JP2009295280A - Optical head device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a wideband 1/2 wave plate functioning as a 1/2 wave plate for incident light having a plurality of wavelengths for use in recording and reproduction of optical information, wherein a material to be used is not limited to a material having small wavelength dispersion, and to provide an optical head device. <P>SOLUTION: The wideband 1/2 wave plate includes: transparent substrates 11a, 11b; alignment films 13a, 13b formed on substrate surfaces at one side of the transparent substrates 11a, 11b; anisotropic medium layers 14a, 14b comprising a medium having optical anisotropy; seal members 15a, 15b; and an adhesive 16. The anisotropic medium layers 14a, 14b are laminated so that fast axes form a mutual prescribed fast axis crossing angle. The anisotropic medium layer functions as the 1/2 wave plate at least an intermediate retardation wavelength. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a broadband half-wave plate for changing the polarization state of incident light over a wide band and an optical head device using the broadband half-wave plate, and more particularly, to a plurality of specific wavelengths used for recording or reproducing optical information. The present invention relates to a broadband ½ wavelength plate for changing the polarization state of incident light having a wavelength and an optical head device using the broadband ½ wavelength plate.

  Conventionally, in an optical head device, the amount of light output from a semiconductor laser widely used as a light source varies depending on the temperature of the element, etc., so the amount of light output from the semiconductor laser is kept constant by monitoring the amount of light. Control is performed so as to maintain (see, for example, Patent Document 1). In addition, since it is necessary to accurately control the amount of light in the optical head device for recording, a so-called front monitoring method is adopted in which the amount of outgoing light from the semiconductor laser element toward the objective lens is partially extracted to monitor the amount of light. (For example, refer to Patent Document 1).

  In the front monitor method, light that is emitted from a semiconductor laser is usually linearly polarized light, and light that emits incident light to the objective lens side according to the polarization direction using a beam splitter (hereinafter referred to as P-polarized component light). And the light emitted to the monitor side (hereinafter referred to as S-polarized component light), and the S-polarized component light is monitored using a photodiode or the like. The amount of light at the time of recording or reproducing optical information by the front monitor method can be controlled because the amount of S-polarized component light is proportional to the amount of P-polarized component light. This is because the light quantity of the P-polarized component light can be controlled based on this.

  Here, when optical information is recorded by the optical head device, it is necessary to increase the amount of light incident on the optical recording medium as much as possible. Therefore, it is necessary to reduce the amount of S-polarized component light used for the monitor. In addition, in order to appropriately output S-polarized component light having a predetermined amount of the incident light amount to the monitor, a technique for providing and adjusting a half-wave plate between the light source and the beam splitter is known. (For example, refer to Patent Document 1).

  However, the conventional half-wave plate used for this purpose functions as a half-wave plate near a specific wavelength due to wavelength dispersion, but does not function as a half-wave plate at other wavelengths. is there. For this reason, the use of a light source that emits light of a plurality of wavelengths is preferable from the viewpoint of miniaturization of the apparatus, and the like. Cannot output as a percentage.

  A technique is disclosed that is likely to eliminate the influence of such chromatic dispersion on the function of the half-wave plate (see, for example, Patent Document 2). According to the description in Patent Document 2, this technique is applied to a projection-type liquid crystal display device, and a retardation having a wavelength dispersion of 1.08 or less obtained by dividing the retardation value of the phase plate by the wavelength and multiplying by 2π. Is used for the purpose of rotating a polarization direction.

JP 2001-93183 A JP-A-10-90521

  However, in the conventional half-wave plate combining the techniques disclosed in Patent Document 1 and Patent Document 2, a plurality of films having wavelength dispersion in a predetermined range are used, and usable materials There is a problem that is extremely limited. Moreover, after producing a film, there existed a problem that the retardation value of a film was adjusted and a 1/2 wavelength plate could not be obtained. For this reason, there has been a great limitation or difficulty in using it as a half-wave plate used for the above purpose in an optical head device or the like.

  The present invention has been made in order to solve such problems, and is not limited to a material having a small wavelength dispersion, and can be applied to incident light having a plurality of specific wavelengths used for recording or reproducing optical information. A broadband ½ wavelength plate capable of functioning as a ½ wavelength plate and an optical head device using such a broadband ½ wavelength plate are provided.

  In view of the above points, the invention according to claim 1 is a half-wave plate disposed in an optical path in an optical head device having a light source that emits light of a plurality of wavelengths, and is optically anisotropic. A plurality of anisotropic medium layers made of a medium having a property are laminated, and each anisotropic medium layer divides the retardation value at the shortest wavelength among the wavelengths of the light emitted from the light source by the wavelength 2π. The phase difference obtained by multiplying the retardation value at the longest wavelength by the wavelength and dividing by 2π, and the phase difference obtained by multiplying by 2π, is an intermediate phase difference wavelength and functions as a half-wave plate It has the composition to do.

  With this configuration, a plurality of anisotropic medium layers are stacked, and each anisotropic medium layer functions as a half-wave plate at least with an intermediate retardation wavelength, so that the material used is limited to a material with small wavelength dispersion. Without this, it is possible to realize a broadband ½ wavelength plate capable of functioning as a ½ wavelength plate with respect to incident light having a plurality of wavelengths used for recording or reproducing optical information.

  The invention according to claim 2 is the invention according to claim 1, wherein the number of the anisotropic medium layers is two, and each of the anisotropic medium layers has a predetermined fast axis relative to each other. It is arranged so as to form an intersecting angle, and the fast axis intersecting angle is ½ of the polarization rotation angle that is an angle to rotate the polarization direction in the optical head device.

  With this configuration, in addition to the effect of the first aspect, the number of anisotropic medium layers is two, and the fast axis crossing angle is ½ of the polarization rotation angle. A broadband ½ wavelength plate capable of functioning as a / 2 wavelength plate can be realized.

  According to a third aspect of the present invention, in the first or second aspect, the material of the anisotropic medium layer is a polymer liquid crystal.

  With this configuration, in addition to the effect of the first or second aspect, the material of the anisotropic medium layer is a polymer liquid crystal, so that a broadband half-wave plate capable of adjusting the thickness of the anisotropic medium layer is realized. it can.

  According to a fourth aspect of the present invention, in any one of the first to third aspects, a plurality of polarization diffraction gratings are integrated with the broadband half-wave plate.

  According to this configuration, in addition to the effect of any one of claims 1 to 3, since a plurality of polarization diffraction gratings are integrated with the broadband half-wave plate, the number of parts can be reduced and integrated. Adjustment of the polarization direction can be completed simply by adjusting the arrangement of the components and the beam splitter, and a broadband half-wave plate capable of improving productivity can be realized.

  According to a fifth aspect of the present invention, there is provided a light source that emits a plurality of linearly polarized lights having different wavelengths, and a first polarized light obtained by polarizing the incident light in the first polarization direction according to the polarization direction of the incident light. A beam splitter that separates component light into second polarized component light polarized in the second polarization direction and emits the first polarized component light and the second polarized component light to different photodetectors, respectively. A first photodetector for detecting the first polarized component light emitted from the beam splitter; and a second photodetector for detecting the second polarized component light emitted from the beam splitter; A half-wave plate that rotates a polarization direction of light incident on the beam splitter, and records or reproduces optical information using the first polarization component light. A wave plate according to claims 1 to 4. The first half-wave plate comprising the broadband half-wave plate according to any one of the claims, wherein the polarization direction of light incident on the beam splitter is rotated at a plurality of wavelengths of light emitted from the light source and emitted from the beam splitter. The ratio of the amount of light of the first polarization component light and the second polarization component light is fixed to a predetermined value, and the first light is detected based on the amount of the first polarization component light detected by the first photodetector. 2 has a configuration for controlling the amount of polarized component light.

  With this configuration, the effects of any one of claims 1 to 4 can be obtained, and the amount of light of a plurality of wavelengths used for recording or reproducing optical information can be monitored with reduced wavelength dependence. An optical head device with high light utilization efficiency can be realized.

  In the present invention, a plurality of anisotropic medium layers are stacked, and each anisotropic medium layer functions as a half-wave plate at least at an intermediate retardation wavelength. Accordingly, it is possible to provide a broadband half-wave plate and an optical head device that have the effect of functioning as a half-wave plate for incident light having a plurality of wavelengths used for recording or reproducing optical information.

Explanatory drawing for demonstrating the cross-sectional structure of an example of the broadband 1/2 wavelength plate which concerns on the 1st Embodiment of this invention The direction of polarization of light incident on the broadband half-wave plate, the direction of the fast axis of the anisotropic medium layer, the direction of the fast axis of the anisotropic medium layer 14b, and the exit from the broadband half-wave plate 1 Explanatory drawing for demonstrating the relationship with the polarization direction of the light to light Explanatory drawing for demonstrating the effect | action of the broadband 1/2 wavelength plate which concerns on the 1st Embodiment of this invention. The figure which shows the block structure of the optical head apparatus based on the 2nd Embodiment of this invention. The figure which shows the other structural example of the optical head apparatus based on the 2nd Embodiment of this invention. The figure for demonstrating the light of the P polarization component and the light of the S polarization component which are radiate | emitted from a broadband 1/2 wavelength plate The figure which shows the information corresponding to FIG. 6 when the conventional technique which produces a half-wave plate with one anisotropic medium layer is applied.

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

(First embodiment)
FIG. 1 is an explanatory diagram for explaining a cross-sectional structure of an example of a broadband half-wave plate according to the first embodiment of the present invention. In FIG. 1 (c), the broadband half-wave plate 1 includes a first intermediate phase difference half-wave plate 1a shown in FIG. 1 (a) and a second intermediate phase difference 1 shown in FIG. 1 (b). / 2 wavelength plate 1b is laminated using a transparent adhesive 16. As shown in FIGS. 1A and 1B, each of the intermediate phase difference half-wave plates 1a and 1b is formed on one surface of the transparent substrates 11a and 11b and the transparent substrates 11a and 11b, respectively. Alignment films 13a and 13b, anisotropic medium layers 14a and 14b made of a medium having optical anisotropy, and sealing materials 15a and 15b.

  The transparent substrates 11a and 11b are made of a material having optical isotropy such as glass. The anisotropic medium layers 14a and 14b are formed using, for example, polymer liquid crystal, but may be formed using other materials having optical anisotropy. Hereinafter, the anisotropic medium layers 14a and 14b will be described as being formed using a UV curable polymer liquid crystal that is polymerized by irradiation with UV light.

  The alignment films 13a and 13b are aligned so that the alignment of the liquid crystal molecules of the liquid crystal layers 14a and 14b is aligned in a predetermined direction. As the alignment films 13a and 13b, an alignment material such as polyimide is applied to the substrate and the surface is subjected to alignment treatment by rubbing, an inorganic film such as an SiO film is obliquely deposited on the substrate surfaces of the transparent substrates 11a and 11b, Or you may use the photo-alignment film etc. which express alignment ability by irradiating an ultraviolet-ray etc. after apply | coating organic substance to a board | substrate.

  The anisotropic medium layers 14a and 14b are aligned using the alignment films 13a and 13b, thereby holding a fast axis having a normal light refractive index and a slow axis having an extraordinary light refractive index. Here, linearly polarized light (hereinafter referred to as medium incident light) incident on the anisotropic medium layers 14a and 14b is light of a component polarized in the slow axis direction of the polarization direction of the medium incident light. Is delayed by 180 degrees (half wavelength) compared to the light component polarized in the fast axis direction component.

  2 shows the polarization direction of light incident on the broadband half-wave plate 1, the direction of the fast axis of the anisotropic medium layer 14a, the direction of the fast axis of the anisotropic medium layer 14b, and the broadband 1 / It is explanatory drawing for demonstrating the relationship with the polarization direction of the light radiate | emitted from the 2 wavelength plate. Here, the angle formed by the direction of the fast axis of the anisotropic medium layer 14a and the direction of the fast axis of the anisotropic medium layer 14b (hereinafter referred to as a fast axis crossing angle) is an optical head device. The anisotropic medium layers 14a and 14b are laminated so as to be 1/2 of the angle θ at which the polarization direction is rotated (hereinafter referred to as polarization rotation angle) θ.

  The anisotropic medium layers 14a and 14b have the same film thickness and function as a half-wave plate with an intermediate phase difference wavelength. Here, the intermediate phase difference wavelength means that the phase difference obtained by dividing the retardation value at the intermediate phase difference wavelength by the wavelength is obtained by dividing the retardation value at the shortest wavelength among the wavelengths of the light emitted from the light source by the wavelength. The wavelength equal to the average value of the phase difference obtained by multiplying the retardation value obtained by multiplying the retardation value at the longest wavelength by the wavelength. Therefore, each anisotropic medium layer 14a, 14b has a phase difference of 180 degrees (1/2 wavelength) at the intermediate phase difference wavelength.

  The sealing materials 15a and 15b are formed by printing outside the optically effective area of the substrate surface on which the alignment films 13a and 13b of the transparent substrates 11a and 11b are provided. Here, the sealing material 15a sandwiches the anisotropic medium layer 14a between the transparent substrate 11a and the substrate 12a (see FIG. 1A) facing the transparent substrate. As the sealing materials 15a and 15b, a thermosetting polymer such as an epoxy resin, an ultraviolet curable resin, or the like can be used. In order to obtain a desired cell interval, a spacer such as a glass fiber is used for the sealing materials 15a and 15b. It may be mixed. Similarly, the sealing material 15b sandwiches the anisotropic medium layer 14b between the transparent substrate 11b and the substrate 12b (see FIG. 1B) facing the transparent substrate.

Hereinafter, the operation of the broadband half-wave plate 1 according to the first embodiment of the present invention will be described. FIG. 3 is an explanatory diagram for explaining the operation of the broadband half-wave plate 1 according to the first embodiment of the present invention. FIG. 3 shows three different points using a Poincare sphere represented in a space having the three elements S ₁ , S ₂ , S ₃ of the Stokes vector as coordinate axes (S ₁ axis, S ₂ axis, S ₃ axis). A curve is shown that shows how the polarization state changes in the broadband ½ wavelength plate 1 when light of a wavelength is incident on the broadband ½ wavelength plate 1. Here, the curves shown in FIG. 3 using the symbols S (λ ₁ ), S (λ ₂ ), and S (λ ₃ ) are, for example, Stokes vectors of medium incident light having wavelengths near 405 nm, 660 nm, and 785 nm. Is shown.

Here, the light incident on the broadband half-wave plate 1 is linearly polarized light, and the polarization state of this light is expressed as [1, 1, 0, 0] ^t using the Stokes vector. Here, the superscript “t” represents transposition. In addition, the broadband half-wave plate 1 forms an angle θ ₁ that is a quarter of the polarization rotation angle θ with respect to the polarization direction of the light incident on the fast axis of the anisotropic medium layer 14 a, so that the anisotropic medium fast axis of the layer 14b is disposed at an angle theta ₂ of 3/4 of the polarization rotation angle theta with respect to the polarization direction of the incident light.

The fast axis of the anisotropic medium layer 14a is represented as an axis A _x1 ′ that forms an angle 2θ ₁ with the S ₁ axis and intersects the equator on the Poincare sphere, and the fast axis of the anisotropic medium layer 14b is Poincare This is represented as an axis A _x2 ′ that forms an angle 2θ ₂ with the S ₁ axis on the sphere and intersects the equator. The polarization state of the light of each wavelength incident on the broadband half-wave plate 1 is expressed as [1, 1, 0, 0] ^t using the Stokes vector, so that the S ₁ axis and the equator cross each other. When the medium incident light of each wavelength passes through the anisotropic medium layer 14a, it rotates around the axis A _x1 ′ corresponding to the _fast axis of the anisotropic medium layer 14a, and the following equation [St ₁ ].

[St ₁ ] = [T _θ1 ] · [C _Δ1 ] · [T _−θ1 ] · [St ₀ ] (1)
Here, Δ1 is the retardation value (degrees) when the medium incident light of each wavelength passes through the anisotropic medium layer 14a, and [St ₀ ] is Stokes expressed by [1, 1, 0, 0] ^t. Is a vector.

When medium incident light of each wavelength is subsequently transmitted through the anisotropic medium layer 14b, the polarization state represented by [St ₁ ] is around the axis A _x2 ′ corresponding to the _fast axis of the anisotropic medium layer 14b. To change to a polarization state represented by [St] in the following equation.

[St] = [T _θ2 ] · [C _Δ2 ] · [T _−θ2 ] · [St ₁ ] (2)
Here, Δ2 is a retardation value (degrees) when the medium incident light of each wavelength passes through the anisotropic medium layer 14b.

Here, the optical rotator matrices [T _θ1 ] and [T _θ2 ] and the phase shifter matrices [C _Δ1 ] and [C _Δ2 ] are expressed using the following equations (3) and (4), respectively. The

[T _θi ] = [[1], [U _2θi ], [V _2θi ], [4]] (3)
[C _Δi ] = [[1], [2], [D _Δi ], [E _Δi ]] (4)
Here, [1], [2], [4], respectively ^[1,0,0,0] t, [0, 1, 0, 0] t, be a ^{[0,0,0,1] t} , [U _2θi ] is [0, Co _θi , Si _θi , 0] ^t , [V _2θi ] is [0, −Si _θi , Co _θi , 0] ^t , and [D _Δi ] is [0, 0, Co _Δi , −Si _Δi ] ^t , [E _Δi ] is [0, 0, Si _Δi , Co _Δi ] ^t , Co _θi is cos (2θ _i ), and Si _θi is sin (2θ _i ), Co _Δi is cos (Δ _i ), Si _Δi is sin (Δ _i ), and i is 1 or 2.

  Hereinafter, changing the polarization state of light incident on the broadband half-wave plate 1 by the anisotropic medium layers 14a and 14b as described above is referred to as polarization state rotation. Since the broadband half-wave plate 1 of the present invention performs polarization state rotation by a plurality of anisotropic medium layers 14a and 14b, the Stokes vector subjected to the first polarization state rotation by the anisotropic medium layer 14a has wavelength dispersion. Thus, the part away from the equator in the latitudinal direction functions to return to a point close to the equator by the second polarization state rotation by the anisotropic medium layer 14b.

  Further, the broadband half-wave plate 1 of the present invention has a half-wave plate in which each of the anisotropic medium layers 14a and 14b is provided for an intermediate phase difference wavelength of light of each wavelength used for recording or reproducing optical information. Therefore, the Stokes vectors corresponding to the light of two wavelengths having the largest wavelength difference subjected to the polarization state rotation by the anisotropic medium layers 14a and 14b are further brought closer to the vicinity of the equator.

  As described above, the broadband half-wave plate 1 of the present invention has a plurality of anisotropic medium layers stacked on each other, with respect to the intermediate phase difference wavelength of each wavelength of light used for recording or reproducing optical information. Since each anisotropic medium layer 14a, 14b functions as a half-wave plate, substantially linearly polarized outgoing light can be obtained regardless of the wavelength used. Here, “substantially linearly polarized light” means that when the broadband half-wave plate 1 of the present invention is used in an optical head device, the polarization state of light of each wavelength used for recording or reproducing optical information is changed to the same extent. It means a polarization state including an elliptically polarized component to the extent that a desired function of the optical head device can be ensured.

  Specifically, when the broadband half-wave plate 1 of the present invention is used for the purpose of detecting the amount of light of each wavelength used for recording or reproducing optical information, the recording or reproducing of optical information is performed. It includes linearly polarized light to the extent that it can be properly performed. However, the ratio of the linearly polarized light component in the substantially linearly polarized light varies depending on the purpose of use in the optical head device, and is not necessarily limited to the above-described detection of the light amount.

  As described above, the broadband half-wave plate according to the first embodiment of the present invention has a plurality of anisotropic medium layers stacked, and each anisotropic medium layer has at least an intermediate retardation wavelength. Therefore, the material used is not limited to a material having a small wavelength dispersion, and functions as a ½ wavelength plate for incident light having a plurality of wavelengths used for recording or reproducing optical information. can do.

  In addition, since the number of anisotropic medium layers is two and the fast axis crossing angle is ½ of the polarization rotation angle, it can function as a ½ wavelength plate with a simple configuration. .

  Further, since the material of the anisotropic medium layer is a polymer liquid crystal, the thickness of the anisotropic medium layer can be adjusted.

  In the first embodiment of the present invention, the broadband half-wave plate having the configuration in which the intermediate phase difference half-wave plate 1a and the intermediate phase difference half-wave plate 1b are bonded is described. The present invention is not limited to the broadband half-wave plate having such a configuration, and the substrates 12a, 12b and the like are disposed between the intermediate retardation half-wave plate 1a and the intermediate retardation half-wave plate 1b. The structure which pinched | interposed and bonded together may be sufficient.

(Second Embodiment)
FIG. 4 is a diagram showing a block configuration of an optical head device according to the second embodiment of the present invention. In FIG. 4, an optical head device 100 includes a light source 101 that emits a plurality of wavelengths, a collimator lens 102, a diffraction element 103 that converts light emitted from the light source 101 into three beams, and S-polarized component light from incident light. A polarization stabilizer 104, which is a diffractive polarizer to be removed, the broadband half-wave plate 1 of the present invention, and a first that separates the S-polarized component light and the P-polarized component light of the incident light and emits them in different directions. Beam splitter 105, first photodetector 106 that detects S-polarized component light emitted from first beam splitter 105, broadband quarter-wave plate 107, objective lens 108, and second beam A splitter 109 and a second photodetector 110 that detects light emitted from the second beam splitter 109 are provided.

  The light source 101 is composed of, for example, a semiconductor laser, and emits a linearly polarized divergent light beam. It is assumed that P-polarized light is emitted from the light source 101. The light source 101 also emits light beams having a plurality of wavelengths, for example, near 405 nm (for HD), near 660 nm (for DVD), and near 785 nm (for CD).

  However, the present invention is not necessarily limited to the configuration in which the light source 101 emits a light beam having the above-described wavelength, and light having a wavelength different from the above is also used in a configuration in which light having two wavelengths or four or more wavelengths is emitted. The structure which radiates | emits may be sufficient. Here, the wavelengths near 405 nm, 660 nm, and 785 nm mean wavelengths in the ranges of 385 nm to 430 nm, 640 nm to 675 nm, and 760 nm to 800 nm, respectively.

  The light source 101 may be configured to have a so-called hybrid two-wavelength laser light source or three-wavelength laser light source by mounting two or three semiconductor laser chips on the same substrate in the same package. The light source 101 also has a monolithic two-wavelength laser light source (for example, see Japanese Patent Application Laid-Open No. 2004-39898) or three light emitting points having two light emitting points that emit light of different wavelengths. A monolithic three-wavelength laser light source may be used.

  For example, the diffractive element 103 may be configured to have a diffraction grating that diffracts incident light and separates it into three beams of zero-order light and ± first-order light. Since such a diffraction grating is well known, its description is omitted.

  The polarization stabilizer 104 removes S-polarized component light from incident light. The light emitted from the light source 101 may be polarized in a direction deviated from the P-polarized light depending on the operating temperature, the amount of light, or the like when the S-polarized light component is included because the optical system is not properly arranged. The polarization stabilizer 104 removes the light of the S polarization component generated in such a case. The polarization stabilizer 104 may diffract the S-polarized component light and remove it from the optical path, or remove it by other methods. When the polarization stabilizer 104 is configured using a diffraction grating, the diffraction grating may be a polarization diffraction grating, and a polarization diffraction grating for each wavelength used for recording or reproducing optical information may be stacked.

  The broadband half-wave plate 1 of the present invention is configured to rotate the polarization direction of P-polarized light by a predetermined angle such as a polarization rotation angle of 18 degrees, for example. Specifically, the fast axis of the anisotropic medium layer 14a forms an angle θ1 that is a quarter of the polarization rotation angle θ with respect to the polarization direction of the incident light, and the fast axis of the anisotropic medium layer 14b is It is arranged so as to make an angle θ2 (a fast axis crossing angle θ / 2) of 3/4 of the polarization rotation angle θ with respect to the polarization direction of the incident light.

  The first beam splitter 105 separates the S-polarized component light and the P-polarized component light of the light emitted from the broadband half-wave plate 1 of the present invention, and sends the S-polarized component light to the first photodetector 106 side. The P-polarized component light is emitted to the optical recording medium 120 side.

  The first photodetector 106 is configured by, for example, a photodiode, and detects the S-polarized component light emitted from the first beam splitter 105 and converts it into an electrical signal corresponding to the amount of light to control the light source 101. The electric signal is output to the illustrated control means. A control unit (not shown) acquires information on the amount of light used for recording or reproducing optical information based on the electrical signal, and controls the light source 101 so as to obtain a predetermined amount of light.

  The second beam splitter 109 is constituted by a polarization HOE (Holographic Optical Element) beam splitter using a holographic element, for example, and is return light that is reflected by the optical recording medium 120 and incident via the first beam splitter 105. The beam is diffracted and separated, and emitted toward different positions in the second photodetector 110.

  The operation of the optical head device 100 according to the second embodiment of the present invention will be described below. The divergent light emitted from the light source 101 is collimated by the collimator lens 102, converted into three beams by the diffraction element 103, the S-polarized component light is removed by the polarization stabilizer 104, and the P-polarized light is converted into a broadband half-wave plate. 1 is incident. The P-polarized light incident on the broadband half-wave plate 1 is rotated by a predetermined angle and polarization direction, for example, 18 degrees, by the broadband half-wave plate 1 so that the S-polarized component and the P-polarized component The S-polarized component light is emitted to the first photodetector 106 side by the first beam splitter 105, and the P-polarized component light is emitted to the optical recording medium 120 side. Is emitted. The S-polarized component light emitted from the first beam splitter 105 is converted into an electric signal by the first photodetector 106. On the other hand, the P-polarized component light emitted from the first beam splitter 105 receives 90-degree retardation at the quarter-wave plate 107 and becomes circularly-polarized light, and is condensed on the optical recording medium 120 by the objective lens 108. Then, the return light reflected by the optical recording medium 120 and directed to the first beam splitter 105 is returned to parallel light by the objective lens 108 and is subjected to retardation of 90 degrees by the quarter wavelength plate 107 and is S-polarized light. Thus, the light beam is reflected by the first beam splitter 105 toward the second beam splitter 109 and converted into an electric signal by the second photodetector 110.

  In the above description, the configuration in which the polarization stabilizer 104 and the broadband half-wave plate 1 are separately disposed has been described. However, the configuration shown in FIG. The optical head device 200 may be used. In this case, the polarization stabilizer 104 and the broadband half-wave plate 1 are integrated into a broadband half-wave plate 2, and the broadband half-wave plate 2 is composed of a plurality of polarization diffraction gratings and a broadband half-wave plate. 1 and the polarization stabilizer 104 is removed.

  As described above, the wideband half-wave plate according to the second embodiment of the present invention has a wideband half-wavelength in addition to the effect of the first embodiment of the present invention. Since it is integrated with the plate, the number of components can be reduced, and the adjustment with respect to the polarization direction can be completed only by adjusting the arrangement of the integrated components and the beam splitter, so that productivity can be improved.

  In addition, the optical head device according to the second embodiment of the present invention has the same effect as that of the broadband half-wave plate of the present invention, and has a plurality of wavelengths used for recording or reproducing optical information. The amount of light can be monitored with reduced wavelength dependence.

  Specific examples based on the above-described embodiments of the present invention will be described below.

(First embodiment)
A broadband half-wave plate according to a first embodiment of the present invention will be described with reference to FIG. The broadband half-wave plate according to the first embodiment of the present invention has a polarization direction of 18 degrees when the wavelength of light used for recording or reproducing optical information is 405 nm, 660 nm, and 785 nm, respectively. It is used for the purpose of rotating to become. The broadband half-wave plate 1 according to the first embodiment of the present invention includes a first intermediate retardation half-wave plate 1a and a second intermediate retardation half-wave plate 1b that are transparent adhesives. 16 is laminated.

  First, production of the intermediate phase difference half-wave plates 1a and 1b will be described. Alignment films 13a and 13b are respectively formed on one substrate surface of the transparent substrates 11a and 11b, and the alignment films are rubbed to perform an alignment process. Next, the sealing materials 15a and 15b are formed by a printing method on the substrate surface on which the alignment films 13a and 13b of the transparent substrates 11a and 11b are formed, and the liquid crystal cell is solidified by thermocompression facing the transparent substrates 12a and 12b. Form.

The thickness d of the liquid crystal cell is the thickness at which the intermediate phase difference half-wave plates 1a and 1b function as half-wave plates at the intermediate phase difference wavelength described in the first embodiment of the present invention, that is, d = λ ₀ / (2Δn (λ ₀ )). Here, lambda ₀ is the intermediate phase difference wavelength, Δn (λ ₀₎ is the refractive index difference obtained by subtracting the ordinary refractive index n _o of the extraordinary refractive index n _e at a wavelength lambda _0.

Further, the intermediate phase difference wavelength λ ₀ is determined as follows, for example. First, the wavelength dependence of a phase difference (hereinafter referred to as a standard phase difference) R (= 2πΔn (λ) / λ) obtained by dividing the retardation value per unit length by the wavelength and multiplying by 2π is calculated as the shortest The standard phase difference R _min when the wavelength λ _min is 405 nm and the standard phase difference R _max when the longest wavelength λ _max is 785 nm are determined based on the wavelength dependence of the standard phase difference R. Then, standard phase difference _{R min,} calculates an intermediate value of _{R max} as standard phase difference _{R 0,} determines the wavelength of standard phase difference _{R 0} is obtained as the intermediate phase difference wavelength lambda _0.

In this embodiment, the standard phase difference R _min when the shortest wavelength λ _min is 405 nm is 0.000115 nm ⁻¹ , and the standard phase difference R _max when the longest wavelength λ _max is 785 nm is 0.000049 nm ⁻¹ . . As a result, standard phase difference _{R 0} in an intermediate phase difference wavelength lambda ₀ is the intermediate phase difference wavelength lambda ₀ which 0.000082Nm ^-1 next, is 0.000082Nm ^-1 obtained became 505 nm.

Therefore, the thickness d of the liquid crystal cell for the intermediate phase difference half-wave plates 1a and 1b to function as the half-wave plates at the intermediate phase difference wavelength λ ₀ described above is 6.1 μm.

  After the liquid crystal cell is formed, UV curable polymer liquid crystal having the above refractive index as the anisotropic medium layers 14a and 14b is injected into each liquid crystal cell using a vacuum injection method and sealed. After UV curable polymer liquid crystal is injected into each liquid crystal cell, UV light is irradiated to polymerize the UV curable polymer liquid crystal, and the transparent substrates 12a and 12b are removed. As a result, intermediate phase difference half-wave plates 1a and 1b are obtained.

  The broadband half-wave plate 1 includes the first intermediate retardation half-wave plate 1a and the second intermediate retardation half-wave plate 1b manufactured as described above, and the alignment direction of the liquid crystal is polarized. It is obtained by adhering the two facing each other so as to be half the rotation angle, that is, 9 degrees, using the adhesive 16.

In the first embodiment of the present invention, an angle θ ₁ formed by the polarization direction of light incident on the broadband half-wave plate 1, the fast axis of the anisotropic medium layer 14 a and the polarization direction of incident light, FIG. 2 shows an angle θ ₂ formed by the fast axis of the anisotropic medium layer 14b and the polarization direction of the incident light. FIG. 6 is a diagram for explaining the P-polarized component light and the S-polarized component light emitted from the broadband half-wave plate 1.

In FIG. 6, for each wavelength of 405 nm, 505 nm, 660 nm, and 785 nm, the angle θ ₁ , the angle θ ₂ , and the intermediate phase difference ½ wavelength plates 1 a and 1 b according to the first embodiment of the present invention are obtained. Phase difference (value in the left column is in units of angle and right column is in units of wavelength), Stokes vector [St ₀ ] (indicated as “In Stokes”) representing the polarization state of incident light. Stokes vector [St] representing the polarization state of the outgoing light (denoted as “out Stokes”), ellipticity (κ) of the polarization state of the outgoing light, the amount of P-polarized component in the outgoing light, and S in the outgoing light A value obtained by dividing the light amount of the polarization component and the light amount of the P polarization component by the light amount of the S polarization component (P / S; hereinafter referred to as P / S ratio) is shown.

  As shown in FIG. 6, the ratio obtained by dividing the phase difference by the wavelength remains wavelength-dependent, and the shortest wavelength and the longest wavelength are more than twice, whereas the P / S ratio is the recording of optical information or A value of approximately 7 is obtained at each wavelength of 405 nm, 660 nm, and 785 nm used for reproduction.

  FIG. 7 is a diagram showing information corresponding to FIG. 6 when a conventional technique for producing a half-wave plate with one anisotropic medium layer is applied. FIG. 7 shows that the P / S ratio at a wavelength of 660 nm and the P / S ratio at a wavelength of 785 nm are different values by 22% or more. This value is much larger than 6% or less, which is a value obtained with the broadband half-wave plate 1 of the present invention shown in FIG. Further, in the half-wave plate to which the conventional technique is applied, the elliptically polarized component of the emitted light is also larger than the elliptically polarized component of the emitted light obtained by the broadband half-wave plate 1 of the present invention.

(Second embodiment)
An optical head device according to a second embodiment of the present invention will be described with reference to FIG. In the optical head device according to the second embodiment of the present invention, as the light source 101, the light emitted from the single wavelength laser light source and the two wavelength laser light source is combined by the combining prism, and the light of each wavelength is collimated by the collimator lens 102. The one having a structure that makes the light incident on is used. The light source 101 emits light having wavelengths of 405 nm, 658 nm, and 784 nm.

  As the polarization stabilizer 104, one having a configuration in which three layers of diffraction gratings functioning as polarization diffraction gratings are stacked for each wavelength of light emitted from the light source 101 is used. As the first beam splitter 105, a polarization beam splitter that reflects S-polarized component light toward the first photodetector 106 and reflects P-polarized component light toward the second photodetector 110 is used.

The broadband half-wave plate 1 according to the first embodiment of the present invention is used as the broadband half-wave plate 1, and the amount of S-polarized component light and the P-polarized component light incident on the first beam splitter 105 are used. The polarization direction of the incident P-polarized light is rotated by 18 degrees (polarization rotation angle) so that the ratio to the light quantity becomes 1: 9 at the intermediate phase difference wavelength. In addition, the broadband half-wave plate 1 forms an angle θ ₁ that is a quarter of the polarization rotation angle θ with respect to the polarization direction of the light incident on the fast axis of the anisotropic medium layer 14 a, so that the anisotropic medium fast axis of the layer 14b is disposed at an angle theta ₂ of 3/4 of the polarization rotation angle theta with respect to the polarization direction of the incident light. As the first photodetector 106, a photodiode is used.

  The broadband half-wave plate according to the present invention is not limited to a material having a small wavelength dispersion, and is used as a half-wave plate for incident light having a plurality of wavelengths used for recording or reproducing optical information. The present invention can also be applied to applications such as a broadband half-wave plate and an optical head device that are useful in functioning.

DESCRIPTION OF SYMBOLS 1 Broadband half wave plate 2 Optical element which integrated polarization stabilizer and wideband half wave plate 1a, 1b Intermediate phase difference half wave plate 11a, 11b Transparent substrate 12a, 12b Substrate 13a, 13b Alignment film 14a , 14b Anisotropic medium layer 15a, 15b Sealing material 16 Adhesive 100, 200 Optical head device 101 Light source 102 Collimator lens 103 Diffraction element 104 Polarization stabilizer 105, 109 Beam splitter 106, 110 Photo detector 107 1/4 wavelength plate 108 Objective lens 120 Optical recording medium S (λ ₁ ), S (λ ₂ ), S (λ ₃ ) The locus of Stokes vectors of light of each wavelength incident on the broadband half-wave plate

Claims (5)

  A half-wave plate disposed in the optical path of an optical head device having a light source that emits light of a plurality of wavelengths, and a plurality of anisotropic medium layers made of a medium having optical anisotropy are laminated, Each of the anisotropic medium layers has at least a retardation value obtained by dividing the retardation value at the shortest wavelength among the wavelengths of light emitted from the light source by the wavelength and multiplying by 2π, and the retardation value at the longest wavelength. A broadband half-wave plate which functions as a half-wave plate at an intermediate phase difference wavelength having a phase difference equal to the average value of the phase difference obtained by dividing by 2π by the wavelength.   The number of the anisotropic medium layers is two, and each of the anisotropic medium layers is arranged so that the fast axes make a predetermined fast axis crossing angle, and the fast axis crossing angle The broadband ½ wavelength plate according to claim 1, wherein ½ is a polarization rotation angle that is an angle to rotate the polarization direction in the optical head device.   The broadband half-wave plate according to claim 1 or 2, wherein the material of the anisotropic medium layer is a polymer liquid crystal.   4. The broadband half-wave plate according to claim 1, wherein a plurality of polarization diffraction gratings are integrated with the broadband half-wave plate. 5.   A light source that emits a plurality of linearly polarized light beams having different wavelengths, a first polarized component light that is polarized in the first polarization direction and a second polarization direction that is polarized according to the polarization direction of the incident light. A beam splitter that separates the first polarized component light and the second polarized component light to different photodetectors, and the first polarized component light emitted from the beam splitter. A first photodetector that detects one polarization component light; a second photodetector that detects the second polarization component light emitted from the beam splitter; and polarization of light incident on the beam splitter. 5. An optical head device comprising a half-wave plate for rotating the direction, and recording or reproducing optical information using the first polarized component light, wherein the half-wave plate is from 1 to 4. The broadband according to any one of The first polarization component light and the second light emitted from the beam splitter by rotating the polarization direction of the light incident on the beam splitter at a plurality of wavelengths of light emitted from the light source. The ratio of the amount of light to the polarized component light is fixed to a predetermined value, and the amount of the second polarized component light is controlled based on the amount of the first polarized component light detected by the first photodetector. An optical head device comprising:

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