JP2010146605A - Wide-band wavelength plate and optical head device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a wide-band wavelength plate receiving a plurality of light rays different in wavelength and emitting them by controlling the polarization state of the light rays, and to provide an optical head device. <P>SOLUTION: The wide-band wavelength plate 10 includes a first wavelength plate 11, a second wavelength plate 12 and a third wavelength plate 13 parallel to each other. The wavelength plate 10 emits light of wavelengths of λ<SB>1</SB>and λ<SB>2</SB>as circularly polarized light, and emits light of a wavelength of λ<SB>3</SB>as linearly polarized light (wherein λ<SB>1</SB><λ<SB>2</SB><λ<SB>3</SB>) to linearly polarized light of different wavelengths of λ<SB>1</SB>, λ<SB>2</SB>and λ<SB>3</SB>by adjusting retardation values Rd<SB>1</SB>, Rd<SB>2</SB>and Rd<SB>3</SB>of the three wavelength plates, and angles θ<SB>1</SB>, θ<SB>2</SB>and θ<SB>3</SB>formed by the vibration direction of incident light and each of the three wavelength plates. A compact optical head device and high-quality recording and playback can be achieved. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a broadband wavelength plate for controlling a polarization state that is transmitted with respect to incident light. In particular, as an optical system that handles optical storage, an optical recording medium such as a CD, a DVD, a magneto-optical disk, and the like. The present invention also relates to an optical head device for recording and reproducing information on a high-density optical recording medium such as “Blu-ray” (registered trademark: BD).

  In recent years, for example, as an optical system that handles optical storage, not only optical recording media such as CDs, DVDs, and magneto-optical disks, but also high-density optical recording media such as BDs (hereinafter referred to as “optical disks”) The development of an optical head device that performs reproduction (hereinafter referred to as “recording / reproduction”) is progressing, and the shorter the wavelength of the laser beam used, the higher the recording density, so the shorter wavelength (405 nm wavelength band). Is underway. On the other hand, among many optical discs that have been used so far, DVD is a laser beam having a long wavelength of laser light in the red region (660 nm wavelength band), and CD is a laser beam having a long wavelength in the near infrared region (785 nm wavelength band) (unnecessary). Is recorded and played back. As an optical head device, a compatible optical head device capable of recording / reproducing with respect to optical discs of different standards which are recorded / reproduced with laser beams of different wavelength bands is desired. Various schemes for securing it have been proposed.

  As described above, as one of methods for ensuring compatibility with not only high-density optical recording media but also conventional optical disks, in addition to light sources emitting short wavelengths, light sources in the red and near infrared regions There is also a method of installing them together. For this reason, there is an increasing need for optical components having a certain characteristic with respect to a wide-band wavelength including three different wavelengths, and an optical head device capable of recording / reproducing BD, DVD, CD, etc. can be configured with few optical components. Thus, downsizing is required.

  FIG. 6 is a schematic diagram showing a configuration of a conventional optical head device 100 using laser beams of three different wavelengths as an example. Light in the 405 nm wavelength band emitted from the light source 101 such as a semiconductor laser is reflected by the polarization beam splitter 107, becomes parallel light by the collimator lens 110, and becomes circularly polarized light by the quarter wavelength plate 112. The circularly polarized light is reflected by the mirror 114 and condensed on the information recording surface of the optical disk 118 such as a BD by the objective lens 116. Then, the reflected light including the information of the pits formed on the information recording surface travels in the reverse direction on the above path. Here, the optical path from the light source to the optical disk is described as “outward path”, and the optical path from the optical disk to the optical detector and “return path” are described. Due to reflection on the optical disk, the light in the return path becomes circularly polarized light whose electric field rotates in the direction opposite to the circular polarization in the forward path, and the light in the return path that passes through the quarter-wave plate 112 has an oscillation direction of the electric field It becomes linearly polarized light orthogonal to the linearly polarized light, passes straight through the polarization beam splitter 107, and reaches the photodetector 119. The photodetector 119 reads out information recorded on the optical disc 118 by detecting pit information from the light reflected by the optical disc.

  Similarly, light in the 660 nm wavelength band emitted from the light source 102 such as a semiconductor laser passes straight through the dichroic prism 108, and light in the 785 nm wavelength band emitted from the light source 103 is reflected by the dichroic prism 108, and each light Are linearly polarized light having the same vibration direction of the electric field, enter the polarization beam splitter 109, and pass straight through. Then, the collimator lens 111 becomes parallel light, the quarter-wave plate 113 becomes circularly polarized light, is reflected by the mirror 115, and is condensed on the optical disk 118 by the objective lens 117. The light on the return path becomes linearly polarized light that is orthogonal to the linearly polarized light on the forward path, is reflected by the polarization beam splitter 109, and reaches the photodetector 120. Elements 104, 105, and 106 are diffractive elements, and are optical elements for expressing light from a light source as a main beam and three beams, for example, two sub-beams that serve as tracking servo signals. In the optical path between the forward light source and the polarizing beam splitter or dichroic prism.

  As described above, the optical head device 100 shown in FIG. 6 has two sets of optical elements such as a polarizing beam splitter and a photodetector, each having a large number of parts, and time is required for downsizing and assembly adjustment of the device. There is such a problem. On the other hand, for example, FIG. 7 reports an optical head device 200 as another configuration example using laser beams of three different wavelengths (Patent Document 1).

  The optical head device 200 shown in FIG. 7 is provided with a collimator lens 210 and a quarter wavelength plate 211 that can be used in common with respect to laser beams having three different wavelengths with respect to the optical head device 100. . A dichroic prism 208 is disposed in the optical path of light emitted from the light source 201 in the 405 nm wavelength band, the light source 202 in the 660 nm wavelength band, and the light source 203 in the 785 nm wavelength band, and the dichroic prism 207 is disposed in the optical path of the light source 202 and the light source 203. The light of each wavelength is transmitted or reflected. The dichroic prism 207 has a function of transmitting light in the 660 nm wavelength band and reflecting light in the 785 nm wavelength band, and the dichroic prism 208 transmits light in the 660 nm wavelength band and the 785 nm wavelength band and 405 nm wavelength band. It has a function of reflecting the light. Further, light of three wavelengths passes through the collimator lens 210 and the quarter wavelength plate 211 and enters the dichroic prism 212. The dichroic prism 212 has a function of reflecting light in the 405 nm wavelength band and transmitting light in the 660 nm wavelength band and the 785 nm wavelength band in a straight line. The light in the 405 nm wavelength band is reflected on the optical disk 216 such as a BD by the objective lens 214. Condensed on the information recording surface.

  On the other hand, light in the 660 nm wavelength band and the 785 nm wavelength band passes straight through the dichroic prism 212, is reflected by the mirror 213, and is condensed by the objective lens 215 on the information recording surface of the optical disk 216 such as a DVD or CD. The 405 nm wavelength band, 660 nm wavelength band, and 785 nm wavelength band return light reflected by each optical disk is converted into linearly polarized light orthogonal to the forward linearly polarized light by the quarter wavelength plate 211, and the polarization beam splitter. The light is reflected at 209 and reaches the photodetector 217. In Patent Document 1, the quarter wavelength plate 211 is a broadband wavelength plate that functions as a quarter wavelength plate that generates a phase difference that is an odd multiple of π / 2 in all of the 405 nm wavelength band, the 660 nm wavelength band, and the 785 nm wavelength band. It has been reported as an optical head device characterized in that there is.

  As described above, in order to realize a compatible optical head device capable of recording / reproducing information of optical discs of different standards such as BD, DVD, CD, etc., by entering light of three different wavelengths, When light of three different wavelengths is condensed as circularly polarized light as in Document 1, there is a problem that CD recording / reproduction cannot be performed particularly well. The reason for this is that some CDs have a large birefringent material as a resin material that protects the information recording surface compared to BD and DVD. For example, when the light reflected from the CD becomes the same circularly polarized light as the light path (outward path) where the light reflected due to the birefringence of the resin material enters, the light that is reflected by the polarization beam splitter and reaches the photodetector The amount of light is greatly reduced, and information cannot be reproduced satisfactorily. For this reason, in order to perform recording / reproduction on such an optical disc satisfactorily, in order to reduce the influence of the birefringence, light having a wavelength for CD (785 nm wavelength band) is linearly polarized in a CD state. Means for condensing light on the information recording surface is used.

  Specifically, as an example of condensing with circularly polarized light for the BD and DVD wavelength bands and linearly polarized light for the CD wavelength band, as shown in FIG. An optical head device 300 having another configuration used has been reported (Patent Document 2). The optical head device 300 includes dichroic prisms 308 and 309 and a non-polarized diffraction grating 307 arranged in the optical path of light emitted from a light source 301 having a wavelength band of 405 nm, a light source 302 having a wavelength band of 660 nm, and a light source 303 having a wavelength band of 785 nm. In the optical path between the lens 310 and the objective lens 313, a polarization diffraction grating 311 and a (broadband) wave plate 312 are provided. In addition, photodetectors 304, 305, and 306 are arranged for the light of each wavelength band, and the light reflected by the optical disk 315 is detected.

  Here, although light in any wavelength band is incident on the wave plate 312 in the state of linear polarization, the wave plate 312 emits light in the 405 nm wavelength band and the 660 nm wavelength band as circularly polarized light, and light in the 785 nm wavelength band. Has a function of transmitting linearly polarized light, and the light emitted from the wave plate 312 is condensed on the information recording surface of the optical disk 315. In this way, light in the 405 nm wavelength band and 660 nm wavelength band is condensed on the information recording surface of the BD and DVD as circularly polarized light, and light on the 785 nm wavelength band is condensed on the information recording surface of the CD as linearly polarized light. By doing so, it is possible to realize an optical head device in which the recording / reproducing quality is not deteriorated by the birefringent material having a large CD.

JP 2006-1114080 A JP 2006-236549 A

  However, the wave plate of the optical head device of Patent Document 2 can obtain a desired phase difference with respect to light having a wavelength of 405 nm, light having a wavelength of 660 nm, and light having a wavelength of 785 nm, respectively. When the wavelength of light is λ, a 9λ / 4 plate that gives a phase difference of (9/4) λ to 405 nm light, and a 5λ / plate that gives a phase difference of (5/4) λ to 660 nm light. It is designed to be a four-plate, one-wavelength plate that gives a phase difference of λ to 785 nm light. If the wavelength of the oscillating light fluctuates due to variations in the operating temperature and individual elements of the semiconductor laser, etc. Since the phase difference of the light passing through the light fluctuates greatly with respect to the desired phase difference, the polarization state of the light passing through is not stable, the amount of light reaching the photodetectors 304, 305, 306 is not stable, and the light The recording / playback quality of the head device There is a problem that the reduction.

The present invention has been made in view of the above points, and is linearly polarized light that is incident at _three wavelengths λ ₁ , λ ₂ , and λ ₃ (λ ₁ <λ ₂ <λ ₃ ) having predetermined different bands. In the broadband wavelength plate that changes the polarization state of the first wavelength plate, the broadband wavelength plate includes a first wavelength plate, a second wavelength plate, and a third wavelength plate in order from the side on which the linearly polarized light is incident. The fast axis of the first wave plate, the fast axis of the second wave plate, and the fast axis of the third wave plate with reference to the direction of oscillation of the electric field of the linearly polarized light incident on the wave plate Or a combination of angles with the slow axis of the first wave plate, the slow axis of the second wave plate, and the slow axis of the third wave plate, respectively, θ ₁ [° ], θ _{2 [°],} when the θ _{3 [°],} the wave linearly polarized light is transmitted through the broadband wave plate the incident lambda ₁ ellipticity kappa ₂ light ellipticity kappa ₁ and the wavelength lambda ₂ of light is 0.65 or more, light ellipticity kappa ₃ of the wavelength lambda ₃ transmitted through the broadband wave plate is 0.4 or less retardation value Rd 1 of the first wave plate so that _the retardation value Rd 2 of the second wave _plate, the third retardation value Rd ₃ and wave plate, the theta _1, the theta _2, the A broadband wave plate in which θ ₃ is set is provided.

Also, an azimuth angle that is an angle between a direction in which the electric field of linearly polarized light incident at the wavelength λ ₃ vibrates and a direction in which the amplitude of the electric field of the light transmitted at the wavelength λ ₃ becomes the largest is −10 ° to +10. Provided is the above-mentioned broadband wave plate having a range of °.

The wavelength λ ₁ is a 405 nm wavelength band that is a wavelength range of 390 to 420 nm, the wavelength λ ₂ is a 660 nm wavelength band that is a wavelength range of 640 to 680 nm, and the wavelength λ ₃ is 785 nm that is a wavelength range of 765 to 805 nm. is the wavelength band, for the wavelength lambda _1, the retardation value of the first wave plate, the second retardation value of the wave plate, the retardation value of the third wave plates, respectively, Rd 1 _{(lambda 1)} [ nm], Rd ₂ (λ ₁ ) [nm], Rd ₃ (λ ₁ ) [nm],
156 ≦ Rd ₁ (λ ₁ ) ≦ 248
And 802 ≦ Rd ₂ (λ ₁ ) ≦ 872
And 172 ≦ Rd ₃ (λ ₁ ) ≦ 231
Furthermore, when the incident linearly polarized light is transmitted through the broadband wave plate, the range of 0 ° to 180 ° counterclockwise with respect to the direction of oscillation of the electric field of the linearly polarized light is added (+). When the range of 0 ° to −180 ° in the clockwise direction is minus (−) and the value of θ ₁ having the smallest absolute value is greater than zero, θ ₂ is
2 × θ ₁ −14.2 ≦ θ ₂ ≦ 2 × θ ₁ +12.1
And θ ₃ is
(2 × θ ₁ +45) -14.5 ≦ θ ₃ ≦ (2 × θ ₁ +45) +12.1
The broadband wave plate satisfying the above is provided.

The wavelength λ ₁ is a 405 nm wavelength band that is a wavelength range of 390 to 420 nm, the wavelength λ ₂ is a 660 nm wavelength band that is a wavelength range of 640 to 680 nm, and the wavelength λ ₃ is 785 nm that is a wavelength range of 765 to 805 nm. is the wavelength band, for the wavelength lambda _1, the retardation value of the first wave plate, the second retardation value of the wave plate, the retardation value of the third wave plates, respectively, Rd 1 _{(lambda 1)} [ nm], Rd ₂ (λ ₁ ) [nm], Rd ₃ (λ ₁ ) [nm],
156 ≦ Rd ₁ (λ ₁ ) ≦ 248
And 802 ≦ Rd ₂ (λ ₁ ) ≦ 872
And 172 ≦ Rd ₃ (λ ₁ ) ≦ 231
Furthermore, when the incident linearly polarized light is transmitted through the broadband wave plate, the range of 0 ° to 180 ° counterclockwise with respect to the direction of oscillation of the electric field of the linearly polarized light is added (+). When the range of 0 ° to −180 ° in the clockwise direction is minus (−) and the value of θ ₁ having the smallest absolute value is smaller than zero, θ ₂ is
2 × θ ₁ −12.1 ≦ θ ₂ ≦ 2 × θ ₁ +14.2
And θ ₃ is
(2 × θ ₁ −45) −12.1 ≦ θ ₃ ≦ (2 × θ ₁ −45) +14.5
The broadband wave plate satisfying the above is provided.

  Furthermore, at least one light source that emits light of three different wavelengths, an objective lens that condenses the light emitted from the light source on the optical recording medium, and light that detects light reflected from the optical recording medium A wideband wavelength as described above in an optical path between the light source and the objective lens or in an optical path between the objective lens and the photodetector. An optical head device in which a plate is disposed is provided.

  The present invention provides a broadband wave plate capable of changing the polarization state of each light by entering light having a plurality of different wavelengths and stably obtaining desired optical characteristics in a certain range of wavelength bands. In addition, in an optical head device that uses light of three different wavelengths, it is possible to achieve downsizing by reducing the number of components of the optical element and improving the quality of recording and reproduction.

(First embodiment)
FIG. 1 is a schematic diagram showing a broadband wavelength plate 10 according to the present embodiment. Among these, FIG. 1A is a schematic cross-sectional view of a broadband wave plate 10 in which three wave plates 11, 12, and 13 showing birefringence are integrated with transparent substrates 14a and 14b. It is. Further, a transparent medium such as another transparent substrate (not shown) or an optically isotropic adhesive (not shown) may be disposed between the transparent substrates 14a and 14b and the wave plates 11, 12, and 13, respectively. Good. Further, the broadband wave plate 10 is not limited to an integrated structure, and does not include the transparent substrates 14a and 14b, and is composed of only the wave plates 11, 12, and 13, or the wave plates 11, 12 And 13 may be arranged apart from each other.

  The transparent substrates 14a and 14b can be made of various materials such as a resin plate and a resin film as long as they are transparent to incident light, but have optically isotropic properties such as glass and quartz glass. Use of a material is preferable because it does not affect the birefringence of transmitted light. As described above, when at least one of the transparent substrates 14a and 14b and the wave plates 11, 12, and 13 are integrated, the surface can be smoothed and the adhesion can be maintained. An increase in the amount of wavefront aberration can be suppressed. In addition, when a broadband wave plate is used alone without being laminated and integrated with other optical elements, a configuration in which it is sandwiched between at least two transparent substrates 14a and 14b is preferable from the viewpoint of reducing wavefront aberration and ensuring strength.

The wave plates 11, 12 and 13 made of a material having birefringence are, for example, polymer liquid crystals obtained by polymerizing liquid crystals, or organic materials such as polycarbonate, polyolefin, and PVA that are birefringent by stretching. In addition, a single crystal having optical anisotropy such as quartz, LiNbO ₃ , LiTaO ₃ , KDP, or the like may be used. Moreover, when it consists of a polymer liquid crystal, the orientation film which is not shown in figure may be given. The optical axes of these wave plates are all configured to be parallel to the XY plane and aligned in the thickness direction (Z direction).

  In addition, an adhesive film (not shown), a UV curable adhesive, or a thermosetting adhesive can be used for bonding the three wave plates 11, 12, and 13. Here, in order to reduce the wavefront aberration of the broadband wavelength plate 10 and to improve the temperature characteristics and reliability, it is desirable to bond as a thin adhesive layer as much as possible, and the thickness of the adhesive layer (not shown) should be 10 μm or less. Especially desirable.

  The broadband wave plate 10 configured as described above has a wavelength of 405 nm, a wavelength of 660 nm, light having a wavelength of 405 nm, light having a wavelength of 660 nm, and light having a wavelength of 785 nm incident as linearly polarized light whose electric field vibrates in the same direction. The function as a one-wave plate having a function of a quarter-wave plate for light in the band and not changing the polarization state for light in the wavelength band of 785 nm is realized. Here, specifically, the 405 nm wavelength band indicates the range of 390 to 420 nm, the 660 nm wavelength band indicates the range of 640 to 680 nm, and the 785 nm wavelength band indicates the range of 765 to 805 nm.

The quarter-wave plate has a function of generating a phase difference that is an integral multiple of λ / 4 with respect to light having a wavelength λ. It has a function of converting to linearly polarized light. When the incident light is linearly polarized light, its performance can be evaluated by the ellipticity of the polarization state of the transmitted light. The closer the ellipticity is to 1, the closer the polarization state is to circular polarization. It can be said that the performance of the board is high. Broadband wave plate 10 of the present invention, 660 nm wavelength incident at 405nm ellipticity of light transmitted to light in the wavelength band (the ratio of the minor axis of the ellipse to major ellipse) kappa ₁ and linearly polarized light incident linearly polarized light preferably if the ellipticity kappa ₂ of light passing through each 0.65 or more to the band of light, more preferably as long as 0.75 or more, more preferably equal to 0.8 or more.

For example, when the direction parallel to the vibration direction of the electric field of linearly polarized light is set to the A direction, the light having the intensity component of 100% and the light intensity of 1 at the wavelength of 405 nm or 660 nm is incident on the broadband wavelength plate 10. The transmitted light is in a polarization state close to circularly polarized light. At this time, the ellipticity of light in the 405 nm wavelength band that transmits through the broadband wavelength plate 10 is κ ₁ , and the ellipticity of light in the 660 nm wavelength band that transmits through the broadband wavelength plate 10 is κ ₂ . Then, when reflected by the optical disk and transmitted through the broadband wavelength plate 10 again, the intensity component of the electric field whose direction oscillates in the B direction orthogonal to the A direction is converted into substantially linearly polarized light. When the ellipticities κ ₁ and κ ₂ are 0.65 or more, the intensity component of the electric field oscillating in the B direction of the light is 0.8 or more. Here, in the optical head device, out of the light reflected from the optical disk by emitting linearly polarized light whose electric field oscillates in the A direction from the light source, the greater the proportion of the intensity component of the electric field oscillating in the B direction, the more in the optical path. It is possible to increase the amount of light that is deflected by the arranged polarization beam splitter and reaches the photodetector. For this reason, as the ellipticities κ ₁ and κ ₂ approach 1, the utilization efficiency becomes higher.

On the other hand, the single wavelength plate has a function of generating a phase difference that is an integral multiple of λ with respect to light of wavelength λ. Good. In the broadband wavelength plate 10 of the present invention, for example, when linearly polarized light in the 785 nm wavelength band where the intensity component of the electric field oscillating in the A direction is 100% and the light intensity is 1 is incident on the broadband wavelength plate 10, the transmitted light is A It is preferable that the intensity component of the electric field oscillating in the direction is the main light. At this time, the ellipticity κ _{3 with} respect to light in the 785 nm wavelength band is preferably 0.4 or less, and more preferably 0.3 or less. Also, the direction of the major axis of the elliptical polarization of the elliptically polarized light with respect to the A direction is defined as an azimuth angle ψ. When the transmitted light is linearly polarized light (κ ₃ = 0), the direction in which the electric field vibrates is the azimuth angle ψ. It corresponds to. In the case of elliptically polarized light, the angle formed by the A direction and the direction in which the amplitude of the electric field is the largest can be the azimuth angle ψ. In the broadband wave plate 10 of the present invention, the azimuth angle of the transmitted light with respect to linearly polarized light whose electric field vibrates in the A direction incident in the 785 nm wavelength band should be within a range of −10 ° to + 10 ° with respect to the A direction. If it is in the range of −5 ° to + 5 °, it is more preferable.

  Next, the configuration of the broadband wavelength plate 10 for obtaining the function of a quarter wavelength plate or the function of a single wavelength plate by entering light of three different wavelength bands will be described. At this time, it is assumed that light in these wavelength bands is linearly polarized light whose electric field oscillates in the X direction and enters the broadband wave plate 10 of FIG. 1 while traveling in the Z direction from the transparent substrate 14a side. . Hereinafter, the wave plates 11, 12, and 13 are referred to as the first wave plate 11, the second wave plate 12, and the third wave plate 13 based on the order in which the linearly polarized light having the respective wavelengths are incident. Hereinafter, the direction of vibration of the electric field of linearly polarized light will be simply referred to as “the direction of linearly polarized light” or “linearly polarized light in the direction A” by abbreviating “vibration of the electric field”.

FIG. 1B is a schematic plan view of the broadband wavelength plate 10, and the direction of the linearly polarized light 15 incident on the broadband wavelength plate 10 (= X direction) and the optical axis 21 of the first wavelength plate 11 are formed. The angle is θ ₁ . Furthermore, the angle formed by the direction of the linearly polarized light 15 incident on the broadband wavelength plate 10 (= X direction) and the optical axis 22 of the second wavelength plate 12 is θ ₂ , and the linearly polarized light 15 incident on the broadband wavelength plate 10 An angle formed between the direction (= X direction) and the optical axis 23 of the third wave plate 13 is defined as θ ₃ . The combination of the optical axes here is considered in the combination of the fast axes or the slow axes of the three wave plates. Hereinafter, the setting conditions of the parameters of the respective wave plates are particularly optimal. The conditions will be described.

1. Setting the angle of the optical axis of the wave plate First, a specific relationship between the angles θ ₁ , θ _2, and θ ₃ (each unit is [°]) will be described with reference to FIG. Here, the signs of θ ₁ , θ _2, and θ ₃ are plus (+) in the counterclockwise direction with respect to the X direction on the light transmission side surface (XY plane) of the broadband wave plate 10, The clockwise direction is defined as minus (−), the range that plus (+) can take is 0 ° to + 180 °, and the range that minus (−) can take is 0 ° to −180 °. Note that the origin of coordinates on the XY plane corresponds to a point that intersects the optical axis (the center of light). The relationship between θ ₁ and θ ₂ is that θ ₁ is a value with respect to the fast axis or the slow axis having the smallest absolute value, and the combination of the fast axes or the slow axes at that time is as follows. ,
θ ₂ = 2 × θ ₁ (1)
To be. Furthermore, the relationship between θ ₂ and θ ₃ is
θ ₃ = θ ₂ +45 (2a)
Or
θ ₃ = θ ₂ −45 (2b)
Is set so that either one of the above is established. Therefore, _once θ ₁ is determined, θ ₂ and θ ₃ are determined by equation (1) and either equation (2a) or equation (2b).

2. Retardation value of wave plate For example, the birefringent materials constituting the first wave plate 11, the second wave plate 12, and the third wave plate 13 are the same, and the ordinary light refraction of the birefringent material the rate _{n o,} the extraordinary refractive index refractive index anisotropy _{Δn (= | n e -n o} |) when formed into a _{n e} is expressed by the product of the thickness of _{d 1} of the first wave plate Let the _first retardation value be Rd ₁ (= Δn · d ₁ ). Further, the refractive index anisotropy Δn is designed in consideration of wavelength dispersion, that is, Δn that varies depending on the wavelength of incident light even with the same birefringent material.

Then, the thickness of the second wave plate 12 is d ₂ , the retardation value at this time is Rd ₂ (= Δn · d ₂ ), the thickness of the third wave plate 13 is d _3, and the retardation at this time The value is Rd ₃ (= Δn · d ₃ ). In addition, although the birefringent material which comprises the 1st waveplate 11, the 2nd waveplate 12, and the 3rd waveplate 13 is demonstrated as the same thing, the combination of a different material may be sufficient. In addition, it is preferable that two materials of the three wave plates are the same, and if all the three materials are the same, the thermal expansion coefficients are all the same with respect to changes in the operating temperature, so that the distortion of the broadband wave plate 10 is reduced. It is more preferable because it can be done.

Next, the conditions for the retardation values of these three wave plates will be described. When light of three different wavelengths λ ₁ , λ ₂ and λ ₃ (λ ₁ <λ ₂ <λ ₃ ) is incident on the broadband wave plate 10, the retardation value Rd ₁ for the wavelength λ ₁ in the first wave plate 11. (Λ ₁ ) is
Rd ₁ (λ ₁ ) = λ _1/2 (3)
The birefringent material and the thickness d ₁ are adjusted so as to satisfy the above. Further, in the third wave plate 13, the retardation value Rd ₃ (λ ₁ ) for the wavelength λ ₁ is
_{Rd 3 (λ 1) = λ} 1/4 ··· (4)
To meet, to adjust the birefringent material and the thickness d _3.

Here, when the broadband wavelength plate 10 is configured so as to satisfy the conditions of the above formulas (1) to (4) (one of the formulas (2a) and (2b)), the broadband wavelength A change in the polarization state of light incident on the plate 10 will be described with reference to FIG. As shown in the schematic plan view of FIG. 1B, when the linearly polarized light 15 having the wavelength λ _{1 in} the X direction is incident, first, the retardation value Rd ₁ (λ ₁ ) of the first wave plate 11 is λ ₁ / 2 and a phase difference of π occurs, so that the light transmitted through the wave plate 11 is linearly polarized light in the direction of 2 × θ ₁ , that is, θ ₂ with respect to the direction of the linearly polarized light 15 from Equation (1). It becomes linearly polarized light in a direction that forms an angle. Since the direction of the linearly polarized light having the wavelength λ ₁ incident on the second wave plate 12 coincides with the optical axis 22 of the second wave plate 12, the light is transmitted without changing the polarization state, and the third wave plate 13 is incident. Accordingly, the retardation value Rd ₂ (λ ₁ ) of the second wave plate can be arbitrarily determined for the light with the wavelength λ ₁ . It desired the value of Rd 2 _{(λ 1)} is, [Delta] n for the wavelength lambda ₂ of light, [Delta] n for the wavelength lambda ₃ of the light, i.e. well considering the wavelength dispersion characteristics, light of the wavelength lambda ₂ and wavelength lambda ₃ is incident It is good to set the value so that the polarization state becomes.

Then, an angle formed by the direction of the linearly polarized light having the wavelength λ ₁ incident on the third wave plate 13 so as to coincide with the optical axis 22 of the second wave plate 12 and the optical axis of the third wave plate 13 (= theta ₃ - [theta] ₂₎ is 45 [°], further retardation value Rd ₃ of the third wave plate 13 (λ _1), since a lambda _1/4 to generate a phase difference of [pi / 2 From the linearly polarized light to the circularly polarized light, the broadband wave plate 10 is transmitted. Thus, in order to transmit light having a wavelength λ ₁ incident as linearly polarized light with circularly polarized light having an ellipticity κ ₁ of 1, it is sufficient to satisfy the conditions of the expressions (1) to (4) (expression ( 2a) and Equation (2b) are either one of these), and satisfy these conditions, and set conditions under which the light transmitted through the light of wavelengths λ ₂ and λ ₃ has a desired polarization state. .

Here, in the light of wavelengths λ ₂ and λ ₃ , linearly polarized light in the X direction travels in the Z direction and enters the broadband wave plate 10, and the light of wavelength λ ₂ is transmitted as circularly polarized light, and the wavelength λ ₃ light to be transmitted in the X-direction as linearly polarized light, the retardation values Rd ₁ (λ ₁ ), Rd ₂ (λ ₁ ) and Rd ₃ (λ ₁ ) of each wave plate, and θ ₁ , θ The conditions of ₂ and θ ₃ may be obtained, and the design method will be described below. The retardation values Rd ₁ (λ ₁ ), Rd ₂ (λ ₁ ), and Rd ₃ (λ ₁ ) with respect to the wavelength λ ₁ may be simply expressed as Rd ₁ , Rd _2, and Rd ₃ hereinafter.

Specifically, a condition for setting the ellipticity κ ₂ close to 1 for the light of wavelength λ ₂ and setting the ellipticity κ ₃ close to 0 for the light of wavelength λ ₃ is set. At this time, the polarization state of light transmitted through the first wave plate 11, the second wave plate 12, and the third wave plate 13 constituting the broadband wave plate 10 is represented by a Stokes parameter S, and a Mueller matrix described later is solved. Thus, the conditions of Rd ₁ , Rd ₂ and Rd _{3 and} the conditions of θ ₁ , θ ₂ and θ ₃ are obtained. The Stokes parameter S can be represented by a normal (S ₀ , S ₁ , S ₂ , S ₃ ) four-dimensional vector, where S ₀ is the light intensity, and S ₁ is in the 0 ° direction (X-axis direction). The intensity of the oscillating electric field, S ₂ means the intensity of the electric field oscillating in the 45 ° direction, and S ₃ means the intensity of circularly polarized light. Hereinafter, the Stokes parameter S will be described as a three-dimensional vector (S ₁ , S ₂ , S ₃ ) with the light intensity S ₀ omitted.

First, when k is a wavelength type (k = 1, 2, 3) and a wavelength λ _k is given, the light of the three wavelengths λ _k is linearly polarized light in the same direction (X-axis direction) and the broadband wave plate 10. Therefore, the Stokes parameter S _INk of the incident light is
(S _1INk , S _2INk , S _3INk ) = (1, 0, 0) (5)
Can be given in

Then, the Stokes parameters of the light of each wavelength transmitted through the broadband wave plate 10 are (S _1OUTk , S _2OUTk , S _3OUTk ). First, the light having the wavelength λ _{1 to be} transmitted is circularly polarized light according to the condition (1), which is a combination of the optical axes of the three wavelength plates, and the condition (2a) or (2b). Because it ’s ideal,
(S _1OUT1 , S _2OUT1 , S _3OUT1 ) = (0, 0, 1) (6a)
Or
(S _1OUT1 , S _2OUT1 , S _3OUT1 ) = (0, 0, −1) (6b)
Either of the above is satisfied.

Next, circularly polarized light is ideal for the light of wavelength λ ₂ that passes through the broadband wave plate 10.
(S _1OUT2 , S _2OUT2 , S _3OUT2 ) = (0, 0, 1) (7a)
Or
(S _1OUT2 , S _2OUT2 , S _3OUT2 ) = (0, 0, −1) (7b)
Either of the above is satisfied.

And, since the light of wavelength λ ₃ transmitted through the broadband wave plate 10 is ideally linearly polarized light,
(S _1OUT3 , S _2OUT3 , S _3OUT3 = (1, 0, 0) (8)
Is established.

Next, for the optical characteristics of the first wave plate 11, the second wave plate 12, and the third wave plate 13, Mueller matrices WP _1k (θ ₁ , δ _1k ), WP _2k (θ ₂ , δ _2k ), respectively. And WP _3k (θ ₃ , δ _3k ). Here, δ _1k , δ _2k and δ _3k are phase differences generated in the respective wave plates, and are determined depending on the wavelength λ _k of the incident light (k = 1, 2, 3). That means
δ _1k = 360 · Rd ₁ (λ _k ) / λ _k [°] (9a)
δ _2k = 360 · Rd ₂ (λ _k ) / λ _k [°] (9b)
δ _3k = 360 · Rd ₃ (λ _k ) / λ _k [°] (9c)
Given in. Each Mueller matrix can be given by the following equations (10a) to (10c) using θ ₁ , θ ₂ , θ ₃ , δ _1k , δ _2k and δ _3k . In the general formula of the 3 × 3 matrix, for example, “23k” of A _23k among the components of the formula (10a) is “row number”, “column number”, “(short wavelength) from the left. It is written so that it is regularly arranged in the order of “kth wavelength”.

Here, in FIG. 1B, when the linearly polarized light in the X direction passes through the first wave plate 11, the second wave plate 12, and the third wave plate 13 of the broadband wave plate 10, the wavelength λ ₁ in light formula (6a) or formula (6b), wherein the wavelength lambda ₂ of the light (7a) or formula (7b), to satisfy the equation (8) in the light of the wavelength lambda _3, the formula (10a) ~ formula ( Using the determinant using 10c), the following equations (11a) to (11c) can be given.

At this time, the function of the half-wave plate is satisfied by the first wave plate 11 that satisfies the formula (1), the formula (2a), or the formula (2b) and the light having the wavelength λ ₁ is incident. By transforming equation (3) to have
δ ₁₁ = 180 [°] (12a)
And the expression (4) is modified so that the third wavelength plate 13 when the light of wavelength λ ₁ is incident has the function of a quarter wavelength plate.
δ ₃₁ = 90 [°] (12b)
The other parameters θ ₁ , θ ₂ , θ ₃ , δ _1k , δ _2k and δ _3k can be determined by optimal calculation.

Next, the above-mentioned conditions satisfy perfect circular polarization (ellipticity κ ₁ , κ ₂ = 1) and perfect linear polarization (ellipticity κ ₃ = 0) in light having three different wavelengths to be transmitted. In the present invention, it is sufficient that the polarization state of the transmitted light satisfies the condition of a constant ellipticity with respect to the light in each wavelength band. Specifically, when the wavelength λ ₁ is a 405 nm wavelength band (390 to 420 nm), the wavelength λ ₂ is a 660 nm wavelength band (640 to 680 nm), and the wavelength λ ₃ is a 785 nm wavelength band (765 to 805 nm), the wavelength λ ₁ ellipticity kappa ₂ for ellipticity kappa ₁ and wavelength lambda ₂ of light is 0.65 or more with respect to light, and ellipticity kappa ₃ for the wavelength lambda ₃ of the light may be any satisfying those 0.4 .

As conditions for the ellipticity κ ₁ of the broadband wavelength plate 10 to be 0.65 or more, θ ₁ , θ _2, and θ ₃ are the conditions of either the expression (1), the expression (2a), or the expression (2b), Rd ₃ (λ ₁ ) is fixed under the condition of Equation (4), and the range that Rd ₁ (λ ₁ ) can take is considered. At this time, the minimum value of Rd ₁ (λ ₁ ) is a retardation value Rd ₁ (390) = 390 / 2−39 = 156 [nm] with respect to light with a wavelength of 390 nm among possible wavelengths λ ₁ . Rd ₁ (λ ₁ ) may be 156 [nm] or more. Furthermore, since the maximum value of Rd ₁ (λ ₁ ) is a retardation value Rd ₁ (420) = 420/2 + 38 = 248 [nm] for light having a wavelength of 420 nm among the possible wavelengths λ ₁ , Rd ₁ ( λ ₁ ) may be 248 [nm] or less. Accordingly, the retardation value Rd ₁ (λ ₁ ) [nm] at the wavelength λ ₁ of the first wave plate 11 is
156 ≦ Rd ₁ (λ ₁ ) ≦ 248 (13)
It is good to set in the range.

Next, when the wavelength λ _{1 is set} to the 405 nm wavelength band (390 to 420 nm), θ ₁ , θ _2, and θ ₃ are expressed by the following equation (1) as a condition that the ellipticity κ ₁ of the broadband wavelength plate 10 is 0.65 or more. Then, one of the conditions (2a) or (2b), Rd ₁ (λ ₁ ) is fixed under the condition of Expression (3), and the possible range of Rd ₃ (λ ₁ ) is considered. At this time, the minimum value of Rd ₃ (λ ₁ ) is a retardation value Rd ₃ (390) = 390 / 2-23 = 172 [nm] with respect to light with a wavelength of 390 nm among possible wavelengths λ ₁ . Rd ₃ (λ ₁ ) may be 172 [nm] or more. Further, the maximum value of Rd ₃ (λ ₁ ) is a retardation value Rd ₃ (420) = 420/2 + 21 = 231 [nm] for light having a wavelength of 420 nm among possible wavelengths λ ₃ , so that Rd ₃ ( λ ₁ ) may be 231 [nm] or less. Accordingly, the retardation value Rd ₃ (λ ₁ ) [nm] at the wavelength λ ₃ of the third wave plate 13 is
172 ≦ Rd ₃ (λ ₁ ) ≦ 231 (14)
It is good to set in the range.

Next, when the wavelength λ _{1 is set} to the 405 nm wavelength band (390 to 420 nm), Rd ₁ (λ ₁ ) and Rd ₃ (λ ₁ ) are the conditions for the ellipticity κ ₁ of the broadband wavelength plate 10 to be 0.65 or more. Is fixed under the conditions of equations (3) and (4), and the possible range of θ ₂ is considered. Here, when θ ₁ > 0, the wavelength λ ₁ = 390 [nm] and −14.2 [°] with respect to the optimum condition of θ ₂ in which the expressions (1) and (2a) are established. Under such conditions, the ellipticity κ ₁ is 0.65. In addition, the ellipticity κ ₁ is 0.65 under the condition of +12.1 [°] with respect to the optimum condition of θ ₂ where the equations (1) and (2a) are established at the wavelength λ ₁ = 405 [nm]. It becomes. From this, when θ ₁ > 0, equations (1) and (2a) are respectively
2 × θ ₁ -14.2 ≦ θ ₂ ≦ 2 × θ ₁ +12.1 (15)
(Θ ₃ −45) −14.2 ≦ θ ₂ ≦ (θ ₃ −45) +12.1 (16)
Can be given under the condition that the inequality of When θ ₁ > 0, a solution for θ ₂ cannot be obtained under the conditions of equations (1) and (2b).

On the other hand, when θ ₁ <0, the condition of +14.2 [°] is satisfied with respect to the optimum condition of θ ₂ in which the equations (1) and (2b) are satisfied at the wavelength λ ₁ = 390 [nm]. The ellipticity κ ₁ becomes 0.65. In addition, the ellipticity κ ₁ is 0.1 in the condition of −12.1 [°] with respect to the optimum condition of θ ₂ in which the expressions (1) and (2b) are established at the wavelength λ ₁ = 405 [nm]. 65. From this, when θ ₁ <0, the equations (1) and (2b) are respectively
2 × θ ₁ −12.1 ≦ θ ₂ ≦ 2 × θ ₁ +14.2 (17)
(Θ ₃ −45) −12.1 ≦ θ ₂ ≦ (θ ₃ −45) +14.2 (18)
Can be given under the condition that the inequality of When θ ₁ <0, the solution of θ ₂ cannot be obtained under the conditions of the equations (1) and (2a).

Next, when the wavelength λ _{1 is set} to the 405 nm wavelength band (390 to 420 nm), Rd ₁ (λ ₁ ) and Rd ₃ (λ ₁ ) are the conditions for the ellipticity κ ₁ of the broadband wavelength plate 10 to be 0.65 or more. Is fixed under the conditions of equations (3) and (4), and the possible range of θ ₃ is considered. Here, when θ ₁ > 0, the ellipticity κ ₁ is -14.5 [°] under the condition of −14.5 [°] with respect to the optimum condition of θ ₃ in which the formula (2a) is satisfied at the wavelength λ ₁ = 390 [nm]. 0.65. Further, the ellipticity κ ₁ is 0.65 under the condition of +12.1 [°] with respect to the optimum condition of θ ₃ where the formula (2a) is established at the wavelength λ ₁ = 405 [nm]. From this, equation (2a) becomes
(Θ ₂ +45) -14.5 ≦ θ ₃ ≦ (θ ₂ +45) +12.1 (19)
If the inequality of
(2 × θ ₁ +45) −14.5 ≦ θ ₃ ≦ (2 × θ ₁ +45) +12.1 (20)
Can be given under the condition that the inequality of When θ ₁ > 0, the solution of θ ₃ cannot be obtained under the conditions of the equations (1) and (2b).

On the other hand, when θ ₁ <0, the condition of +14.5 [°] is satisfied with respect to the optimum condition of θ ₃ where the equations (1) and (2b) are satisfied at the wavelength λ ₁ = 390 [nm]. The ellipticity κ ₁ becomes 0.65. In addition, the ellipticity κ ₁ is 0.1 in the condition of −12.1 [°] with respect to the optimum condition of θ ₂ in which the equations (1) and (2a) are established at the wavelength λ ₁ = 405 [nm]. 65. From this, when θ ₁ <0, the equation (2b) becomes
(Θ ₂ −45) −12.1 ≦ θ ₃ ≦ (θ ₂ −45) +14.5 (21)
If the inequality of
(2 × θ ₁ −45) −12.1 ≦ θ ₃ ≦ (2 × θ ₁ −45) +14.5 (22)
Can be given under the condition that the inequality of When θ ₁ <0, a solution for θ ₃ cannot be obtained under the conditions of equations (1) and (2a).

As other parameters, the value of θ ₁ and the retardation value Rd ₂ (λ ₁ ) of the second wave plate 12 are the light beams in the 660 nm wavelength band (640 to 680 nm) that becomes the wavelength λ ₂ and the broadband wave plate Similarly, the ellipticity κ _{2 of the} light transmitted through 10 is 0.65 or more, and the ellipticity of the light transmitted through the broadband wavelength plate 10 when light in the 785 nm wavelength band (765 to 805 nm) having the wavelength λ ₃ enters. kappa ₃ as is 0.4 or less, it may be set. In this case, the retardation value Rd ₂ (λ ₁ ) of the second wave plate 12 is
802 ≦ Rd ₂ (λ ₁ ) ≦ 872 (23)
It is good to set in the range.

(Second Embodiment)
As a second embodiment, an optical head device 20 using the broadband wavelength plate 10 according to the first embodiment will be described. FIG. 2 is a schematic diagram of an optical head device 20 according to the second embodiment. Light in a 405 nm wavelength band emitted from a light source 21 such as a semiconductor laser is a diffraction element 24, a dichroic prism 27, a polarization beam splitter. 28, passes through the dichroic prism 29, becomes parallel light by the collimator lens 30, and becomes circularly polarized light by the broadband wavelength plate 10 of the present invention. The circularly polarized light passes through the dichroic prism 31, is reflected by the mirror 32, and is collected on the information recording surface of the optical disk 35 such as a BD by the objective lens 34. The light on the return path is circularly polarized light whose electric field rotates in the opposite direction to the circularly polarized light on the forward path, and the direction of the linearly polarized light on the return path exiting the broadband wave plate 10 is orthogonal to the direction of the linearly polarized light on the forward path. The light is reflected by the splitter 28 and reaches the photodetector 36. The photodetector 36 reads information recorded on the optical disc 35 by detecting pit information from the light reflected by the optical disc 35.

  Similarly, light in the 660 nm wavelength band emitted from the light source 22 such as a semiconductor laser is transmitted through the diffraction element 25, reflected by the dichroic prism 27, transmitted through the polarization beam splitter 28 and dichroic prism 29, and collimated by the collimator lens 30. It becomes parallel light and becomes circularly polarized light by the broadband wave plate 10 of the present invention. The circularly polarized light is reflected by the dichroic prism 31 and focused on the information recording surface of the optical disk 35 such as a DVD by the objective lens 33. The light on the return path becomes circularly polarized light whose electric field rotates in the opposite direction to the circularly polarized light on the forward path, and enters the broadband wave plate 10. The direction of the linearly polarized light on the return path that exits the broadband wave plate 10 is orthogonal to the direction of the linearly polarized light on the outbound path, and is reflected by the polarization beam splitter 28 and reaches the photodetector 36.

  The light in the 785 nm wavelength band emitted from the light source 22 such as a semiconductor laser is transmitted through the diffraction element 26, transmitted through a part of incident light, and transmitted through a transmission / reflection element 27 such as a half mirror that reflects a part of the incident light. Then, the light is reflected by the dichroic prism 29, becomes parallel light by the collimator lens 30, and passes through the broadband wavelength plate 10 of the present invention with almost no change in the polarization state of the linearly polarized light. The linearly polarized light is reflected by the dichroic prism 31 and is condensed by the objective lens 23 on the information recording surface of the optical disk 35 such as a DVD in a linearly polarized state. The light on the return path is linearly polarized in the same direction as the linearly polarized light on the outward path incident on the broadband wave plate 10, is reflected by the dichroic prism 29, is reflected by the transmission / reflection optical element 37, and reaches the photodetector 38.

  As described above, by using the broadband wavelength plate 10, it is possible to obtain good circularly polarized light with respect to light in the 405 nm wavelength band and 660 nm wavelength band, and further to make linearly polarized light with respect to light in the 785 nm wavelength band. Since the optical head device can be realized, it is possible to realize a downsized optical head device with high recording / reproducing quality with respect to the optical disc.

(Example)
As an example, a method for producing a broadband wavelength plate 40 according to the present invention will be described with reference to FIG. The broadband wave plate 40 of the example has a structure in which a transparent substrate is formed between three wave plates and integrated with the broadband wave plate 10 according to the first embodiment. This is the same as the first embodiment. First, a quartz glass substrate was used as the transparent substrate 44a, and an antireflection film (not shown) was formed on one surface using a vacuum deposition method. Polyimide was applied to the surface of the transparent substrate 44a opposite to the antireflection film, and a horizontal alignment process was performed by rubbing to form an alignment film (not shown). Next, a thermosetting epoxy sealant (not shown) is applied to the outer peripheral portion of the transparent substrate 44a, and then SiO ₂ beads (not shown) having a diameter of 5.0 μm are provided on the alignment film at a density of 10 / mm ^2. Sprayed as. Thereafter, similarly, a quartz glass substrate with a polyimide alignment film (not shown) was prepared, and a water repellent treatment was performed on the polyimide alignment film of the quartz glass substrate with a polyimide alignment film (not shown).

  Then, the transparent substrate 44a with a polyimide alignment film and a quartz glass substrate with a polyimide alignment film (not shown) are opposed to each other so that the alignment directions of the alignment films coincide with each other, and fixed with the above-described epoxy sealant, and two quartz A liquid crystal cell having a gap between glass substrates of 5.0 μm was formed. A nematic liquid crystal monomer having birefringence was injected from an injection port (not shown) into a 5.0 μm space between the two quartz glass substrates opposed to each other in this manner.

  As the liquid crystal monomer, the refractive index anisotropy Δn in the polymer (polymer liquid crystal state) after polymerization / curing is 0.040 for light having a wavelength of 405 nm, 0.035 for light having a wavelength of 660 nm, and 785 nm for wavelength. What used 0.034 with respect to light was used.

  Thereafter, the entire liquid crystal monomer material is irradiated with UV light having a wavelength of 365 nm to polymerize and solidify the entire liquid crystal monomer composition, and then subjected to a heat treatment at 140 ° C. for 30 minutes to form a horizontally oriented polymer having a thickness of 5.0 μm. A liquid crystal layer was formed. Then, the quartz glass substrate with the polyimide alignment film subjected to the water repellent treatment is removed, and the first wavelength plate 41 made of a polymer liquid crystal having a thickness of 5.0 μm is horizontally aligned on the transparent substrate 44a with the polyimide alignment film. Formed.

  By the same process as described above, the second wave plate 42 made of polymer liquid crystal having a thickness of 7.6 μm was formed on the alignment film (not shown) on the transparent substrate 44b with the polyimide alignment film. As the liquid crystal monomer of the second wave plate 42, the refractive index anisotropy in the polymer after polymerization / curing is 0.109 for light with a wavelength of 405 nm, 0.095 for light with a wavelength of 660 nm, and light with a wavelength of 785 nm. In contrast, 0.092 was used.

Further, by the same process, a third wave plate 43 made of a polymer liquid crystal having a thickness of 2.5 μm was formed on an alignment film (not shown) on a transparent substrate 44c with a polyimide alignment film. The polymer liquid crystal material constituting the third wave plate 43 is the same as the polymer liquid crystal material of the first wave plate 41 and is different from the polymer liquid crystal material constituting the second wave plate 42. It was supposed to be. At this time, the retardation value Rd ₁ (λ ₁ ) of the first wave plate 41 with respect to light having the wavelength λ ₁ = 405 nm is about 203 nm, the retardation value Rd ₂ (λ ₁ ) of the second wave plate 42 is about 829 nm, The retardation value Rd ₃ (λ ₁ ) of the _third wavelength plate 43 was about 102 nm.

  Next, as shown in FIG. 3B, when viewed from the light transmission side, the angle formed by the fast axis 46 of the first wave plate 41 and the fast axis 47 of the second wave plate 42 is 18 degrees. [°] was opposed to each other and bonded with a transparent adhesive (not shown) that is optically isotropic. Next, the transparent substrate 44b with the polyimide alignment film and the third wave plate 43 so that the fast axis 47 of the second wave plate 42 and the fast axis 48 of the third wave plate 43 are 45 [°]. Were bonded together with an optically isotropic transparent adhesive (not shown) to produce a broadband wavelength plate 40.

Linearly polarized light was incident in the direction of travel perpendicular to the surface of the transparent substrate 44a with the polyimide alignment film (antireflection film forming surface) of the produced broadband wavelength plate 40. At this time, the angle (= θ ₁ ) between the direction 45 of the incident linearly polarized light and the fast axis 46 of the first wave plate was set to 18 [°]. Here, linearly polarized light of 405 nm wavelength band, 660 nm wavelength band, and 785 nm wavelength band was incident on the broadband wavelength plate 40, and the optical characteristics with respect to light of each wavelength were examined. FIG. 4 is a graph showing optical characteristics obtained for linearly polarized light in each wavelength band. As a result, the polarization state of the light transmitted through the broadband wavelength plate 40 has an ellipticity of 0.9 or more with respect to the light in the 405 nm wavelength band, as shown in FIG. 4A, and is shown in FIG. Thus, it was found that the ellipticity of the light in the 660 nm wavelength band is 0.8 or more.

  On the other hand, as shown in FIG. 4 (c), it was found that the ellipticity is 0.35 or less for light in the 785 nm wavelength band. Then, when the direction of the incident linearly polarized light is used as a reference, the azimuth angle of the transmitted 785 nm wavelength light is stable at 6 [°] or less as seen from the transmitting side as shown in FIG. Value. The polarization state with an ellipticity of 0.35 or less and an azimuth angle of + 6 ° or less is elliptically polarized light and not completely linearly polarized light, but the intensity of the component parallel to the direction of the incident linearly polarized light (polarizer intensity) is It can be considered that the polarization state is hardly changed with respect to incident polarized light.

  Thus, the broadband wave plate of the present invention functions as a quarter wave plate for light in the 405 nm wavelength band and 660 nm wavelength band, and changes the polarization state substantially for light in the wavelength 785 nm wavelength band. It functions as a wave plate that does not exist (maintains a linear polarization state). By disposing the broadband wave plate in an optical path common to light of these three different wavelengths, it is possible to realize an optical head device that can be downsized and has a high quality function.

(Comparative example)
As a comparative example, Patent Document 2 using two wave plates having a function of a 9λ / 4 plate for the 405 nm wavelength band, a 5λ / 4 plate for the 660 nm wavelength band, and a 1 wave plate for the 785 nm wavelength band. The characteristics of the broadband wave plate described in 1 are shown. Specifically, when the structure of the two wave plates is the first wave plate and the second wave plate from the light incident side, the thickness of the first wave plate is 75.0 [μm], θ _The angle corresponding to ₁ is 12 [°], the thickness of the second wave plate is 66.0 [μm], and the angle corresponding to θ ₂ is 65 [°]. Further, as the liquid crystal monomer, the refractive index anisotropy Δn in the polymer after polymerization / curing (polymer liquid crystal state) is 0.0140 for light having a wavelength of 405 nm, 0.0118 for light having a wavelength of 660 nm, and 785 nm for wavelength. The calculation was performed on the assumption that 0.0114 was used for light.

  At this time, in the conditions of the comparative example, the ellipticity when linearly polarized light in the 405 nm wavelength band, the 660 nm wavelength band, and the 785 nm wavelength band is incident and the azimuth angle results in the 785 nm wavelength band are shown in FIG. 5A is a graph showing the ellipticity in the 405 nm wavelength band, FIG. 5B is a graph showing the ellipticity in the 660 nm wavelength band, and FIG. 5C is a graph showing the ellipticity in the 785 nm wavelength band. FIG. 5D is a graph showing the azimuth angle of the 785 nm wavelength band.

  From this result, in the comparative example, characteristics with an ellipticity of 0.65 or more cannot be obtained particularly in the 405 nm wavelength band. For this reason, when this broadband wave plate is arranged in an optical head device, the influence of the birefringence of the disc can be reduced in the recording / reproducing of the CD, but at the time of recording / reproducing of the high-density optical recording medium such as BD. Since the polarization state in the 405 nm wavelength band does not become good circularly polarized light, the light utilization efficiency of the return path reflected by BD or the like is lowered, and stable recording / reproduction cannot be performed. Even if the parameters of the broadband wave plate composed of two wave plates as in the comparative example are changed, the ellipticity with respect to light in the 405 nm wavelength band and the 660 nm wavelength band is 0.65 or more and the light in the 785 nm wavelength band. The condition that the ellipticity with respect to the angle is 0.4 or less and the azimuth angle is in the range of −10 ° to + 10 ° was not obtained.

As described above, the wavelength λ ₁ having different wavelength bands is obtained by adjusting the retardation value of each wave plate and the crossing angle of each optical axis as a wide band wave plate configured by arranging three wave plates. , lambda to ₂ and lambda ₃ linearly polarized light incident in _{(λ 1 <λ 2 <λ} 3), λ 1, light lambda ₂ can be a polarization state close to high circular polarization ellipticity, lambda ₃ Can be emitted in a polarization state close to linearly polarized light having a low ellipticity. Furthermore, by arranging this broadband wavelength plate in an optical head device for recording / reproducing optical disks of different standards, an optical head device with improved recording / reproducing quality can be realized, which is useful.

The cross-sectional schematic diagram of the broadband wavelength plate which concerns on this invention, and the plane schematic diagram which show relationships, such as the angle of each optical axis of each wavelength plate. The schematic diagram which shows an example of the optical head apparatus using the broadband wavelength plate which concerns on this invention. The cross-sectional schematic diagram of the broadband wavelength plate in the Example of this invention, and the plane schematic diagram which show relationships, such as the angle of each optical axis of each wavelength plate. The optical characteristic result of the ellipticity and azimuth in each wavelength band of the broadband wavelength plate in the Example of this invention. Calculation results of ellipticity and azimuth optical characteristics in each wavelength band of the comparative example. FIG. 6 is a schematic diagram illustrating a configuration of a conventional optical head device that handles light of three wavelengths. Schematic diagram showing the configuration of an optical head device that handles light of three wavelengths (Patent Document 1). Schematic diagram showing the configuration of an optical head device that handles light of three wavelengths (Patent Document 2).

Explanation of symbols

10, 40 Broadband wave plate (present invention)
11, 41 First wave plate 12, 42 Second wave plate 13, 43 Third wave plate 14a, 14b, 44a, 44b, 44c Transparent substrate 15 (with polyimide alignment film) 45, Incident linearly polarized light Direction of oscillation of electric field 21, 46 Optical axis 22, 47 of first wave plate Optical axis 23, 48 of second wave plate Optical axis 20, 100, 200, 300 of third wave plate Optical head device 21, 101, 201, 301 Light source (405 nm wavelength band)
22, 102, 202, 302 Light source (660 nm wavelength band)
23, 103, 203, 303 Light source (785 nm wavelength band)
24, 25, 26, 104, 105, 106, 204, 205, 206 Diffraction element 27, 29, 31, 108, 207, 208, 212, 308, 309 Dichroic prism 38, 107, 109, 209 (polarization) beam splitter 30, 110, 111, 210, 310 Collimator lens 31, 114, 115, 213 Mirror 33, 34, 116, 117, 214, 215, 313 Objective lens 35, 118, 216, 315 Optical disk 36, 38, 119, 120, 217, 304, 305, 306 Photodetector 37 Transmission / reflection optical element 112, 113, 211 1/4 wavelength plate 307 Non-polarization diffraction grating 311 Polarization diffraction grating 312 (broadband) wavelength plate

Claims (5)

In a broadband wave plate that changes the polarization state of linearly polarized light incident at _three wavelengths λ ₁ , λ ₂ , λ ₃ (λ ₁ <λ ₂ <λ ₃ ) having predetermined different bands,
The broadband wave plate includes a first wave plate, a second wave plate, and a third wave plate in order from the side on which the linearly polarized light is incident.
The fast axis of the first wave plate, the fast axis of the second wave plate, and the third wave plate based on the direction of oscillation of the electric field of the linearly polarized light incident on the first wave plate Or a combination of angles with the slow axis of the first wave plate, a slow axis of the second wave plate, and a slow axis of the third wave plate, respectively. When θ ₁ [°], θ ₂ [°], and θ ₃ [°],
The incident linearly polarized light the ellipticity kappa ₂ broadband wave plate has passed through the wavelength lambda ₁ light ellipticity kappa ₁ and the wavelength lambda ₂ of light is 0.65 or more, said transmitted through the broadband wave plate retardation value Rd 1 of the first wavelength plate so that light ellipticity kappa _third wavelength lambda ₃ becomes 0.4 or _less, the second retardation value Rd 2 wave _plate, the third wave plate retardation value Rd ₃ and the theta _1, the theta _2, the wideband-wave plate in which the theta ₃ is set. The azimuth angle, which is the angle between the direction of oscillation of the linearly polarized electric field incident at the wavelength λ ₃ and the direction in which the amplitude of the electric field transmitted through the wavelength λ ₃ is the largest, is −10 ° to + 10 °. The broadband wave plate according to claim 1, which falls within a range. The wavelength λ ₁ is a 405 nm wavelength band that is a wavelength range of 390 to 420 nm, the wavelength λ ₂ is a 660 nm wavelength band that is a wavelength range of 640 to 680 nm, and the wavelength λ ₃ is a 785 nm wavelength band that is a wavelength range of 765 to 805 nm. And
With respect to the wavelength lambda _1, the retardation value of the first wave plate, the second retardation value of the wave plate, the retardation value of the third wave plates, _{respectively, Rd 1 (λ 1) [} nm], Rd 2 (Λ ₁ ) [nm], Rd ₃ (λ ₁ ) [nm],
156 ≦ Rd ₁ (λ ₁ ) ≦ 248
And 802 ≦ Rd ₂ (λ ₁ ) ≦ 872
And 172 ≦ Rd ₃ (λ ₁ ) ≦ 231
Meet, and
When viewed from the side where the incident linearly polarized light is transmitted through the broadband wave plate, the range of 0 ° to 180 ° in the counterclockwise direction with respect to the direction of oscillation of the electric field of the linearly polarized light is plus (+), and the clockwise direction is 0. When the range of ° to −180 ° is minus (−) and the value of θ ₁ having the smallest absolute value is larger than zero, θ ₂ is
2 × θ ₁ −14.2 ≦ θ ₂ ≦ 2 × θ ₁ +12.1
While satisfying
The θ ₃ is
(2 × θ ₁ +45) -14.5 ≦ θ ₃ ≦ (2 × θ ₁ +45) +12.1
The broadband wave plate according to claim 1 or 2, which satisfies the following conditions. The wavelength λ ₁ is a 405 nm wavelength band that is a wavelength range of 390 to 420 nm, the wavelength λ ₂ is a 660 nm wavelength band that is a wavelength range of 640 to 680 nm, and the wavelength λ ₃ is a 785 nm wavelength band that is a wavelength range of 765 to 805 nm. And
With respect to the wavelength lambda _1, the retardation value of the first wave plate, the second retardation value of the wave plate, the retardation value of the third wave plates, _{respectively, Rd 1 (λ 1) [} nm], Rd 2 (Λ ₁ ) [nm], Rd ₃ (λ ₁ ) [nm],
156 ≦ Rd ₁ (λ ₁ ) ≦ 248
And 802 ≦ Rd ₂ (λ ₁ ) ≦ 872
And 172 ≦ Rd ₃ (λ ₁ ) ≦ 231
Meet, and
When viewed from the side where the incident linearly polarized light is transmitted through the broadband wave plate, the range of 0 ° to 180 ° in the counterclockwise direction with respect to the direction of oscillation of the electric field of the linearly polarized light is plus (+), and the clockwise direction is 0. When the range of ° to −180 ° is minus (−) and the value of θ ₁ having the smallest absolute value is smaller than zero, θ ₂ is
2 × θ ₁ −12.1 ≦ θ ₂ ≦ 2 × θ ₁ +14.2
While satisfying
The θ ₃ is
(2 × θ ₁ −45) −12.1 ≦ θ ₃ ≦ (2 × θ ₁ −45) +14.5
The broadband wave plate according to claim 1 or 2, which satisfies the following conditions. At least one light source that emits light of three different wavelengths;
An objective lens for condensing the light emitted from the light source on an optical recording medium;
An optical head device comprising a photodetector for detecting light reflected from the optical recording medium,
The broadband wave plate according to any one of claims 1 to 4, wherein the broadband wave plate is disposed in an optical path between the light source and the objective lens, or in an optical path between the objective lens and the photodetector. Optical head device.

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