JP4876814B2 - Phase difference element and optical head device - Google Patents

Description

  The present invention relates to a phase difference element that changes the polarization state of light, and an optical head device that includes the phase difference element and performs recording or reproduction on an optical recording medium such as a CD or a DVD (hereinafter referred to as “optical disk”). .

  Conventionally, this type of optical head device reflects the light beam emitted from the light source and the light detector so as to guide the light beam toward the optical disc and transmits the reflected light from the optical disc so as to guide it toward the light detector. An optical path separating element such as a beam splitter, and an objective lens arranged to face the information recording surface of the optical disc are provided. In particular, the polarization state of the light beam is changed between the objective lens and the optical path separation element, that is, the polarization plane of the light beam irradiated on the information recording surface of the optical disc and the light beam reflected by the information recording surface is rotated 1 Generally, a / 4 wavelength plate is provided.

  In the optical head device provided with the quarter-wave plate as described above, a region having a predetermined intensity or higher in the light spot irradiated on the information recording surface (that is, a so-called far field region in the light spot). ) Is an elliptical shape. The major axis direction in the elliptical shape is set to be orthogonal or parallel to the direction of the recording track formed by the information pits on the information recording surface.

  As a problem that may occur when the optical head device configured as described above reproduces an optical disk, there is a problem that the S / N of a reproduction signal decreases due to birefringence on the information recording surface. Such birefringence causes a phenomenon that light constituting the reflected light is doubled in a transparent layer such as a protective layer of the optical disk. This birefringence is caused by an optical disk manufacturing process using a stamper or the like, and unevenness occurs in the refractive index distribution in the information recording surface on the inner and outer circumferences of the optical disk. As a result, there arises a problem that the jitter amount, that is, the fluctuation in the time axis direction of the reproduction signal is out of the allowable range, and correct reproduction of information becomes difficult.

  In order to solve this problem, for example, an optical head device disclosed in Patent Document 1 has been proposed. The optical head device disclosed in Patent Document 1 includes a semiconductor laser that emits a linearly polarized light beam and an optical system that includes a reflection mirror and a half mirror. The semiconductor laser is a predetermined centering on the light beam emission direction. The recording direction is formed by information pits on the information recording surface, and the polarization direction of the light emitted from the semiconductor laser is controlled by controlling the phase difference with respect to the reflected light of the reflection mirror or half mirror. The reproduction characteristic of the optical disk is improved by setting it at an angle of 45 degrees with respect to the track direction.

JP 2004-259328 A

  However, although the optical head device disclosed in Patent Document 1 is configured to control the phase difference between the S-polarized component and the P-polarized component at the reflecting mirror and the half mirror, the phase difference must be accurately controlled. Is difficult, and there is a problem that the variation in the polarization direction tends to be large. In addition, when a plurality of different wavelengths are used, it is more difficult to control the phase difference for each of the plurality of wavelengths, resulting in a decrease in yield and an increase in cost. In addition, there is a method using a polarizing plate that controls the polarization direction of the light beam. However, when the polarizing plate is used, light in the unnecessary polarization direction is absorbed by the polarizing plate, so that the light use efficiency decreases. There was a problem.

  The present invention has been made to solve the conventional problems, and improves the reproduction characteristics of an optical disc without increasing the manufacturing cost and reducing the light utilization efficiency even when a plurality of different wavelengths are used. An object of the present invention is to provide a phase difference element and an optical head device that can be made to operate.

The inventor of the present invention has conducted various studies on a phase difference element in which at least two phase plates are combined. As a result, by using a phase difference element satisfying a predetermined condition, two wavelengths incident on the phase difference element are obtained. The linearly polarized light of λ ₁ and λ ₂ becomes elliptically polarized light, and the major axis direction of the elliptically polarized light after passing through the phase difference element with respect to the polarization direction of the incident light is approximately 45 degrees with respect to the light of wavelength λ ₁ or generally it indicates 135 degrees, found to exhibit an approximately 0 degrees or about 90 degrees with respect to the wavelength lambda ₂ of the light, has solved the conventional problems. In the present specification, the angle indicated by the word “generally” means that the angle is within a range of ± 15 degrees. For example, approximately 45 degrees refers to an angle in the range of 30 degrees to 60 degrees.

That is, the optical head device of the present invention includes a light source that emits linearly polarized light having at least two different wavelengths λ ₁ and λ ₂ , an objective lens that focuses the light on the optical recording medium, and the optical recording medium And a phase difference element that is provided between the light source and the objective lens and transmits the light. The phase difference element has first and second birefringence. comprise a plurality of phase plates containing 2 phase plate, a phase difference angle Rd ₁₁ is 90 ± 15 degrees of the first phase plate with respect to the wavelength lambda ₁ of the light, the with respect to the wavelength lambda ₂ of the light first phase difference angle Rd ₂₂ of the phase plate 2 is a 180 ± 15 degrees, the first through the wavelength lambda ₁ of the light incident from the side of the phase plate, generally 45 degrees to the polarization direction of the incident light Or elliptically polarized light having a major axis in a direction inclined approximately 135 degrees Conversion to is emitted, an ellipse having a major axis of the wavelength lambda ₂ of the light incident from the side, in a direction inclined approximately 0 degrees or about 90 degrees to the polarization direction of the incident light of the first phase plate The first phase plate has a configuration closer to the light source than the second phase plate .

With this configuration, the phase difference element included in the optical head device of the present invention is configured such that linearly polarized light having wavelengths λ ₁ and λ ₂ is incident from the first phase plate side of the first and second phase plates. Both incident lights are changed to elliptically polarized light and output. The major axis direction of the elliptically polarized light after passing through the phase difference element is approximately 45 degrees or approximately 135 degrees with respect to the light with the wavelength λ ₁ and approximately 0 degrees or approximately 90 degrees with respect to the light with the wavelength λ _2. Will be shown. Therefore, the phase difference element provided in the optical head device of the present invention can improve the reproduction characteristics of the optical disc without increasing the manufacturing cost and reducing the light utilization efficiency even when a plurality of different wavelengths are used. it can. Therefore, the optical head device of the present invention can cope with a plurality of types of optical disks having different recording / reproducing wavelengths. Further, in the optical head device of the present invention, the phase difference element rotates the polarization directions of light having a plurality of different wavelengths, respectively, and generates elliptically polarized light, thereby improving the reproduction characteristics of the optical disk. be able to.

Here, the optical head device of the present invention, the phase difference element, angle of theta ₁ of the fast axis of the polarization direction of the wavelength lambda ₁ of the incident light first phase plate, the wavelength lambda ₂ the angle between the fast axis of the polarization direction of the incident light and the second phase plate when theta _{2, and, (θ 1, θ 2)} = (45 ± 10 °, 0 ± 10 degrees), (theta ₁ , θ ₂ ) = (45 ± 10 degrees, 90 ± 10 degrees), (θ ₁ , θ ₂ ) = (135 ± 10 degrees, 0 ± 10 degrees) and (θ ₁ , θ ₂ ) = (135 ± 10 Or 90 ± 10 degrees) is preferable.

The optical head device of the present invention has a configuration in which the wavelength λ ₁ and the wavelength λ ₂ are two different wavelengths selected from a 405 ± 20 nm band, a 660 ± 20 nm band, and a 790 ± 20 nm band. Yes.

With this configuration, the optical head device of the present invention can cope with optical disks having different recording / reproducing wavelengths such as Blu-ray, HD-DVD, DVD, and CD. Further, the optical head device of the present invention is improved, the phase difference element, rotates a plurality of different wavelengths of light in the polarization direction to each other in the optimum direction, and so to generate a polarization ellipse, the reproduction characteristics of the optical disc Can be made.

Furthermore, the optical head device of the present invention has a configuration in which the plurality of phase plates are laminated and integrated.

With this configuration, in the optical head device of the present invention, the phase difference element can handle a plurality of phase plates including the first and second phase plates as one component. This is preferable because the number of parts can be reduced. In addition, the first and second phase plates are preferably bonded together in advance, so that the optical head device of the present invention is preferable because the phase difference element can accurately convert the polarization state.

Further, the optical head device of the present invention, the light transmitted through the phase difference element, compared polarization direction of the linearly polarized light incident on the retardation element, the light of the wavelength lambda ₁ is approximately 45 degrees or approximately 135 It is elliptically polarized light having a major axis in a direction inclined by a degree, and the light of wavelength λ ₂ has a configuration that is elliptically polarized light having a major axis in a direction inclined by approximately 0 degrees or approximately 90 degrees.

With this configuration, the optical head device of the present invention, the polarization direction on the optical disc surface, the slope in approximately 45 degrees or approximately 135 degrees at the wavelength lambda ₁ of the light, in the wavelength lambda ₂ of light 0 ° or approximately 90 Since the phase difference element emits elliptically polarized light that is inclined at a degree, it is possible to improve the reproduction characteristics of the optical disc with respect to light of two different wavelengths.

  Furthermore, the optical head device of the present invention has a configuration in which the light transmitted through the retardation element is elliptically polarized light having an ellipticity of 0.10 to 0.90.

  With this configuration, the optical head device of the present invention has an elliptical polarization state on the surface of the optical disk, so that it is possible to suppress signal light amount fluctuations due to the birefringence of the optical disk.

  The present invention provides a phase difference element and an optical head having an effect of improving the reproduction characteristics of an optical disc without causing an increase in manufacturing cost and a decrease in light utilization efficiency even when a plurality of different wavelengths are used. An apparatus can be provided.

  Hereinafter, an embodiment of the present invention will be described with reference to the drawings. FIG. 1 is a conceptual diagram conceptually showing a configuration example of the phase difference element 10 in the present embodiment.

  As shown in FIG. 1, the phase difference element 10 in the present embodiment includes a first phase plate 11 and a second phase plate 12 having birefringence, and a first phase plate 11 and a second phase plate. And an adhesive layer 13 for adhering 12 to each other.

The first phase plate 11 and the second phase plate 12 are made of a birefringent material having birefringence, for example, a polymer liquid crystal obtained by polymerizing liquid crystal. However, the present invention is not limited to this, and may be composed of a single crystal material having birefringence such as quartz or LiNbO ₃ . Moreover, it can also be comprised with the resin film which has birefringence, for example, a polycarbonate and an olefin resin. Note that the first phase plate 11 and the second phase plate 12 are preferably made of a polymer liquid crystal or resin film, so that the thickness can be made thinner than that made of a single crystal material.

  The adhesive layer 13 is made of, for example, an adhesive film, a UV curable adhesive, a thermosetting adhesive, or the like. In order to reduce wavefront aberration and improve temperature characteristics and reliability of the phase difference element 10, the adhesive layer 13 is preferably as thin as possible, and the thickness of the adhesive layer 13 is particularly preferably 10 μm or less.

  In FIG. 1, the first phase plate 11 and the second phase plate 12 are bonded by the adhesive layer 13, but the first phase plate 11 and the first phase plate 11 can be provided without providing the adhesive layer 13. The second phase plate 12 and the second phase plate 12 may be arranged in the optical path.

  Moreover, in FIG. 1, although the example which comprised the phase difference element 10 with the two phase plates of the 1st phase plate 11 and the 2nd phase plate 12 was shown, the phase difference element of this invention is limited to this. It can be configured by combining three or more phase plates. For example, the first phase plate 11 is composed of two phase plates 11 a and 11 b, and the phase difference obtained by combining the phase difference of the phase plate 11 a and the phase difference of the phase plate 11 b is a phase difference due to the first phase plate 11. It can also be set as the structure equal to. In addition, for example, a half-λ phase plate or a λ phase plate can be added to the phase difference element 10 shown in FIG. 1 to form three or more phase plates.

  Furthermore, the phase difference element according to the present invention can be configured as shown in FIG. 2, for example. That is, the phase difference element 20 shown in FIG. 2 is obtained by sandwiching the phase difference element 10 between two transparent substrates 21. Here, the transparent substrate 21 can be configured using, for example, glass or resin, and preferably has a small birefringence. With this configuration, the phase difference element 20 has improved rigidity compared to the phase difference element 10, which facilitates handling in the manufacturing process and improves surface flatness, thereby improving transmitted wavefront aberration. Furthermore, it is preferable because reliability is improved.

  Alternatively, only one transparent substrate 21 may be provided, and a phase plate may be provided on one or both surfaces of the single transparent substrate 21. With this configuration, the number of transparent substrates 21 can be reduced, which is preferable because the manufacturing cost of the phase difference element 20 can be reduced.

  Next, specific configurations of the first phase plate 11 and the second phase plate 12 will be described.

  In general, the phase difference Rd of the birefringent phase plate is equal to the difference in refractive index (Δn) with respect to light in the polarization direction parallel to the optical axis (fast axis) of the phase plate and perpendicular to the optical axis. The product Δn · d with the thickness d is divided by the wavelength of light λ, and can be expressed as Rd = (Δn · d / λ) × 360 (degrees). Usually, a phase plate made of one birefringent layer has different phase differences depending on the wavelength, but the optical axis direction is the same, so that two or more lights having different wavelengths can each realize a desired polarization state. Was difficult.

  Therefore, the inventor of the present invention has made various studies on the phase difference element in which at least two phase plates are combined, and uses the first phase plate 11 and the second phase plate 12 that satisfy the following conditions to form the phase difference element 10. By constructing, it has been found that even when a plurality of different wavelengths are used, a desired polarization state can be realized together.

First, let the wavelengths of two different lights incident on the phase difference element 10 be wavelengths λ ₁ and λ ₂ , respectively. The phase difference Rd ₁₁ of the first phase plate 11 with respect to the light having the wavelength λ ₁ is set to a value shown in the equation (1).

Rd ₁₁ = 90 ± 15 (degrees) (1)

Since the wavelength λ ₁ and the wavelength λ ₂ are different, the phase difference Rd ₁₂ for the light of the wavelength λ ₂ of the first phase plate 11 is different from the phase difference Rd ₁₁ .

On the other hand, the phase difference Rd ₂₂ of the second phase plate 12 with respect to the light having the wavelength λ ₂ is set to a value shown in the equation (2).

Rd ₂₂ = 180 ± 15 (degrees) (2)

Since the wavelength λ ₁ and the wavelength λ ₂ are different, the phase difference Rd ₂₁ for the light of the wavelength λ ₁ of the second phase plate 12 is different from the phase difference Rd ₂₂ .

Note that the phase difference between the first phase plate 11 and the second phase plate 12 is not limited to that shown in the equations (1) and (2), respectively. The phase difference Rd ₂₂ may be 180 + 360 × i (degrees) (i is an integer).

The angle θ ₁ formed by the polarization direction of the incident light having the wavelength λ _{1 and} the fast axis of the first phase plate 11, and the angle θ formed by the polarization direction of the incident light having the wavelength λ _{2 and} the fast axis of the second phase plate 12. ₂ can be four combinations shown in the equations (3) to (4). In the equations (3) to (4), the direction formed by the angles θ ₁ and θ ₂ is not particularly defined. The angles θ ₁ and θ ₂ are preferably in the same direction, but may be in different directions.

(Θ ₁ , θ ₂ ) = (45 ± 10 degrees, 0 ± 10 degrees) (3)
(Θ ₁ , θ ₂ ) = (45 ± 10 degrees, 90 ± 10 degrees) (4)
(Θ ₁ , θ ₂ ) = (135 ± 10 degrees, 0 ± 10 degrees) (5)
(Θ ₁ , θ ₂ ) = (135 ± 10 degrees, 90 ± 10 degrees) (6)

Specifically, 405 ± 20 nm band light used for Blu-ray (or HD-DVD) is BD light, 660 ± 20 nm band light used for DVD is DVD light, and 790 ± 20 nm band light used for CD is used. When light is expressed as CD light, for example, the following combinations are possible. In addition, “45 ± 10 degrees” described below can be replaced with “135 ± 10 degrees”.
1) θ ₁ = 45 ± 10 degrees with BD light, θ ₂ = 0 ± 10 degrees or 90 ± 10 degrees with CD light 2) θ ₁ = 45 ± 10 degrees with DVD light, θ _{2 with} CD light = 0 ± 10 degrees or 90 ± 10 degrees 3) θ ₁ = 45 ± 10 degrees with BD light, θ ₂ = 0 ± 10 degrees or 90 ± 10 degrees with DVD light 4) θ ₁ = 45 with DVD light ± 10 degrees, θ ₂ with BD light = 0 ± 10 degrees or 90 ± 10 degrees 5) θ ₁ with CD light = 45 ± 10 degrees, θ ₂ with DVD light = 0 ± 10 degrees or 90 ± 10 degrees

Next, some specific design examples of the retardation element according to the present invention are shown in Tables 1 to 3. The design examples of the phase difference element shown in Table 1 and Table 2 are cases where light of wavelength λ ₁ = 660 nm and wavelength λ ₂ = 790 nm is incident on the phase difference element as linearly polarized light in the same direction. Table 1 shows a design example when the phase difference Rd ₁₁ is changed around 90 degrees and the phase difference Rd ₂₂ is changed around 180 degrees. Table 2 shows a design example when the phase difference Rd ₁₁ is 90 degrees and the phase difference Rd ₂₂ is 180 degrees, and the example 1 shown in Table 1 is also shown in duplicate. . The design example of the phase difference element shown in Table 3 is a case where two wavelengths are selected and combined from wavelengths of 405 nm, 660 nm and 790 nm, and light of the wavelength is incident as linearly polarized light in the same direction. Yes, the example 1 shown in Table 1 is also duplicated.

For example, in Table 1, in Example 1, the phase difference Rd ₁₁ = 90 degrees for the light with the wavelength λ ₁ of the first phase plate 11, and the phase difference Rd ₂₂ = for the light with the wavelength λ ₂ of the second phase plate 12 = A phase difference element of 180 degrees, angle θ ₁ = 45 degrees with respect to the incident polarization direction of the fast axis of the first phase plate 11, and angle θ ₂ = 90 degrees with respect to the incident polarization direction of the fast axis of the second phase plate 12 It is. The transmitted light of wavelength λ ₁ transmitted through the phase element of Example 1 is elliptically polarized light, the angle of the major axis direction of the elliptically polarized light with respect to the incident polarization direction is 45 degrees, and the ellipticity = 0.52. Similarly, the transmitted light having the wavelength λ ₂ that has been transmitted through the phase difference element of Example 1 becomes elliptically polarized light, and the angle of the major axis direction of the elliptically polarized light with respect to the incident polarization direction is 0 degree and the ellipticity = 0.77. Here, the ellipticity means the ratio of the minor axis to the major axis of the elliptically polarized light (short axis / major axis), where ellipticity = 1 indicates circularly polarized light and ellipticity = 0 indicates linearly polarized light.

As shown in Tables 1 to 3, the light of wavelength λ ₁ is elliptically polarized light having a major axis in a direction inclined approximately 45 degrees or approximately −45 degrees with respect to the incident polarization direction, and light of wavelength λ ₂ is approximately 0. It is elliptically polarized light having a major axis in a direction inclined by approximately 90 degrees. The light transmitted through the phase difference element is elliptically polarized light having an ellipticity of approximately 0.10 to 0.90.

In the case of determining the wavelength lambda ₁ and wavelength lambda ₂ from a plurality of wavelengths, both large and small relation of the _<but preferably the wavelength lambda _2, the wavelength lambda _1> wavelength lambda ₁ may as a wavelength lambda _2. As shown in Examples 12 and 13 in Table 3, when wavelength λ ₁ > wavelength λ ₂ , the major axis direction of elliptically polarized light is rotated 90 degrees with respect to the case of wavelength λ ₁ <wavelength λ _2. Yes.

As described above, the phase difference element 10 according to the present embodiment can emit elliptically polarized light when _two linearly polarized lights having different wavelengths λ ₁ and λ ₂ are incident, and the incident polarization direction. Each elliptically polarized light can be oriented approximately 45 degrees or approximately 135 degrees and approximately 0 degrees or approximately 90 degrees, respectively.

In addition, the polarization directions of the incident light having the wavelengths λ ₁ and λ ₂ are preferably parallel to each other, but can also be orthogonal to each other.

  Next, a case where the phase difference element 10 in the present embodiment is applied to an optical head device will be described. FIG. 3 is a schematic view showing an example of an optical head device according to the present invention.

  As shown in FIG. 3, the optical head device 30 in the present embodiment includes light sources 31 and 32 that respectively emit laser beams having different wavelengths, and a combining prism 33 that combines light from the light sources 31 and 32. The beam splitter 34 that guides incident light to the optical disk 37 or the light detection system 38, the collimator lens 35 that converts the incident light into parallel light, the phase difference element 10 that converts the incident light into elliptically polarized light, and the optical disc 37. Objective lens 36 and a light detection system 38 for detecting the reproduction light from the optical disk 37.

The light sources 31 and 32 are constituted by, for example, semiconductor lasers, and emit linearly polarized laser light to the combining prism 33. Here, for example, the wavelength λ ₁ of the laser light emitted from the light source 31 is a 660 ± 20 nm band, and the wavelength λ ₂ of the laser light emitted from the light source 32 is a 790 ± 20 nm band. A so-called twin laser that emits two wavelengths from one semiconductor laser may be used. A light source that emits light having a third wavelength different from the wavelengths λ ₁ and λ ₂ may be added.

  The combining prism 33 is made of a light-transmitting material, such as glass or resin, and includes a reflecting surface that transmits the laser light from the light source 31 and reflects the laser light from the light source 32. With this configuration, the combining prism 33 combines the laser beams from the light sources 31 and 32 and emits them to the beam splitter 34.

  The beam splitter 34 is made of a light-transmitting material, such as glass or resin, and includes a reflecting surface that transmits the laser light from the light sources 31 and 32 and reflects the reproduction light from the optical disk 37.

  The collimator lens 35 is made of a translucent material, such as glass or resin, and converts incident laser light into parallel light.

  As described above, the phase difference element 10 includes the first phase plate 11 and the second phase plate 12 shown in FIG. Although not shown in FIG. 3, the first phase plate 11 is disposed closer to the light sources 31 and 32 than the second phase plate 12. Note that the phase difference element 20 shown in FIG. 2 may be used instead of the phase difference element 10. Further, the arrangement location of the phase difference element 10 is not limited to the position shown in FIG. 3, and it may be arranged in the optical path between the light sources 31 and 32 and the objective lens 36.

  The objective lens 36 is composed of, for example, a resin lens having a predetermined NA (numerical aperture), and condenses the elliptically polarized light incident from the phase difference element 10 onto the recording layer of the optical disc 37, and from the recording layer. The reflected light is captured. The phase difference element 10 may be integrated with an objective lens, another diffraction element, or an aperture limiting element.

  The light detection system 38 includes, for example, a lens and a photodiode, and converts the reproduction light from the optical disk 37 reflected by the reflection surface of the beam splitter 34 into an electric signal.

As described above, the optical head device 30 according to the present embodiment includes the phase difference element 10, and the phase difference element 10 receives linearly polarized light having two different wavelengths λ ₁ and λ ₂ . Both of them emit elliptically polarized light, and the direction of the major axis of each elliptically polarized light can be approximately 45 degrees or approximately 135 degrees, and approximately 0 degrees or approximately 90 degrees with respect to the incident polarization direction at that time. That is, the angle of elliptically polarized light with respect to the pits of the optical disk 37 can be set appropriately.

  Next, setting of the angle of elliptically polarized light will be described with reference to FIG. FIG. 4 schematically shows the state of elliptically polarized light with respect to the pits of the optical disk 37.

  According to the examination results of the inventor of the present invention, as shown in FIG. 4, the inclination is 45 degrees with respect to the longitudinal direction of the pits of an optical disc which is a high-density optical recording medium using a DVD or a blue laser. It has been found that the reproduction characteristics (for example, CNR) of the optical disk are improved by irradiating with polarized light. On the other hand, the polarization state of the light applied to the optical disk 37 is preferably closer to the circularly polarized light (ellipticity = 1) than the linearly polarized light (ellipticity = 0) because the reduction in jitter is suppressed with respect to the birefringence of the optical disk. I understood it. Therefore, it is possible to irradiate the optical disc 37 with light having an elliptical polarization angle of approximately 45 degrees or approximately 135 degrees with respect to the longitudinal direction of the pits of the optical disk and having an ellipticity of 0.10 to 0.90. Even when the birefringence is relatively large, a change in signal intensity is small, and a reduction in jitter is suppressed. Further, it has been found that the ellipticity is more preferably 0.20 to 0.80, and most preferably 0.3 to 0.75.

Therefore, for example, for light in the wavelength λ ₁ = 660 ± 20 nm band used for DVDs, the major axis direction is approximately 45 degrees or approximately 135 degrees with respect to the radial direction or pit direction of the optical disk as shown on the left side of FIG. It is preferable to irradiate the optical disk as elliptically polarized light because it can improve the reproduction characteristics of the optical disk, particularly jitter and CNR. In the case of a CD having a relatively low recording density, the effect of improving the reproduction characteristics of the disc by changing the polarization direction with respect to the pits of the optical disc is not so great.

  On the other hand, although not shown in FIG. 3, for example, a rising mirror is provided between the phase difference element 10 and the objective lens 36, and a reflection film is provided on the surface of the rising mirror. In this configuration, the polarization state may change due to the phase difference due to the reflection film on the mirror surface, which may be a problem. However, the phase difference due to the reflection film on the mirror is suitable for two different wavelengths. It is particularly difficult to control. When the polarization direction with respect to the mirror is close to P-polarized light or S-polarized light, the change in the polarization state due to the phase difference due to the reflection film of the mirror can be made difficult. This P-polarized light or S-polarized light is parallel to the radial direction or tangential direction of the optical disk, that is, the major axis direction is a direction of 0 degree or 90 degrees. In other words, by irradiating the optical disk with the light having the second polarization state shown in FIG. 4, it is possible to make the mirror less susceptible to the phase difference caused by the reflection film of the mirror. This is preferable.

  Next, the operation of the optical head device 30 in the present embodiment will be described.

First, the light sources 31 and 32 emit linearly polarized light having different wavelengths λ ₁ and λ ₂ .

Next, the laser beam having the wavelength λ ₁ emitted from the light source 31 and the laser beam having the wavelength λ ₂ emitted from the light source 32 are multiplexed by the multiplexing prism 33 and emitted to the beam splitter 34.

Further, the laser beams of wavelengths λ ₁ and λ ₂ are transmitted by the beam splitter 34 and emitted to the collimator lens 35, converted into parallel light by the collimator lens 35, and emitted to the phase difference element 10.

Subsequently, linearly polarized light having wavelengths λ ₁ and λ ₂ is converted into elliptically polarized light by the phase difference element 10 and emitted to the objective lens 36. Here, the transmitted light having the wavelength λ ₁ transmitted through the phase difference element 10 becomes elliptically polarized light. For example, the angle of the major axis direction of the elliptically polarized light with respect to the incident polarization direction is 45 degrees, and the ellipticity is 0.52. The transmitted light having the wavelength λ ₂ is also elliptically polarized light. For example, the angle of the major axis direction of the elliptically polarized light with respect to the incident polarization direction is 0 degree and the ellipticity = 0.77.

  Next, the objective lens 36 collects the elliptically polarized light emitted from the phase difference element 10 onto the optical disc 37, and the reflected light corresponding to the pits on the optical disc 37 is reflected from the optical disc 37. The reflected light from the optical disk 37 is sequentially transmitted through the objective lens 36, the phase difference element 10, and the collimator lens 35 and is incident on the beam splitter 34.

  The reflected light from the optical disk 37 is reflected by the reflecting surface of the beam splitter 34 and is incident on the light detection system 38, and information recorded on the optical disk 37 is acquired as an electrical signal by the light detection system 38.

As described above, according to the optical head device 30 of the present embodiment, the phase difference element 10 emits elliptically polarized light when _two linearly polarized lights having different wavelengths λ ₁ and λ ₂ are incident. Since the direction of elliptical polarization can be 45 degrees and 0 degrees with respect to the incident polarization direction, even when using a plurality of different wavelengths, the manufacturing cost increases and the light utilization efficiency The reproduction characteristics of the optical disc 37 can be improved without causing a decrease in the above.

  As described above, the retardation element and the optical head device according to the present invention improve the reproduction characteristics of the optical disc without increasing the manufacturing cost and reducing the light use efficiency even when using a plurality of different wavelengths. It is useful as a phase difference element that changes the polarization state of light, an optical head device that performs recording or reproduction on an optical disk such as a CD or a DVD, and the like.

Sectional drawing which shows notionally the structural example of the phase difference element in one embodiment of this invention Sectional drawing which shows notionally the other structural example of the phase difference element in one embodiment of this invention Schematic diagram showing a configuration example of an optical head device according to an embodiment of the present invention. The figure which shows typically the relationship between the pit of an optical disk, and a polarization state in the optical head apparatus in one embodiment of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10, 20 Phase difference element 11 1st phase plate 12 2nd phase plate 13 Adhesive layer 21 Transparent substrate 30 Optical head apparatus 31, 32 Light source 33 Combined prism 34 Beam splitter 35 Collimator lens 36 Objective lens 37 Optical disk 38 Optical detection system

Claims (6)

A light source that emits linearly polarized light of at least two different wavelengths λ ₁ and λ ₂ , an objective lens that focuses the light on the optical recording medium, and a detector that detects reflected light from the optical recording medium; A phase difference element that is provided between the light source and the objective lens and transmits the light;
Wherein the phase difference element includes first and second comprise a plurality of phase plates containing phase plate, wherein for said wavelength lambda ₁ of the light first retardation angle Rd ₁₁ of the phase plate having birefringent 90 15 ° ±, the phase difference angle _{Rd 22} of the second phase plate with respect to the wavelength lambda ₂ of light is a 180 ± 15 degrees,
Emission by converting the wavelength lambda ₁ of the light incident from the side of the first phase plate, the elliptically polarized light having a long axis in a direction inclined approximately 45 degrees or approximately 135 degrees to the polarization direction of the incident light Let
Emission by converting the wavelength lambda ₂ of the light incident from the side of the first phase plate, the elliptically polarized light having a long axis in a direction inclined approximately 0 degrees or about 90 degrees to the polarization direction of the incident light It is what
The optical head device characterized in that the first phase plate is closer to the light source than the second phase plate. The retardation element, the wavelength lambda ₁ the angle fast axis θ of the _first polarization direction of the incident light first phase plate, the wavelength lambda ₂ of the polarization direction of the incident light the second phase When the angle formed by the fast axis of the plate is θ ₂ ,
(Θ ₁ , θ ₂ ) = (45 ± 10 degrees, 0 ± 10 degrees)
(Θ ₁ , θ ₂ ) = (45 ± 10 degrees, 90 ± 10 degrees)
(Θ ₁ , θ ₂ ) = (135 ± 10 degrees, 0 ± 10 degrees)
And (θ ₁ , θ ₂ ) = (135 ± 10 degrees, 90 ± 10 degrees)
The optical head device according to claim 1, wherein any one of the relationships is satisfied . 3. The optical head device according to claim 1, wherein the wavelength λ ₁ and the wavelength λ ₂ are two different wavelengths selected from a 405 ± 20 nm band, a 660 ± 20 nm band, and a 790 ± 20 nm band . 4. The optical head device according to claim 1, wherein the plurality of phase plates are laminated and integrated . Elliptical light transmitted through the phase difference element, compared polarization direction of the linearly polarized light incident on the phase difference element, having a major axis in the direction of light of the wavelength lambda ₁ is inclined approximately 45 degrees or approximately 135 degrees 5. The optical head device according to claim 1, wherein the optical head device is polarized light, and the light having the wavelength λ ₂ is elliptically polarized light having a major axis in a direction inclined at approximately 0 degrees or approximately 90 degrees . 6. The optical head device according to claim 1, wherein the light transmitted through the phase difference element is elliptically polarized light having an ellipticity of approximately 0.10 to 0.90 .

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