JP4876826B2 - 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 disk and transmits the light reflected from the optical disk 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 made various studies on a phase difference element in which at least two phase plates are combined. As a result, (1) by using a phase difference element that satisfies a predetermined condition, the light enters the phase difference element. The linearly polarized light of the _two wavelengths 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 shows approximately 45 degrees with respect to the polarization direction of the incident light, (2) 45 The conventional problem was solved by finding that the reproduction characteristics of the optical disk are improved by irradiating the optical disk with polarized light inclined at a certain degree. In the present specification, an angle indicated by the word “generally” means that the angle is within a range of ± 10 degrees. For example, approximately 45 degrees refers to an angle in the range of 35 degrees to 55 degrees.

That is, the optical head device of the present invention includes a light source that emits linearly polarized light having at least two wavelengths of 660 nm and 790 nm, an objective lens that focuses the light on an optical recording medium, and the light source. A phase difference element provided between the objective lens and transmitting the light, the phase difference element comprising a plurality of phase plates including first and second phase plates having birefringence. When the wavelength of the incident light in the wavelength 660 nm band is λ ₁ , the wavelength of the incident light in the wavelength 790 nm band is λ ₂ , and the wavelength between the wavelengths λ ₁ and λ ₂ is λ _m , the wavelength The phase difference of the first phase plate with respect to the incident light of λ _m is Rd _1m , and the angle between the polarization direction of the incident light of the wavelength λ _m and the fast axis of the first phase plate is θ ₁ , the second position of the phase plate with respect to incident light of wavelength lambda _m The difference Rd _2m, when the angle between the fast axis of the second phase plate and the polarization direction of the incident light of the wavelength lambda _m was theta _2, represented by the following formula (1) or (2) ( against A1, B1, A2, B2) a set _of, Rd 1 m, _{Rd 2m,} theta ₁ and theta ₂ is meets the following relationship formula (3) to (6), the first phase plate, the An optical head device, wherein the optical head device is located closer to the light source than the second phase plate.
(A1, B1, A2, B2) = (− 0.209 ± 0.02, 93.8 ± 5, −0.256 ± 0.02, 214.7 ± 5) (1)
(A1, B1, A2, B2) = (− 0.209 ± 0.02,183.8 ± 5, −0.259 ± 0.02,125.1 ± 5) (2)
Rd _1m = 180 (degrees) (3)
Rd _2m = 100 to 170 (degrees) or 190 to 223 (degrees) (4)
θ ₁ = A1 × Rd _2m + B1 (degrees) (5)
θ ₂ = A2 × Rd _2m + B2 (degrees) (6)

With this configuration, the phase difference element included in the optical head device of the present invention changes both to elliptically polarized light when the linearly polarized light having the same polarization state of wavelengths λ ₁ and λ ₂ is incident, and changes the polarization direction of the incident light. On the other hand, the major axis direction of the elliptically polarized light after passing through the phase difference element is approximately 45 degrees. 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, in the optical head device of the present invention, the polarization direction of the light in the wavelength of 660 nm that is the recording / reproducing wavelength of DVD and the light in the wavelength of 790 nm that is the recording / reproducing wavelength of CD are the same by one retardation plate. In addition, since the elliptical polarized light can be obtained, the reproduction characteristics of the optical disk can be improved, which is preferable.

The retardation element included in the optical head device of the present invention has a configuration in which the wavelength λm is 690 nm to 740 nm.

With this configuration, the phase difference element included in the optical head device of the present invention can change linearly polarized incident light having polarization directions parallel to each other at least in the wavelength 660 nm band and the wavelength 790 nm band to substantially the same elliptically polarized light. it can.

Furthermore, the phase difference element provided in 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, the phase difference element included in the optical head device of the present invention can handle a plurality of phase plates including the first and second phase plates as one component, so that the phase difference element is not integrated. This is preferable because the number of parts can be reduced. In addition, the phase difference element provided in the optical head device of the present invention can be converted with high accuracy by pasting the first and second phase plates with high accuracy in advance, which is preferable.

  In the optical head device of the present invention, the light transmitted through the retardation element is elliptically polarized light having a major axis in a direction inclined by approximately 45 degrees with respect to the polarization direction of linearly polarized light when entering the retardation element. It has the composition which is.

  With this configuration, the optical head device of the present invention can increase the polarization direction on the optical disk surface to approximately 45 degrees with respect to light of two wavelengths by using one phase plate, thereby improving the reproduction characteristics of the optical disk. It is preferable because it is possible.

  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 composed of a birefringent layer made of a birefringent material having birefringence. As the birefringent material, for example, polymer liquid crystal obtained by polymerizing liquid crystal is used. However, the present invention is not limited to this, and a single crystal material having birefringence such as quartz or LiNbO ₃ may be used. Further, a resin film having birefringence, such as a polycarbonate or an olefin resin, can also be used. 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. Alternatively, the first phase plate 11 and the second phase plate 12 may be directly bonded without providing the adhesive layer 13.

  In FIG. 1, the first phase plate 11 and the second phase plate 12 are bonded and integrated by the adhesive layer 13. However, the first phase plate 11 and the second phase are integrated. It can also be set as the structure which arrange | positions the board 12 separately in an 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 adding the phase difference of the phase plate 11 a and the phase difference of the phase plate 11 b is the phase difference due to the first phase plate 11. It can also be set as the structure which becomes equal. Further, for example, even if a phase difference element 10 shown in FIG. 1 is added with a ½λ phase plate or a λ phase plate to constitute three or more phase plates, the same function is obtained.

  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 phase plate with respect to light of wavelength λ is the product of the refractive index difference Δn for linearly polarized light oscillating in the optical axis directions perpendicular to each other of the birefringent layer of the phase plate and the thickness d of the birefringent layer. Δn · d is divided by the wavelength of light λ, and can be expressed as Rd = (Δn · d / λ) × 360 (degrees). A phase plate made of one birefringent layer has a problem in that the output polarization state differs because the phase difference differs depending on the wavelength.

  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. As a result, the output of elliptically polarized light having a major axis in the direction rotated by 45 degrees is obtained for incident light having a wavelength of 660 nm and 790 nm, which is linearly polarized light whose polarization directions are parallel to each other. I came to find that I could get the light. The 660 nm band is a wavelength band suitable for DVD recording and reproduction at a wavelength of 660 ± 30 nm, and the 790 nm band is a wavelength band suitable for CD recording and reproduction at a wavelength of 790 ± 30 nm.

In FIG. 1, linearly polarized light having a wavelength of 660 nm band and a wavelength of 790 nm band incident on the phase difference element 10 is incident from the first phase plate 11 side and proceeds to the second phase plate 12 side, and has a wavelength of 660 nm. The wavelength of the incident light in the band is λ ₁ , the wavelength of the incident light in the 790 nm band is λ ₂ , and the wavelength between the wavelengths λ ₁ and λ ₂ is λ _m . Further, the phase difference of the first phase plate 11 with respect to the incident light having the wavelength λ _m is Rd _1m , the polarization direction of the incident light having the wavelength λ ₁ (vibration direction of the electric field) and the optical axis of the first phase plate 11 (phase advance). the angle between the axes) and theta _1, a phase difference with respect to the incident light of wavelength lambda _m the second phase plate 12 Rd _2m, the wavelength lambda ₂ of the polarization direction of the incident light (electric field vibration direction) and the second of the angle between the optical axis of the phase plate 12 (fast axis) and theta _2. In addition, the polarization directions of the incident light having the wavelengths λ ₁ and λ ₂ are parallel to each other.

First, the phase difference Rd _1m of the first phase plate 11 with respect to the light with the wavelength λ _m is set to a value shown in Expression (7).
Rd _1m = 180 (degrees) (7)

Here, since the wavelengths λ ₁ and λ ₂ are different from the wavelength λ _m , the first phase plate 11 does not show a half-wave phase difference with respect to the light of the wavelengths λ ₁ and λ ₂ .

Next, the phase difference Rd _2m of the second phase plate 12 with respect to the light with the wavelength λ _m is set to a value shown in Expression (8).
Rd _2m = 100 to 170 (degrees) or 190 to 220 (degrees) (8)

Note that Rd _1m shown in Expression (7) may be Rd _1m = 180 + 360 × i (degrees), and Expression (7) shows a representative value of Rd _1m . Similarly, Rd _2m shown in the equation (8) may be Rd _2m = (100 to 170) + 360 × j (degrees), or Rd _2m = (190 to 220) + 360 × j (degrees). ) Represents a representative value of Rd _2m . Here, i and j are integers. If i, j = 0, the thickness of the birefringent layer can be reduced, the tolerances of the phase differences Rd _1m and Rd _2m can be kept small, and the phase difference element 10 can be made thinner, which is preferable.

Furthermore, the relationship between the angles θ ₁ and θ ₂ and the phase difference Rd _{2m of} the second phase plate 12 is shown in equations (9) and (10).
θ ₁ = A1 × Rd _2m + B1 (degrees) (9)
θ ₂ = A2 × Rd _2m + B2 (degrees) (10)

Here, A1, B1, A2 and B2 are values shown in the formula (11) or (12).
(A1, B1, A2, B2) = (− 0.209 ± 0.02, 93.8 ± 5, −0.256 ± 0.02, 214.7 ± 5) (11)
(A1, B1, A2, B2) = (− 0.209 ± 0.02,183.8 ± 5, −0.259 ± 0.02,125.1 ± 5) (12)

Note that the wavelength λ _m is preferably 690 nm to 740 nm, and by setting the wavelength λ _{m to} 700 nm to 730 nm, the light transmitted through the phase plate can obtain the same desired polarization state with respect to the two wavelengths of light. Therefore, it is preferable.

Here, the relationship among Rd _2m , θ _1, and θ ₂ satisfying the expressions (7), (9), (10), and (12) is illustrated in FIG. FIG. 4 illustrates the relationship among Rd _2m , θ _1, and θ ₂ that satisfies the expressions (7) and (9) to (11). The light transmitted through the phase difference element satisfying any one of the conditions shown in FIG. 3 or FIG. 4 is in the polarization direction of the incident light (hereinafter referred to as “incident polarization direction”) with respect to the wavelengths λ ₁ and λ ₂ . The output light is elliptically polarized light having a major axis direction in a direction inclined 45 degrees, linearly polarized light oscillating in a direction inclined 45 degrees, or circularly polarized light.

FIG. 5 shows the relationship between the ellipticity of the emitted light and the phase difference Rd _2m of the second phase plate. 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. That is, the angle θ is satisfied so as to satisfy the relationship shown in the equations (7) and (9) to (11) or the equations (7), (9), (10), and (12) and to satisfy the equation (8). by choosing ₁ and theta _2, the phase difference element 10 may be obtained elliptically polarized light having a long axis direction in a direction inclined approximately 45 degrees or -45 degrees with respect to the incident polarization direction.

Next, Tables 1 and 2 show specific design examples regarding the relationship between Rd _2m , θ ₁ , θ ₂ and ellipticity. In Tables 1 and 2, those with a circle in the remarks column have an ellipticity of 0.3, which is most preferable. In addition, although what was described as a reference in the remarks column cannot obtain elliptically polarized light, it is described for comparison and reference. Here, λ _m was set to 720 nm.

  As described above, the phase difference element 10 according to the present embodiment is approximately 45 with respect to the incident polarization direction when linearly polarized light having a polarization direction parallel to each other at least in the wavelength 660 nm band and the wavelength 790 nm band is incident. Ellipsoidal polarized light having a major axis direction in a direction inclined at a degree or −45 degrees can be obtained.

  Next, a case where the phase difference element 10 in the present embodiment is applied to an optical head device will be described. FIG. 6 is a schematic view showing an example of an optical head device according to the present invention.

  As shown in FIG. 6, 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. 6, 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 position of the phase difference element 10 is not limited to the position shown in FIG. 6, 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, other diffraction elements, 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.

  When the phase difference element 10 of the present invention is used as described above, elliptically polarized light whose major axis is inclined by 45 degrees with respect to the longitudinal direction of the pits of the optical disk is irradiated, and the reproduction characteristic (for example, CNR) of the optical disk is improved. It turns out that it improves. On the other hand, the polarization state is preferably closer to circularly polarized light (ellipticity = 1) because the reduction in jitter is suppressed with respect to the birefringence of the optical disk. Therefore, it is preferable that the ellipticity is 0.2 to 0.7, because even if the birefringence of the optical disk is relatively large, the change in signal intensity is small, and the reduction in jitter is suppressed. By adopting the above-described design for the phase difference element 10, it is possible to emit such elliptical light. Further, it has been found that the ellipticity is most preferably 0.3 to 0.5.

  Therefore, as shown in FIG. 7, it is preferable to irradiate the optical disk as elliptically polarized light having a major axis direction of 45 degrees with respect to the radial direction or the pit direction of the optical disk because the reproduction characteristics of the optical disk, particularly jitter and CNR can be improved. .

  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 of the wavelengths λ ₁ and λ ₂ transmitted through the phase difference element 10 is both 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 = 0.3. It is.

  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 in the present embodiment, the phase difference element 10 receives linearly polarized light having polarization directions parallel to each other at least in the wavelength 660 nm band and the wavelength 790 nm band. Since the direction of each elliptical polarization with respect to the incident polarization direction is an elliptical polarization having a major axis direction inclined by 45 degrees, even when a plurality of different wavelengths are used, the manufacturing cost is increased and the light utilization efficiency is increased. The reproduction characteristics of the optical disk can be improved without causing a decrease.

  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 In one embodiment of the present invention, the formula (7), (9), illustrate _{Rd 2m,} theta ₁ and theta ₂ of the relationship satisfying (10) and (12) Fig. In one embodiment of the present invention, illustrating the _{Rd 2m,} theta ₁ and theta ₂ relations satisfying the formula (7) and (9) to (11) FIG. In one embodiment of the present invention, diagram illustrating a relationship between the phase difference Rd _2m of ellipticity of elliptically polarized light and the second phase plate 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 (4)

A light source that emits linearly polarized light of at least two different wavelengths in the 660 nm band and the 790 nm band, an objective lens that condenses the light on an optical recording medium, and is provided between the light source and the objective lens A retardation element that transmits the light,
The phase difference element is
A plurality of phase plates including first and second phase plates having birefringence,
When the wavelength of the incident light in the 660 nm band is λ ₁ , the wavelength of the incident light in the 790 nm band is λ ₂ , and the wavelength between the wavelengths λ ₁ and λ ₂ is λ _m ,
The phase difference of the first phase plate with respect to the incident light with the wavelength λ _m is Rd _1m , and the angle between the polarization direction of the incident light with the wavelength λ _m and the fast axis of the first phase plate is θ _1. ,
The phase difference of the second phase plate with respect to the incident light with the wavelength λ _m is Rd _2m , and the angle between the polarization direction of the incident light with the wavelength λ _m and the fast axis of the second phase plate is θ ₂ Then, Rd _1m , Rd _2m , θ ₁ and θ ₂ are represented by the following formulas (3) to (6) with respect to the set of (A1, B1, A2, B2) represented by the following formula (1) or (2). relationship meets a),
The optical head device characterized in that the first phase plate is closer to the light source than the second phase plate.
(A1, B1, A2, B2) = (− 0.209 ± 0.02, 93.8 ± 5, −0.256 ± 0.02, 214.7 ± 5) (1)
(A1, B1, A2, B2) = (− 0.209 ± 0.02,183.8 ± 5, −0.259 ± 0.02,125.1 ± 5) (2)
Rd _1m = 180 (degrees) (3)
Rd _2m = 100 to 170 (degrees) or 190 to 223 (degrees) (4)
θ ₁ = A1 × Rd _2m + B1 (degrees) (5)
θ ₂ = A2 × Rd _2m + B2 (degrees) (6) 2. The optical head device according to claim 1, wherein the wavelength [lambda] _m is 690 nm to 740 nm. The optical head device according to claim 1, wherein the plurality of phase plates are laminated and integrated. 4. The light transmitted through the phase difference element is elliptically polarized light having a major axis in a direction inclined by approximately 45 degrees with respect to the polarization direction of linearly polarized light when entering the phase difference element. the optical head apparatus according to 1, wherein.

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