, the rectangular diffraction grating 23 is placed in a region where the diffraction efficiency of the stepped diffraction grating 22 is smaller than the diffraction efficiency of the rectangular diffraction grating 23, and other regions. A step-like diffraction grating 22 was formed.
[0042]
Here, as the wavelength plate 25, a polycarbonate uniaxially stretched to induce a retardation value corresponding to a quarter wavelength with respect to the center wavelength of light transmitted through the diffractive optical element of this example was used.
Next, an adhesive 26 was applied to the wavelength plate 25 and a transparent substrate 27 made of glass was laminated, and then the adhesive 26 was cured to produce the diffractive optical element of this example.
[0043]
According to the diffractive optical element of this example configured as described above, a diffractive optical element having both wavelength selectivity and polarization selectivity and high diffraction efficiency could be produced.
[0044]
Note that the planar pattern of the distribution of the stepped diffraction grating and the rectangular diffraction grating is not limited to the above, and may be variously changed according to the use situation.
[0045]
【The invention's effect】
As described above, when a CD / DVD compatible optical head device is reduced in size and weight, a step-like diffraction grating is used as a wavelength-selective diffractive optical element according to the conditions depending on the grating pitch, incident polarization, and incident angle. By using a diffractive optical element in which a rectangular diffraction grating is mixed, an optical head device with increased light utilization efficiency can be realized.
[Brief description of the drawings]
FIG. 1 is a sectional view of a diffractive optical element according to a first embodiment of the present invention.
FIG. 2 is a sectional view of a diffractive optical element according to a second embodiment of the present invention.
FIG. 3 is an optical layout diagram of an optical head device according to a third embodiment of the present invention.
FIG. 4 is a conceptual diagram showing an example of a plane pattern of a diffraction grating formed in the diffractive optical element of the present invention.
[Explanation of symbols]
1A, 1B: Semiconductor laser 2A, 2B: Photodetector 3: Hologram element 4: Beam splitter 5: Collimator lens 6: Diffractive optical element 7: Objective lens 8: Optical disks 11, 21, 27: Transparent substrates 12, 22: Stairs Shaped diffraction gratings 13 and 23: rectangular diffraction gratings 24 and 26: adhesive 25: wave plates A ₁ , A ₂ , B ₁ , B ₂ : divided areas of the planar pattern

Claims (4)

A diffractive optical element comprising at least one transparent substrate and a diffraction grating formed on at least one surface of the transparent substrate, wherein the diffraction grating comprises a grating having a stepped section and a rectangular section. A diffractive optical element characterized by having a wavelength selectivity of diffracting light of one wavelength out of two light of different wavelengths incident on the optical element and transmitting light of the other wavelength. 2. The diffractive optical element according to claim 1, wherein the diffractive optical element has polarization selectivity that diffracts linearly polarized light incident as one of abnormal light and ordinary light and transmits linearly polarized light incident as one of the other. element. The diffractive optical element according to claim 1, wherein a wave plate is integrated with the diffractive optical element. Two light sources for emitting light of different wavelengths, an objective lens for condensing the light of different wavelengths on an optical recording medium, and two lights for detecting light collected by the objective lens and reflected by the optical recording medium An optical head device comprising a detector, wherein the optical head device comprises the diffractive optical element according to claim 1 or 2 in an optical path between the light source and the objective lens. Optical head device.

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