JP2001093180A - Phase-difference element and optical head device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a thin type phase-difference element generating desired phase difference and to provide a small-sized optical head device. SOLUTION: An organic thin film 1 having a function to generate the phase difference is fixed to a fixing substrate 3 having a reflecting function by using an adhesive 2 after adjusting the retardation value of the film 1 so as to generate the desired phase difference to obtain the phase-difference element. In the optical head device, this phase-difference element is arranged between a semiconductor laser and an object lens so that the beam emitting from the semiconductor laser is reflected and guided toward a recording medium.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

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

[0002]

2. Description of the Related Art An optical head device provided with an optical element such as a phase difference element or a diffractive element for recording and reproducing optical information such as an optical recording medium such as an optical disk such as a CD or DVD or a magneto-optical disk. Used. The optical head device guides emitted light from a semiconductor laser to an optical recording medium, diffracts or deflects reflected light from the optical recording medium by an optical element such as a diffraction element or a beam splitter, and detects reflected light with a detector. It has become. In addition, the phase difference element, which is an optical element, alone or integrated with the diffraction element is usually provided between the semiconductor laser and the optical recording medium.

Conventionally, as a transmission type retardation element, an element obtained by polishing an inorganic single crystal such as quartz is used.
However, an inorganic single crystal has a strong birefringence and therefore has a strong incident angle dependence of a phase difference, and is not suitable as a phase difference element having a function of adding a desired phase difference. In addition, an inorganic single crystal optical element requires a large number of manufacturing steps, and further involves difficulties in manufacturing.

[0004] The phase difference element made of an organic material does not involve any great difficulty in its production, but the entire phase difference element has been thickened, for example, by being adhered to a substrate. Therefore, it has been difficult to reduce the size of the device when incorporated into an optical head device.

[0005]

As described above, the phase difference element using an inorganic single crystal has a large dependence on the incident angle of the phase difference, so that it is difficult to obtain a desired phase difference. It was difficult. Therefore, it has been difficult to use the optical head device.

Further, since the retardation element made of an organic material is provided on a fixed substrate, there is a problem that the transmission type retardation element becomes thick as a whole. Therefore, when this phase difference element is used in an optical head device, it is difficult to reduce the size of the device.

[0007]

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems, and an organic thin film having a phase difference generating function adjusts a retardation value so as to generate a desired phase difference. In addition, the present invention provides a phase difference element which is provided on a fixed substrate having a reflection function. In addition, the present invention provides the above-described phase difference element integrated with an aperture control element formed so as to transmit light of two or more wavelengths in a central part and to transmit light of one or more wavelengths in a peripheral part surrounding the central part. .

In an optical head device for guiding linearly-polarized light emitted from a semiconductor laser to an optical recording medium and for guiding reflected light from the optical recording medium to a photodetector, the optical recording device reflects the emitted light to form the optical recording medium. An optical head device provided with the above phase difference element so as to reach a medium is provided. Further, the relationship between the slow axis direction of the phase difference element and the polarization direction of the emitted light is specified so that the phase difference generated when the emitted light enters the phase difference element does not depend on the angle of incidence. The above-mentioned optical head device is provided.

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION In the phase difference element of the present invention, the organic thin film having a phase difference generating function is mounted on a fixed substrate having a reflection function, so that a transmission type substrate having the organic thin film fixed to the fixed substrate is provided. The thickness of the organic layer can be reduced to about half that of the phase difference element, and a thin phase difference element can be realized.

Hereinafter, description will be made with reference to the drawings. FIG.
As shown in the figure, the reflection type retardation element 8 has the following configuration. That is, one surface of the organic thin film 1 having a phase difference generating function is fixed to a fixed substrate 3 having a reflecting function by, for example, an adhesive 2. In the configuration of FIG. 1, a non-reflection coating 4 may be applied to the other surface of the organic thin film 1, and the surface reflection loss due to the difference in the refractive index between air and the organic thin film 1 is almost zero.
Becomes Here, an anti-reflection coat 4 is applied.

Further, when a laser beam such as a semiconductor laser is perpendicularly incident and transmitted, the organic thin film 1
/ 4 (m is a natural number).
When the light is reflected by the fixed substrate 3, the reflected light is mπ /
2 can be formed. This mπ / 2 will be described later.

Here, in order to generate a desired phase difference, the retardation value of the organic thin film 1 is adjusted. That is, when the difference between the ordinary refractive index and the extraordinary refractive index of the organic thin film 1 is Δn, the thickness of the organic thin film 1 is d, and the wavelength of the transmitted light is λ, the retardation value is given by Δn · d, and the phase difference 位相Ψ = between
There is a relationship of Δn · d · 2π / λ. Therefore, a desired phase difference can be obtained by adjusting the retardation value for light of a specific wavelength.

The organic thin film is made of a polymer thin film such as polycarbonate, polyimide, polyarylate, polyethersulfone, alicyclic polyolefin, and polyacrylate by imparting birefringence by uniaxial stretching or the like, thereby exhibiting a phase difference generating function. Organic thin films can be used. A resin other than the above may be used as an organic thin film having a phase difference generating function, but it is preferable to use modified polycarbonate, polyimide, polyethersulfone or alicyclic polyolefin from the viewpoint of heat resistance. Here, the modified polycarbonate means a modified polycarbonate in which polyester carbonate is used as a part of a component of the polycarbonate made of bisphenol A. Further, as the organic thin film, a polymer liquid crystal thin film obtained by subjecting a fixed substrate to a normal liquid crystal alignment treatment and applying a polymer liquid crystal monomer can also be used. This polymer liquid crystal may be either a side chain type or a main chain type.

An organic thin film made of polycarbonate or the like is uniaxially stretched so that a polymer chain is oriented, so that a difference occurs between a refractive index in a stretching direction and a refractive index in a direction perpendicular to the stretching direction. Becomes Normally,
The stretching direction is the direction of the slow axis that retards the phase of light, and the slow axis coincides with the direction in which a general birefringent material gives an ordinary refractive index or an extraordinary refractive index. Also, a desired retardation can be obtained by adjusting the stretching.

Further, when light diverging from the light emitted from a semiconductor laser or the like enters the phase difference element, the light has various angles of incidence, and large astigmatism occurs in the light reflected from the phase difference element. However, in the present invention, the thickness of the organic thin film having the phase difference generating function is set to 0.5 mm or less to prevent the deterioration of the information reproduction signal due to astigmatism. The following is preferable because deterioration of the reproduction signal can be further prevented.

As the material of the adhesive, acrylic, epoxy, urethane, polyester, polyimide,
Urea, melamine, furan, isocyanate,
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 preferable that the adhesive is applied smoothly and thinly at a constant thickness in order to reduce the wavefront aberration. 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.

Further, in order to reduce the reflection loss at the interface due to the difference between the refractive index of the adhesive and the refractive index of the organic thin film having a phase difference generating function, an adhesive having a refractive index substantially equal to that of the organic thin film is used. It is desirable to use As a fixed substrate having a reflection function used in the reflection type retardation element of the present invention, a metal thin film such as aluminum or a TiO ₂ and SiO ₂ or Ta ₂ O ₅
A dielectric multilayer thin film made of SiO ₂ and SiO ₂ can be used. A metal thin film can obtain a high reflectance relatively easily, and it is desirable to use this. On the other hand, a dielectric multilayer mirror can obtain a reflectance of almost 100% with respect to the wavelength of light to be used depending on the design of the optical film, and it is desirable to use this.

In FIG. 2, instead of the anti-reflection coating 4 applied to one surface of the organic thin film 1 in the phase difference element of FIG. 1, an optical cover glass 5 of a parallel transparent substrate provided with an anti-reflection coating is used as an adhesive. 6 is adhered. In FIG. 2, components having the same reference numerals as those in FIG. 1 mean the same components. By bonding the optical cover glass 5 to the organic thin film 1, the organic thin film 1
Since the influence of the light on the wavefront based on the surface roughness can be removed, the wavefront aberration characteristics can be improved. In order to reduce reflection at the interface between the optical cover glass 5 and the adhesive 6 and at the interface between the adhesive 6 and the organic thin film 1, the refractive index n ₆ of the adhesive 6 is changed to the refractive index of the optical cover glass 5. n ₅ and the average refractive index n ₁₁ of the ordinary light refractive index and the extraordinary light refractive index of the organic thin film 1
It is desirable to select such that the relationship of n ₆ ≒ (n ₅ · n ₁ ) ^1/2 is satisfied.

The reflection type retardation element 8 shown in FIG. 3 is obtained by changing the shapes of the fixed substrate 3 having a reflection function and the optical cover glass 5 to prisms. Since the light is vertically incident on the optical cover glass 5 due to the shape of the prism,
The wavelength dependence of the refraction angle due to the refractive index difference between the air and the optical cover glass 5 is removed. Selecting the refractive index of the adhesive 6 to be equal to the refractive index of the optical cover glass 5 can eliminate the wavelength dependence of the incident angle of light on the organic thin film 1 having a phase difference generating function. desirable.

In FIG. 4, components having the same reference numerals as those in FIG. 2 mean the same components. The reflection type phase difference element 8 is an optical element having both a phase difference generation function and a reflection function, is a thin phase difference element, and does not require a mirror, so that the volume occupied in the optical head device is reduced. Is preferable because it can be downsized.

It is also possible to integrate a phase difference element with an aperture control function of transmitting light of two or more wavelengths in the central portion of the transparent substrate and transmitting light of one or more wavelengths in the peripheral portion surrounding the central portion. Although it is an element, it is preferable because it has both functions of aperture control and phase difference generation. It is preferable to incorporate this composite element into an optical head device, because the size of the device can be reduced. Usually, the central portion transmits light of two wavelengths from the semiconductor laser, and the peripheral portion transmits only light of one wavelength. This aperture control function can be obtained by forming a dielectric multilayer film on the periphery of the transparent substrate, a diffraction grating having a cross section having a periodically uneven shape, and the like.

When the phase difference element of the present invention is used in an optical head device for recording and reproducing information on an optical recording medium such as an optical disk such as a CD or DVD or a magneto-optical disk, as shown in FIG. It is installed so that the emitted light of, for example, a semiconductor laser 7 serving as a light source enters the organic thin film 1. When the light emitted from the semiconductor laser 7 is perpendicularly incident on the organic thin film 1 and is reflected and emitted from the fixed substrate, the organic thin film 1 is (2p-1) π
/ 2 (p is a natural number) is used. Thereby, the linearly polarized light emitted from the semiconductor laser is reflected by the phase difference element of the present invention and becomes circularly polarized light.

Further, when the direction of the slow axis of the phase difference element and the linear polarization direction of the light emitted from the semiconductor laser have a specific relationship, the angle of incidence of the emitted light entering the phase difference element is hardly affected. It is preferable because a substantially constant phase difference can be obtained. For example, with respect to light having a wavelength of 650 nm, the direction of the slow axis of the organic thin film 1 is changed so that the ellipticity angle indicating the polarization state of the reflected light from the phase difference element becomes approximately 45 ° regardless of the incident angle. It is adjusted to the polarization direction and fixed to the fixed substrate 3. Thus, the outgoing light reflected by the phase difference element 8 generates a phase difference of (2p-1) π / 2 (p is a natural number),
Can be converted to circularly polarized light.

Here, the ellipticity angle is defined as tan α = ± b / a (−45) where a is the major axis of the elliptically polarized light and b is the minor axis.
(Α ≦ α ≦ 45 °). In particular,
By selecting an organic thin film 1 having a desired retardation value for a specific wavelength, the wavelength 650n used for DVD is selected.
A reflection-type retardation element that gives a phase difference of 5π / 2 for light in the vicinity of m and 2π for light of a wavelength of 780 nm used for CD on the other hand.

That is, when a phase difference element is incorporated in an optical head device using light of two wavelengths, the retardation value is adjusted, and light of one wavelength reflected by the phase difference element is 5π / 2 (ie, 5π / 2). , / 2) to give a circularly polarized light, and to the light of the other wavelength, give a phase difference of 2π (that is, an even number, π / 2) to make it linearly polarized. Further, a straight line can be obtained even with a phase difference of π.

When the phase difference element is placed in the common optical path of the light of 650 nm and the light of 780 nm,
The use efficiency of the light is increased using a polarization hologram or the like. In addition, for the light of 780 nm, the use efficiency of the light of the polarization hologram is reduced, and a separately placed non-polarization hologram can be used.

As described above, by making the polarization states of the lights of the respective wavelengths different from each other (circularly polarized light and linearly polarized light), it becomes easier to control each light for each wavelength. Therefore, in general, it is preferable that light reflected by a phase difference element in which an organic thin film whose retardation value is adjusted is fixed to a fixed substrate having a reflection function generates a phase difference of mπ / 2 (m is a natural number). Here, when m is an odd number, the reflected light becomes circularly polarized light, and when it is an even number, it becomes linearly polarized light. In addition, 2p
-1 corresponds to a case where m is an odd number.

[0028]

Example 1 In this example, the reflection type retardation element shown in FIG. 1 was manufactured. Organic thin film 1 having phase difference generating function
A retardation element was manufactured using uniaxially stretched polycarbonate, a polyester-based adhesive as an adhesive 2, and a glass substrate on which aluminum was deposited to a thickness of 100 nm as a fixed substrate 3 having a reflective function.

The organic thin film 1 has a thickness of 45 μm.
A phase difference of 8 nm (that is, 5π / 4), which is doubled to 816 nm when reflected by a phase difference element provided with the organic thin film 1.
(Ie, 5π / 2). Also,
In order to prevent reflection at the interface between the organic thin film 1 and air, a non-reflection coating 4 was applied to the air-side interface of the organic thin film 1.

As shown in FIG. 5, light emitted from a semiconductor laser 7 having an oscillation wavelength of 652 nm is transmitted through a polarizer 9 to be linearly polarized light parallel to the x-axis. This linearly polarized light enters the phase difference element 8 at an arbitrary angle. At this time,
The angle of incidence is the angle between the normal to the surface of the phase difference element 8 and the linearly polarized light.

Here, the slow axis azimuth of the organic thin film 1 is adjusted so that the ellipticity angle becomes approximately 45 °, and as a result, it is oriented in the direction of 42 ° with the x-axis of the coordinate system shown in FIG. φ =
After being rotated by 42 °, it was adhered to the aluminum film side of the fixed substrate 3 using the adhesive 2 described above.

The incident angle dependence of the phase difference of the light reflected by the phase difference element formed as described above was examined. as a result,
The light reflected at the phase difference element 8 at an incident angle θ = 20 to 50 ° has a phase difference of 806 to 824 nm. This is a shift of only about 9 nm at the maximum from the phase difference of 816 nm in the case of normal incidence, and the phase difference is circularly polarized at about 5π / 2.

The dependence of the phase difference on the incident angle was similarly examined using a semiconductor laser 7 having an oscillation wavelength of 780 nm as a light source. The linearly polarized light that reflects the phase difference element 8 at an incident angle θ = 20 to 50 ° is
A phase difference of 9 to 797 nm occurred. This means that the phase difference is only about 9 nm at the maximum from the phase difference of 788 nm in the case of normal incidence, and the phase difference is about 2π, which is linearly polarized light.

Accordingly, the phase difference element having the reflection function converts the linearly polarized light incident in a wide angle range of θ = 20 to 50 ° into substantially uniform circularly polarized light (for a wavelength of 650 nm), The polarized light (with respect to the wavelength of 780 nm) could be stored as it was, and the incident angle dependence of the phase difference of the reflected light could be reduced.

Example 2 In this example, the reflection type retardation element shown in FIG. 2 was manufactured. Glass in which uniaxially stretched polycarbonate is applied as the organic thin film 1 having a phase difference generating function, the same polyester-based adhesive is used as the adhesive 2 and the adhesive 6, and aluminum is deposited to a thickness of 100 nm as the fixing substrate 3 having the reflecting function. A retardation element was manufactured using a substrate having a thickness of 0.3 mm as an optical cover glass 5 and having an anti-reflection coating.

The organic thin film 1 is the same as that of Example 1 and has a thickness of 45 μm. The light having a wavelength of 652 nm transmitted through the organic thin film 1 at normal incidence and transmitted at 408 nm (about 5π / 4).
When the light reflected by the phase difference element provided with the organic thin film 1 was reflected, a phase difference of 816 nm (ie, 5π / 2) was doubled.

The organic thin film 1 was sandwiched between the aluminum film surface of the fixed substrate 3 and the uncoated surface of the optical cover glass 5 using the adhesive 2 and the adhesive 6. Here, the slow axis azimuth of the organic thin film 1 was adjusted so that the ellipticity angle became approximately 45 °, and as a result, the x axis of the coordinate system shown in FIG.
It was rotated in a direction of 5 °, that is, rotated by φ = 45 °, and attached to the aluminum film side of the fixed substrate 3 using the adhesive 2 described above.

The incident angle dependence of the phase difference of the light reflected by the phase difference element formed as described above was examined in the same manner as in Example 1. The light reflected from the phase difference element 8 at an incident angle θ = 10 to 60 ° has a phase difference of 808 to 822 nm. This means that the phase difference is only about 8 nm at the maximum from the phase difference of 816 nm in the case of normal incidence, and the phase difference is a circularly polarized light at about 5π / 2.

Further, the semiconductor laser 7 having an oscillation wavelength of 780 nm was used as a light source, and the incident angle dependence of the phase difference was similarly examined. The linearly polarized light that reflects the phase difference element 8 when the incident angle θ is 10 to 60 ° is 78
A phase difference of 1 to 795 nm occurred. This means that the phase difference is only about 7 nm at the maximum from the phase difference of 788 nm in the case of normal incidence, and the phase difference is about 2π, which is linearly polarized light.

Therefore, the phase difference element having the reflection function converts linearly polarized light incident at a wider angle than in Example 1 with θ = 20 to 50 ° into substantially uniform circularly polarized light (for a wavelength of 650 nm). Such linearly polarized light (with respect to a wavelength of 780 nm) could be preserved, and the incident angle dependence of the phase difference of the reflected light could be further reduced.

Example 3 In this example, the phase difference element of Example 2 was incorporated in an optical head device. This is shown in FIG. The linearly polarized light (in the direction of 14b) emitted from the semiconductor laser 7 is reflected by the reflection type retardation element 8 and is counterclockwise circularly polarized (in the direction of 14c).
Becomes The circularly polarized light (in the direction of 14 c) reflected by the phase difference element 8 is split by the beam splitter 10.
At this time, only the light traveling straight without diffracting the beam splitter 10 is used, and the light (not shown) traveling diffracted is not used.

The circularly polarized light having passed straight through the beam splitter 10 is condensed on an optical disk 13 by an objective lens 11. Further, the light reflected from the optical disk 13 becomes clockwise circularly polarized light (in the direction of 14d) and passes through the objective lens 11 and the beam splitter 10. At this time, the beam splitter 10
Is divided into light traveling toward the photodetector 12 and light traveling toward the semiconductor laser 7. The circularly polarized light split by the beam splitter 10 is reflected by the reflection type retardation element 8 and becomes linearly polarized light (direction of 14a).

This linearly polarized light (in the direction of 14a) is different from the linearly polarized light (in the direction of 14b) emitted from the semiconductor laser 7.
Vibration directions are orthogonal. Therefore, light emitted from the semiconductor laser 7 and light returning to the semiconductor laser 7 did not interfere with each other, and a stable laser output was obtained from the semiconductor laser 7. As a result, a signal detection error called jitter of light detected by the photodetector 12 was reduced, and a good reproduced signal was obtained.

In this embodiment, by incorporating the reflection type phase difference element 8 of the present invention into an optical head device, the number of components of the optical head device can be reduced, the size can be reduced, and good jitter can be achieved in the optical head device. It was confirmed that characteristics were obtained. Further, since the accuracy of the angle of incidence of the laser beam on the reflection type phase difference element is allowed, it is easy to assemble the optical head device.

[0045]

As described above, according to the present invention, an organic thin film having a phase difference generating function is mounted on a fixed substrate having a reflection function by adjusting a retardation value so that a desired phase difference is generated. The reflection type phase difference element is a phase difference element that generates a desired phase difference while being thin.

Also, by using this phase difference element, the number of parts can be reduced, the number of steps of assembling the optical head device can be reduced, and the device can be further downsized. Further, by specifying the slow axis of the phase difference element and the linear polarization direction of the laser light incident on the phase difference element, light having a large degree of freedom of the incident angle of the laser light to the phase difference element (easy to assemble). Can be a head device.

[Brief description of the drawings]

FIG. 1 is a sectional view showing an example of the configuration of a phase difference element of the present invention.

FIG. 2 is a sectional view showing another example of the configuration of the phase difference element of the present invention.

FIG. 3 is a sectional view showing another example of the configuration of the phase difference element of the present invention.

FIG. 4 is an enlarged view of the optical head device of the present invention in which the phase difference element of FIG. 2 is installed.

FIG. 5 is a conceptual diagram showing a measuring optical system and a coordinate system of the phase difference element of the present invention.

FIG. 6 is a cross-sectional view of the optical head device of the present invention in which the phase difference element of FIG. 2 is installed.

[Explanation of symbols]

 1: Organic thin film 2: Adhesive 3: Fixed substrate 4: Non-reflective coating 5: Optical cover glass 6: Adhesive 7: Semiconductor laser 8: Phase difference element 9: Polarizer 10: Beam splitter 11: Objective lens 12: Light Detector 13: Optical disk

Claims (4)

[Claims] 1. A phase difference element, wherein an organic thin film having a phase difference generation function is mounted on a fixed substrate having a reflection function, the retardation value of which is adjusted so as to generate a desired phase difference. . 2. An aperture control element formed to transmit light of two or more wavelengths in a central portion and to transmit light of one or more wavelengths in a peripheral portion surrounding the central portion. Phase difference element. 3. An optical head device which guides linearly polarized light emitted from a semiconductor laser to an optical recording medium and guides reflected light from the optical recording medium to a photodetector. An optical head device provided with the phase difference element according to claim 1 so as to reach a medium. 4. A method according to claim 1, wherein a phase difference between a slow axis direction of the phase difference element and a polarization direction of the emitted light is adjusted so that a phase difference generated when the emitted light is incident on the phase difference element does not depend on an incident angle. The optical head device according to claim 3, wherein the relationship is specified.