JP2001311821A - Phase shifter and optical head device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain an optical head device which stably detects signals by obtaining an optical element producing angles of polarization directions' rotation after transmission with respect to linearly polarized light of both wavebands for CD(compact disk) and DVD(digital versatile disk) type optical disks identical to each other. SOLUTION: The phase element 101 mounted on the optical head device comprises birefringent organic thin films 13A and 13B which have identical retardation values to each other, are superposed on each other so as to make respective optic axes intersect each other and are fixed by adhesives 12A and 12B between transparent substrates 11 and 14.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a phase shifter and an optical head device.

[0002]

2. Description of the Related Art In an optical head device for recording / reproducing information on an optical recording medium such as an optical disk such as a CD or a DVD or a magneto-optical disk, light emitted from a semiconductor laser as a light source is applied to the optical recording medium by a lens. The light is condensed and reflected by the optical recording medium to become return light. This return light is guided to a light receiving element, which is a photodetector, using a beam splitter, and information on the optical recording medium is converted into an electric signal.

[0003] CD / DVD compatible optical head devices have been commercialized for recording and reproducing information on CD and DVD optical disks, which are optical recording media of different standards, using the same optical head device. In an optical disk that uses a medium having a high wavelength dependence with respect to reflection and absorption of light as a recording layer of an optical recording medium, and a semiconductor laser used for a CD has a wavelength band of 790 nm in a CD-R or other optical disk. is there. At this time, a DVD uses a semiconductor laser having a wavelength band of 660 nm.

[0004]

A conventional phase shifter is a CD.
Has a desired retardation value with respect to light in the wavelength band of either semiconductor laser for DVD-based or DVD-based.
The same retarder did not have the same retardation value for light in both the D-system and DVD-system wavelength bands.

[0005] For example, it functions as a half-wave plate for light in the wavelength band for DVD system, and transmits the incident linearly polarized light by rotating the polarization direction, but it transmits one light for light in the wavelength band for CD system.
It does not function as a / 2 wavelength plate, and the emitted light becomes elliptically polarized light.
Or, conversely, it functions as a half-wave plate for light in the CD-system wavelength band, but does not function as a half-wave plate for light in the DVD-system wavelength band, and the emitted light is elliptical. It becomes polarized light.

As a result, the intensity fluctuation of the reflected signal light from the optical disk, which is condensed on the photodetector after passing through the polarization beam splitter, is more elliptical than that of the linearly polarized light due to the undesirable birefringence distribution remaining on the optical disk. There was a problem that the polarization was larger and information could not be recorded / reproduced. For this reason, there has been a demand for an optical element in which the rotation angle of the polarization direction after transmission is the same for linearly polarized light in both the CD and DVD wavelength bands.

[0007]

The present invention solves the above-mentioned problems.
And at least one transparent sheet
A birefringent organic thin film is fixed on the substrate, and the wavelength λ₁and
λ_Two(Λ₁<Λ_Two) In the phaser where linearly polarized light enters
And the transparent substrate has λ₁≦ λ ≦ λ_TwoWavelength λ
Λ / 2 retardation values for linearly polarized light
Two birefringent organic thin films have their optical axes
Are provided so as to intersect, and the wavelength λ ₁And λ_Two
When the linearly polarized light passes through the birefringent organic thin film,
The plane of polarization of each linearly polarized light is rotated by the same angle.
And a phaser characterized by the following.

Further, there are two light sources for emitting linearly polarized light of wavelengths λ ₁ and λ ₂ (λ ₁ <λ ₂ ), and an objective lens for condensing linearly polarized light of wavelengths λ ₁ and λ _{2 on} an optical recording medium. An optical head device for recording / reproducing information on / from the optical recording medium, the optical head device comprising: a light detector for receiving reflected light from the optical recording medium; Provided is an optical head device provided with a child.

[0009]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail with reference to the drawings. As shown in FIG. 1, the retarder 101 of the present invention has two birefringent organic thin films 13A and 13B sandwiched between transparent substrates 11 and 14,
Each member is fixed using adhesives 12A and 12B.

The two organic thin films 13A and 13B have the same retardation value, and the organic thin films 13A and 13B are overlapped so that the optical axes of the respective organic thin films cross each other. When linearly polarized lights of λ ₁ and λ ₂ (λ ₁ <λ ₂ ) enter, the plane of polarization of each linearly polarized light rotates by the same angle.

The organic thin films 13A and 13B are birefringent films whose optical axes are aligned in the stretching direction by stretching an organic material such as polycarbonate. The retardation values R _d of the respective organic thin films are the same, and λ ₁ ≦ λ
For linearly polarized light of wavelength λ having a relationship of ≦ λ ₂ , R _d = 1 /
2λ.

For example, 660n for DVD-type optical disc
m wavelength band (λ₁) And 790 for CD optical discs
nm wavelength band (λ_TwoField that constitutes a phase shifter for light
If R_d= With a retardation value of 330 to 395 nm
Good linearity of incident linearly polarized light can be maintained by using equipment thin films
It is preferable because performance and optical rotation performance can be obtained.
R _dIs an intermediate value in the above range, that is, about 362 nm.
When the above performance is maximized. Also, R_d= 330nm
The above performance is observed even at a smaller value of about 310 nm.

The rotation angles of the polarization directions of the two linearly polarized lights by the phaser are the same, and the rotation angle φ is the same as the organic thin film 13A.
Between the angle θ and the angle θ between the optical axis of 13B and φ = 180 ° −
It is given by the relationship of 2θ. Therefore, the organic thin film 13
By adjusting the angle θ between the optical axis of A and the optical axis of 13B, a desired rotation angle φ can be obtained.

After the desired alignment treatment is applied to the alignment film coated on the transparent substrates 11 and 14 as the organic thin films 13A and 13B, a solution of a liquid crystal monomer as a birefringent material is applied, A polymer liquid crystal polymerized and cured by irradiation with a light source for polymerization can also be used. In this case, the organic thin films 13A and 13B are
And 14 can be formed without using the adhesive 12A fixed to the phase shifter.

The materials of the adhesives 12A and 2B include:
Acrylic, epoxy, urethane, polyester, polyimide, urea, melamine, furan,
Isocyanate-based, silicone-based, cellulose-based, vinyl acetate-based, vinyl chloride-based, rubber-based and mixtures thereof can be used. The adhesive is preferably workable if it is a UV-curing type or a thermosetting type, but is not limited thereto. It is necessary that the adhesive be applied smoothly and thinly at a constant thickness in order to improve the correction of the wavefront aberration of the incident light. As a method of application, it is preferable to use a method such as spin coating or roll coating because the workability is excellent and the thickness can be easily controlled.

As the transparent substrates 11 and 14, an optically flat glass substrate or a plastic substrate can be used. Further, a diffraction grating may be formed on the surface of the transparent substrate by using a technique such as photolithography or etching to exhibit a function of diffracting light.

The phaser constructed as described above can rotate the plane of polarization by the same angle while maintaining the linearity of two different types of linearly polarized light, even if the wavelengths are different.

In the optical head device of the present invention shown in FIG. 2, the linearly polarized lights having different wavelengths emitted from the semiconductor lasers 1A and 1B are reflected by the beam splitters 2 and 3, respectively, become parallel light by the collimating lens 4, and become parallel light.
01 is transmitted. The linearly polarized lights of different wavelengths are
, The polarization direction is rotated by an angle φ, and the light is focused on the recording surface of the optical disk 6 by the objective lens 5. The return light reflected on the recording surface of the optical disk 6 becomes parallel light again by the objective lens 5 and passes through the phaser 101. The return light transmitted through the phaser 101 is focused on the photodetector 8 by the collimator lens 4 after the polarization direction of the return light is matched with the polarization direction of the forward light.

In the optical head device according to the present invention, since the phase shifter according to the present invention is used, the fluctuation in the intensity of the signal light reflected from the optical disk due to the unfavorable birefringence distribution remaining on the optical disk can be reduced and the optical head device can be stabilized. Record and play back information.

[0020]

EXAMPLE 1 Example 1 is a specific example of the phase shifter shown in FIG. Organic thin film 13 in which polycarbonate is drawn on transparent substrate 11 having a refractive index of 1.5 to exhibit birefringence.
A was fixed with a polyester-based UV-curable adhesive 12A. Retardation value R _d of organic thin film 13A
Was 725/2 nm = 362 nm. This value of R _d is half of the wavelength band (wavelength) between the 660 nm wavelength band (λ ₁ ) of the DVD system and the 790 nm wavelength band (λ ₂ ) of the CD system used in the present retarder.

Next, similarly by stretching a polycarbonate to express birefringence, retardation value R _d is a thin organic film 13B is the same 362nm as above were fixed with an adhesive 12B of UV curable polyester. At this time, the angle θ formed between the optical axis of the organic thin film 13A and the optical axis of the organic thin film 13B was fixed at 67.5 °.

Further, a transparent substrate 14 having a refractive index of 1.5 was bonded by using a polyester-based UV-curable adhesive 12A to produce a retarder 101. The linearly polarized light in the two wavelength bands incident from the transparent substrate 11 side of the retarder 101 manufactured as described above was emitted with both polarization directions rotated counterclockwise by 45 °. The linearly polarized light of two wavelength bands incident from the transparent substrate 14 side of the one-way retarder 101 was emitted with both polarization directions rotated clockwise by 45 °.

"Example 2" The retarder 101 produced in Example 1
Was installed between the collimating lens 4 and the objective lens 5 of the optical head device as shown in FIG. 66 of DVD system
The two types of linearly polarized light emitted from the semiconductor laser 1A that oscillates linearly polarized light in the 0 nm wavelength band (λ ₁ ) and the semiconductor laser 1B that oscillates linearly polarized light in the 790 nm wavelength band (λ ₂ ) of the CD system are combined with the phase shifter 101. The transparent substrate 11 (see FIG. 1)
It was incident from the side.

The semiconductor lasers 1A and 1B were set so that the polarization directions of the respective linearly polarized lights became parallel. This polarization direction and the organic thin film 13 constituting the retarder 101
The phase shifter 101 was installed in the optical head device so that the angle formed by the optical axis A (see FIG. 1) with the optical axis was 55 °.

In the optical head device thus configured, the light of the 660 nm wavelength band and the light of the 790 nm wavelength band converged on the optical disk 6 are both linearly polarized light,
The polarization directions were parallel.

FIG. 3 shows the wavelength dependence of the ellipticity angle as a parameter indicating the linearity of polarization. Generally, the polarization is elliptically polarized light. When the amplitude of the electric field in the major axis direction of the ellipse is a and the amplitude of the electric field in the minor axis direction is b, the ellipticity angle χ is tan.
^-1 (± b / a). When the ellipticity angle χ is 0 °, it means linearly polarized light, and when χ = ± 45 ° means circularly polarized light.

The solid line (A) in FIG. 3 shows the phase shifter 10 of the present invention.
1 shows the wavelength dependence of the ellipticity angle of light condensed on the optical disk 6 after passing through the optical disk 1 and having a wavelength of 600 to 900 nm.
Is substantially linearly polarized light in a wide band. On the other hand, the dotted line (B) in FIG. 3 shows the wavelength dependence of the ellipticity angle of the light transmitted through the half-wave plate designed for the conventional wavelength of 730 nm and condensed on the optical disk 6. And the band of substantially linearly polarized light is very narrow.

FIG. 4 shows the wavelength dependence of the polarization direction of the light collected on the optical disk 6. The polarization direction means the major axis direction of the elliptically polarized light. From FIG. 4, the light transmitted through the retarder 101 of the present invention and condensed on the optical disk 6, the solid line (A), and transmitted through the half-wave plate designed for the conventional wavelength of 730 nm. The light condensed on the optical disk 6 and the polarization direction of the dotted line (B) hardly change.
nm wavelength band light and 790 nm wavelength band light,
It can be seen that the polarization direction is approximately 45 °.

[0029]

As described above, the retarder of the present invention is
A single retarder can have the same retardation value with respect to laser light in both wavelength bands for CD optical discs and DVD optical discs while maintaining the state of linearly polarized light. Can be rotated by the same angle.

By mounting the retarder of the present invention on an optical head device, fluctuations of signal light due to undesired birefringence remaining on the optical disk can be reduced, and reading errors during information reproduction on the optical disk and information recording can be prevented. An optical head device that can perform stable signal detection with extremely few write errors can be realized.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view showing a configuration of a retarder according to the present invention, and FIG.
The figure which shows a mode of rotation of the polarization direction when the linearly polarized light of one wavelength is incident, and the figure which shows the state of rotation of the polarization direction when the linearly polarized light of another wavelength is incident.

FIG. 2 is a conceptual diagram showing a configuration of an optical head device according to the present invention.

FIG. 3 is a graph showing the wavelength dependence of the ellipticity angle of light condensed on an optical disc, showing the difference between the present invention and a conventional example.

FIG. 4 is a graph showing the wavelength dependence of the polarization direction of light condensed on an optical disc, showing the difference between the present invention and a conventional example.

[Explanation of symbols]

 101: phase shifter 11, 14: transparent substrate 12A, 12B: adhesive 13A, 13B: organic thin film 1A, 1B: semiconductor laser 2, 3: beam splitter 4: collimating lens 5: objective lens 6: optical disk 8: photodetector

Claims (2)

[Claims] 1. A phase shifter in which a birefringent organic thin film is fixed on at least one transparent substrate and linearly polarized light having wavelengths λ ₁ and λ ₂ (λ ₁ <λ ₂ ) is incident on the transparent substrate. ₁
Two birefringent organic thin films each having a retardation value of λ / 2 with respect to linearly polarized light having a wavelength λ in a relationship of ≦ λ ≦ λ ₂ are provided so as to overlap each other's optical axes, A phase shifter characterized in that, when linearly polarized light of wavelengths λ ₁ and λ ₂ passes through a birefringent organic thin film, the plane of polarization of each linearly polarized light is rotated by the same angle. 2. An object lens for emitting linearly polarized light having wavelengths λ ₁ and λ ₂ (λ ₁ <λ ₂ ), and an objective lens for condensing linearly polarized light having wavelengths λ ₁ and λ ₂ onto an optical recording medium. An optical head device comprising: a light detector that receives reflected light from an optical recording medium; and recording and reproducing information on the optical recording medium.
An optical head device provided with the phaser according to claim 1 in an optical path between a light source and an objective lens.