JP5083014B2 - Broadband wave plate and optical head device - Google Patents

Description

  The present invention is a broadband wave plate for controlling the polarization state of incident light. In particular, as an optical system for handling optical storage, an optical recording medium such as a CD, a DVD, a magneto-optical disk, and “Blu-ray”. The present invention relates to an optical head device that records and reproduces information on a high-density optical recording medium such as (registered trademark: BD) and HD-DVD.

  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 and HD-DVDs (hereinafter referred to as “optical disks”) Development of an optical head device for recording and reproduction is in progress. The shorter the wavelength of the laser light used, the more the recording density can be improved. On the other hand, it is necessary to be able to reproduce with a long wavelength (red region, near infrared region) laser beam for many optical disks that have been widely used so far, and the red region (660 nm wavelength band as in the conventional DVD). ), Various systems having compatibility between near infrared (785 nm wavelength band) laser light such as CD and short wavelength (405 nm wavelength band) laser light have been proposed.

  Thus, in order to ensure compatibility with not only high-density recording media but also conventional optical discs, there is a method in which a light source in the red range and near infrared range is installed in addition to a short wavelength light source. . For this reason, there is a growing need for optical components having certain characteristics for three different broadband wavelengths. Therefore, an optical head device capable of reproducing or recording (hereinafter referred to as “recording / reproducing”) BD / HD-DVD, DVD, CD, etc. is required to be miniaturized so that it can be configured with few optical components.

  FIG. 22 shows an example of the configuration of a conventional optical head device 300 using three different wavelength laser beams. Light in the 405 nm wavelength band emitted from the light source 301 such as a semiconductor laser is reflected by the polarization beam splitter 307, becomes parallel light by the collimator lens 310, and becomes circularly polarized light by the λ / 4 plate 312. The circularly polarized light is reflected by the mirror 314 and is collected by the objective lens 316 on the information recording surface of the optical disk 318 such as BD or HD-DVD. 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. The light on the return path is circularly polarized in the opposite direction to the circularly polarized light on the forward path, and the light on the return path exiting the λ / 4 plate 312 is linearly polarized light orthogonal to the linearly polarized light on the forward path, and is transmitted straight through the polarization beam splitter 307. As a result, the light detector 319 is reached. The light detector 319 reads information recorded on the optical disk 318 by detecting pit information from the light reflected by the optical disk.

  Similarly, light in the 660 nm wavelength band emitted from the light source 302 such as a semiconductor laser and light in the 785 nm wavelength band emitted from the light source 303 are reflected or transmitted through the dichroic prism 308 and transmitted through the polarization beam splitter 309. Then, the collimator lens 311 becomes parallel light, the λ / 4 plate 313 becomes circularly polarized light, is reflected by the mirror 315, and is focused on the optical disk 318 by the objective lens 317. The light on the return path becomes linearly polarized light orthogonal to the linearly polarized light on the forward path, is reflected by the polarization beam splitter 309, and reaches the photodetector 320. The diffractive elements 304, 305, and 306 are optical elements that generate a main beam and two sub-beams that serve as tracking servo signals, for example, and are optical paths between the respective forward light sources and the polarizing beam splitter or dichroic prism. It can also be placed inside.

  As described above, the optical head device 300 shown in FIG. 22 has two sets of optical elements such as a polarization beam splitter and a photodetector, and has a large number of parts, and it takes time to reduce the size of the device and to adjust the assembly. There is a problem. On the other hand, for example, FIG. 23 reports an optical head device 400 as another configuration example using laser beams of three different wavelengths (Patent Document 1).

  This employs a collimator lens 110 and a λ / 4 plate 411 that are commonly used for light of three different wavelengths in the optical head device 300 of FIG. A dichroic prism 107 and a trichroic prism 108 are arranged in the optical path of light emitted from the light source 101 in the 405 nm wavelength band, the light source 102 in the 660 nm wavelength band, and the light source 103 in the 785 nm wavelength band to transmit or reflect light of each wavelength. Further, light of three wavelengths passes through the collimating lens 110 and the λ / 4 plate 411. The trichromatic prism 112 reflects only the light in the 405 nm wavelength band, transmits the light in the 660 nm wavelength band and the 785 nm wavelength band, and collects the light on the information recording surface of the optical disk 116 such as a BD or HD-DVD by the objective lens 114. .

  On the other hand, light in the 660 nm wavelength band and the 785 nm wavelength band passes straight through the trichroic prism 112, is reflected by the mirror 113, and is condensed by the objective lens 115 on the information recording surface of the optical disk 116 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 λ / 4 plate 411, and the polarization beam splitter 109. And reaches the photodetector 117. Patent Document 1 reports an optical head device characterized by a broadband wavelength plate that functions as a λ / 4 plate in each of a 405 nm wavelength band, a 660 nm wavelength band, and a 785 nm wavelength band.

JP 2007-086105 A

  However, as a characteristic of the trichroic prism 112, if the transmittance of light in the 660 nm wavelength band and the 785 nm wavelength band is to be increased regardless of the polarization state of the incident light, when the light in the 405 nm wavelength band is incident, S A phase difference occurs between the polarized light and the P-polarized light. That is, even if it becomes circularly polarized light by the broadband wavelength plate 411 and enters the trichromatic prism 112 as circularly polarized light, light in the 405 nm wavelength band becomes elliptically polarized light due to this phase difference. Therefore, the return light reflected by the optical disk 116 is similarly elliptically polarized, and the return light transmitted through the broadband wavelength plate 411 is not linearly polarized but elliptically polarized. For this reason, for example, even if light on the return path in the 405 nm wavelength band enters the polarization beam splitter 109, the amount of reflected light is reduced, and only a part of the amount of light reaches the photodetector 117. There has been a problem of deteriorating the recording / reproducing characteristics of the optical head device.

  If the polarization dependence of the 405 nm wavelength band is eliminated as a characteristic of the trichromatic prism 112, the light as shown in FIG. 23 is caused by the trade-off problem that the transmission polarization dependence of the 660 nm wavelength band and the 785 nm wavelength band appears. In the head device 400, it is difficult to obtain high-quality recording / reproduction of each optical disc. In order to solve the above-described problem in an optical head device using two or more laser beams having different wavelengths including light having a wavelength of 405 nm as a light source, the object of the present invention is to linearly polarize light having a wavelength of 405 nm. Without changing the state, in the wavelength band on the long wavelength side, a broadband wavelength plate having a function of changing the polarization state of light from linearly polarized light to circularly polarized light is obtained. In particular, two wavelength bands of 660 nm wavelength band and 785 nm wavelength band are obtained. Thus, a broadband wave plate having a function of changing the polarization state of light from linearly polarized light to circularly polarized light is obtained.

The present invention has been made to solve the above-described problem, and includes a first wave plate and a second wave plate arranged in parallel, and m (m is an integer of 2 or more) wavelengths. A broadband wave plate on which light is incident, the optical axis of the first wave plate intersects with the optical axis of the second wave plate, and the incident wavelength is expressed as λ _j <λ _{j + 1} (j = 1 The retardation value of the i-th wave plate with respect to light at the wavelength λ _n (n = integer of 1 to m) is expressed as R _i (λ _n ) [degree] (i Is 1 or 2), R ₁ (λ _{j + 1} ) / R ₂ (λ _{j + 1} ) is in the range of 2 ± 0.1 for at least one set of wavelength λ ₁ and wavelength λ _{j + 1} , and R ₁ (λ ₁ ) / R ₁ (λ _{j + 1} ) is in the range of 2 ± 0.1, k is an odd number, and R ₁ (λ ₁ ) is ( 360 × k) ± 5 [degrees], and the polarization direction in which linearly polarized light having the wavelength λ ₁ and the wavelength λ _{j + 1} is incident in the same polarization direction, and the optical axis of the second wave plate What range der of but the angle theta ₂ is 90 ± 5 [degrees], the first wavelength plate and the polarization direction of the wavelength lambda ₁ and the wavelength lambda _{j + 1} of the linearly polarized light is incident in the same direction of polarization The broadband wave plate in which the angle θ ₁ formed by the optical axis of the light is within the range of 22.5 − {(90−θ ₂ ) / 2} ± 2.5 [degrees] is provided.

With this configuration, for example, the polarization state of light incident at λ ₁ can be emitted as linearly polarized light, and the polarization state of light incident at λ ₂ can be changed from linearly polarized light to circularly polarized light. In addition, the ellipticity with respect to λ ₁ is in the range of 0 ± 0.1, and the absolute value of the ellipticity with respect to λ ₂ can be 0.7 or more.

Further, R ₁ (λ _{j + 1} ) / R ₂ (λ _{j + 1} ) is in the range of 2 ± 0.05, and the angle θ ₁ is 22.5 − {(90−θ ₂ ) / 2} ± 1. The broadband wavelength plate according to the above, on which linearly polarized light having at least one wavelength λ _{j + 1} within a range of 5 degrees is incident. Further, the wavelength λ ₁ , the wavelength λ ₂ and the wavelength λ ₃ (λ ₁ <λ ₂ <λ ₃ ) is a broadband wavelength plate on which linearly polarized light is incident, and the λ ₁ is a 405 nm wavelength band, The broadband wavelength plate according to the above, wherein λ ₂ is a 660 nm wavelength band and λ ₃ is a 785 nm wavelength band.

With this configuration, the ellipticity with respect to λ ₁ is in the range of 0 ± 0.1, and the absolute value of the ellipticity with respect to λ ₂ can be 0.85 or more. In addition, light having three different wavelengths is incident as linearly polarized light, light having a wavelength of λ ₁ does not change the polarization state, and light having wavelengths of λ ₂ and λ ₃ has a high ellipticity (circular) polarized light. The light can be emitted in a state. Further, the three different wavelengths can be set to the 405 nm wavelength band, the 660 nm wavelength band, and the 785 nm wavelength band, respectively, and specifically configured according to the design conditions of the two wave plates. In the present invention, the 405 nm wavelength band is defined as a wavelength band in the range of 405 ± 15 nm, the 660 nm wavelength band is defined as a wavelength band in the range of 660 ± 20 nm, and the 785 nm wavelength band is defined as a wavelength band in the range of 785 ± 20 nm.

A light source that emits light of at least the wavelengths λ ₁ and λ _2; a first separation unit that deflects and separates the light emitted from the light source; and the light emitted from the first separation unit. a second separation unit that deflects and separates the light of λ _{1 and} the light of wavelength λ ₂ into different optical paths; an objective lens that condenses the light emitted from the second separation unit on the optical disc; and An optical head device for detecting reflected light, wherein a λ / 4 plate is disposed in the optical path of the light having the wavelength λ ₁ between the second separating means and the objective lens. In addition, an optical head device is provided in which the broadband wave plate described above is disposed in the optical path between the first separation means and the second separation means. Further, the light source emits the wavelength λ ₁ , the wavelength λ _2, and the wavelength λ _3, and the second separating unit includes an emission direction of light having the wavelength λ ₁ , and the wavelength λ ₂ The optical head device according to the above, which is a trichroic prism that makes the light and the emission direction of the light having the wavelength λ ₃ different.

  With this configuration, in an optical head device that records and plays back optical discs with different standards, light of each wavelength can be condensed onto optical discs of each standard with circularly polarized light with a high ellipticity, resulting in high light utilization efficiency and improved quality. An optical head device can be realized. Further, the optical head device can be reduced in size and weight by using a broadband wave plate in an optical system common to two different wavelengths.

Further, the first separating means is a hologram element comprising filled planarized filler diffraction grating is isotropic materials having birefringence, the refractive index n _s of the filler, the to provide an optical head device according to substantially equal the to the ordinary refractive index n _o or the extraordinary refractive index n _e of the diffraction grating.

  With this configuration, the position of the photodetector can be freely set, the broadband wavelength plate and the hologram element can be integrated, and further miniaturized by integrating with other optical elements. An optical head device can be realized.

  The present invention provides a broadband wavelength plate capable of changing the polarization state of each light by the incidence of light having a plurality of different wavelengths, and a component of an optical element in an optical head device using light of three different wavelengths. Miniaturization can be realized by reducing the number of points.

(First embodiment)
FIG. 1 shows a configuration of a broadband wavelength plate 10 according to the present embodiment. FIG. 1A is a schematic cross-sectional view of the broadband wavelength plate 10, and FIG. 1B is a schematic plan view illustrating a laminated state of two wavelength plates included in the broadband wavelength plate. The broadband wave plate 10 is formed by integrating two wave plates 13a and 13b via transparent substrates 11a and 11b and an adhesive 11c. As will be described later, when the wave plates 13a and 13b are made of polymer liquid crystal, alignment films 12a and 12b may be provided, respectively.

As long as the transparent substrates 11a and 11b and the adhesive 11c are transparent to incident light, various materials such as a resin plate and a resin film can be used. However, optically isotropic materials such as glass and quartz glass can be used. When used, it is preferable because it does not affect the birefringence of transmitted light. The wave plates 13a and 13b are made of an optically birefringent material, and the orientation direction of the liquid crystal is controlled by using a polymer liquid crystal obtained by polymerizing the liquid crystal so that the optical axis direction of the wave plates 13a and 13b is changed. Easy to set and preferred. The material of the wave plates 13a and 13b is not limited to the polymer liquid crystal, but is processed by a single crystal material having optical birefringence such as quartz or LiNbO ₃ (lithium niobate), or birefringent by stretching. Injection molded products of inorganic materials such as TiO ₂ and resins, which are polycarbonate films and organic polymer films that have been provided with a birefringence by a film formation method such as anisotropic vapor deposition on a substrate In such a case, the alignment films 12a and 12b may not be provided.

  For bonding the two wave plates 13a and 13b, an adhesive film, a UV curable adhesive, or a thermosetting adhesive can be used. In order to reduce wavefront aberration and improve temperature characteristics and reliability of the broadband wavelength plate, it is desirable to bond as a thin adhesive layer as much as possible, and it is particularly desirable that the thickness of the adhesive layer be 10 μm or less. Further, it is desirable that the broadband wavelength plate is subjected to a surface smoothing process or an adhesive holding by a substrate in order to avoid deterioration of wavefront aberration of transmitted light. Specifically, it is desirable to use a broadband wavelength plate adhered to at least one transparent substrate. In the case where a broadband wave plate is used alone without being laminated and integrated with other optical elements, a structure sandwiched between two transparent substrates 11a and 11b is particularly desirable from the viewpoint of reducing wavefront aberration and ensuring strength.

Next, conditions for configuring the wave plates 13a and 13b will be described. The light incident on the broadband wave plate 10 is incident from the vertical direction on the transparent substrate 11a side from the transparent substrate 11a side, and the first wave plate 13a and the second wave plate are respectively entered in the order in which the light is incident. 13b. Further, it is assumed that light having at least wavelengths λ ₁ and λ ₂ is incident on the broadband wave plate 10 (λ ₁ <λ ₂ ). Next, when the first wave plate 13a and the second wave plate 13b are overlapped, the relationship between the respective optical axes and the direction of incident linearly polarized light will be described. Here, as the direction 15 of the linearly polarized light incident in FIG. 1 (b), the which the angle between the optical axis 16 of the first wave plate 13a the counterclockwise direction plus the reference and theta _1, second An angle formed with the optical axis 17 of the wavelength plate 13b is θ ₂ . It is assumed that the linear polarization direction of the light with wavelength λ ₁ and the linear polarization direction of the light with wavelength λ ₂ are incident in the same direction. In addition, the optical axis includes a fast axis and a slow axis, but in this embodiment, the combination of the fast axes of the first wave plate 13a and the second wave plate 13b or the slow axes. Effective in combination. Therefore, hereinafter, the angle formed by using the optical axis means that both of the two wave plates are fast axes or both are slow axes.

The first wave plate 13a and the second wave plate 13b each have a retardation value. Here, the retardation value is represented by a phase difference value (unit: [degree]) between the fast axis and the slow axis with respect to incident light. Further, the material constituting these, ordinary refractive index n _o and is the difference between the extraordinary refractive index n _e refractive index anisotropy _{Δn (= | n e -n o} |) varies depending on the wavelength of light that is incident There are characteristics (hereinafter referred to as wavelength dispersion characteristics), and the retardation value varies depending on the wavelength of incident light even if the same material is used. Here, the retardation values of the first wave plate 13a with respect to the light of the wavelength λ ₁ and the wavelength λ ₂ are R ₁ (λ ₁ ) and R ₁ (λ ₂ ), respectively. Also, let R ₂ (λ ₁ ) and R ₂ (λ ₂ ) be the retardation values of the second wave plate 13b for the light of the wavelengths λ ₁ and λ ₂ , respectively. Broadband wave plate 10 of the present embodiment, at least lambda with _one of linear polarized light of a wavelength to be emitted without changing the polarization state is incident, linearly polarized light having a wavelength of lambda ₂ is incident in a circularly polarized light The present invention provides a configuration having the characteristic of emitting light.

As described above, the retardation values of the first wave plate 13a and the second wave plate 13b and θ ₁ and θ ₂ that are the lamination conditions are used to satisfy all the following conditions (A) to (D). A broadband wave plate may be constructed.
(A) R ₁ (λ ₂ ) [degree]> R ₂ (λ ₂ ) [degree], and R ₁ (λ ₂ ) [degree] / R ₂ (λ ₂ ) [degree] is 2 ± 0.1. It is in the range.
(B) R ₁ (λ ₁ ) [degree]> R ₁ (λ ₂ ) [degree], and R ₁ (λ ₁ ) [degree] / R ₁ (λ ₂ ) [degree] is 2 ± 0.1 It is in the range.
(C) R ₁ (λ ₁ ) [degree] is in the range of (360 × k) ± 5 [degree] (k is an odd number: 1, 3, 5,...).
(D) θ ₂ is in a range of 90 ± 5 [degrees].

With this configuration, the broadband wave plate 10 functions as a λ plate that emits light without changing the polarization state with respect to the incident light having the wavelength λ ₁ . On the other hand, when light of wavelength λ ₂ (> λ ₁ ) is incident, the Poincare sphere shown in FIG. 2 takes a polarization state in the vicinity of the meridian passing through S1 to S3. That is, light that is incident as linearly polarized light along the trajectory of the arrow 26 in FIG. 2 can take a wide range of polarization states from elliptically polarized light and linearly polarized light that is orthogonal to light incident through circularly polarized light. The effect of increasing the degree of freedom in optical design when designing the apparatus is brought about.

A specific description will be given using the Poincare sphere shown in FIG. When the polarization direction of the incident light is the pole of S1, the light of wavelength λ ₂ is first polarized by the first wave plate 13a with the polarization state R ₁ (λ ₂ centering on the axis 22 forming an angle of 2θ _{1 with} the S1 axis. ) Move to the point of rotation. When R ₁ (λ ₂ ) = 180 [degrees], the point is located on the equator passing through S1 and S2. Then, the second wave plate 13b moves to a point rotated about R ₂ (λ ₂ ) around the axis 23 that forms an angle of 2θ _{2 with} the S1 axis. Particularly when R ₁ (λ ₂ ) = 180 [degrees], θ ₂ = 90 [degrees], and R ₂ (λ ₂ ) = 90 [degrees], the point is the polarization state on the meridian passing through S1 and S3 Take. That is, light that is incident as linearly polarized light along the trajectory of the arrow 26 can take a wide range of polarization states from elliptically polarized light and linearly polarized light that is orthogonal to light incident through circularly polarized light. In particular, when R ₁ (λ ₂ ) = 180 [degrees], θ ₂ = 90 [degrees], R ₂ (λ ₂ ) = 90 [degrees], and θ ₁ = 22.5 [degrees], the extreme point of S3 And the polarization state is circularly polarized.

Further, the following (E) condition is added to the above conditions (A) to (D), and the broadband wavelength plate is configured to satisfy all the conditions (A) to (E).
(E) θ ₁ is in the range of 22.5 − {(90−θ ₂ ) / 2} ± 2.5 [degrees].
With this configuration, the broadband wave plate 10 emits the linearly polarized light with the wavelength λ ₁ without changing the state, and functions as a λ / 4 plate with respect to the linearly polarized light with the wavelength λ _2. Circularly polarized light can be used.

In the above description, the case where m = 2, that is, two wavelengths are used has been described. When m = 2, the wavelengths are only λ ₁ and λ ₂ , so the above setting is used. On the other hand, when the wavelength used is m ≧ 3, the above conditions (A) and (B) are set so as to satisfy the following relationship with respect to at least one set of wavelength λ ₁ and wavelength λ _{j + 1.} It will be.
(A ′) R ₁ (λ _{j + 1} ) [degree] / R ₂ (λ _{j + 1} ) [degree] is in the range of 2 ± 0.1.
(B ′) R ₁ (λ ₁ ) [degree] / R ₂ (λ _{j + 1} ) [degree] is in the range of 2 ± 0.1.
More specifically, for example, in the case of m = 3, λ ₁ , λ _2, and λ ₃ (λ ₁ <λ ₂ <λ ₃ ), the above conditions (A) and (B) are as follows: Should be set. It may be set so as to satisfy (A1) and (B1) when j = 1, or (A2) and (B2) when j = 2.
(A1) R ₁ (λ ₂ ) [degree] / R ₂ (λ ₂ ) [degree] is in the range of 2 ± 0.1.
(B1) R ₁ (λ ₁ ) [degree] / R ₁ (λ ₂ ) [degree] is in the range of 2 ± 0.1.
(A2) R ₁ (λ ₃ ) [degree] / R ₂ (λ ₃ ) [degree] is in the range of 2 ± 0.1.
(B2) R ₁ (λ ₁ ) [degree] / R ₁ (λ ₃ ) [degree] is in the range of 2 ± 0.1.

Next, description will be made on the assumption that _three light beams having different wavelengths (λ ₁ , λ ₂ and λ ₃ : λ ₁ <λ ₂ <λ ₃ ) are incident on the broadband wave plate 10. The retardation values of the two wave plates (13a and 13b) constituting the broadband wave plate 10 in the present invention are generalized as follows.
R _i (λ _j ) = {Δn _ij · d _i / λ _j } · 360 [degrees] (1)
Here, i represents the order in which light enters (i th wave plate), and j represents the order counted from the shortest wavelength among the wavelengths of incident light. In other words, in the present invention, the value that i can take is either 1 or 2, and the value that j can take is any one of 1, 2, and 3. Δn is the refractive index anisotropy of the wave plate, and d is the thickness of the wave plate. Therefore, Δn _ij represents the refractive index anisotropy of the birefringent material used for the i-th wave plate having a broadband phase difference at the wavelength λ _j , and d _i represents the thickness of the i-th wave plate having a broadband phase difference.

Here, R ₁ (λ ₁ ) is 360 degrees, and R ₁ (λ ₁ ) / R ₁ (λ ₂ ) and R ₁ (λ ₁ ) / R ₁ (λ ₃ ) are 2 ± 0.1. In this range, R ₁ (λ ₂ ) and R ₁ (λ ₃ ) are about 180 [degrees]. Further, assuming that R ₁ (λ ₂ ) / R ₂ (λ ₂ ) and R ₁ (λ ₃ ) / R ₂ (λ ₃ ) are in the range of 2 ± 0.1, R ₂ (λ ₂ ) and R ₂ (λ ₃ ) is about 90 degrees. At this time, as described above, the linearly polarized light having the wavelengths λ ₂ and λ ₃ transmitted through the broadband wave plate 10 is substantially circularly polarized.

Here, using the Mueller matrix representing the polarization state of light, the polarization state until linearly polarized light is incident on the broadband wave plate and transmitted is described. If the Stokes vector of the light incident on the broadband wave plate is S _i and the Stokes vector of the light emitted from the broadband wave plate is S _o , the Stokes vector S _o is expressed using Equation (2).
_{S o = G 2 (θ 2} , Γ 2) × G 1 (θ 1, Γ 1) × S i ··· (2)
The Stokes vector S _i is [1, 1, 0, 0] ^T. However, the symbol T means transposition.

Also, the conversion matrices G ₁ (θ ₁ , Γ ₁ ) and G ₂ (θ ₂ , Γ ₂ ) represent changes in the polarization state generated in the first wave plate 13a and the second wave plate 13b, respectively. , Θ ₁ and θ ₂ are angles formed by the incident polarization direction S _i and the optical axis of the birefringent layer, and Γ ₁ and Γ ₂ are retardation values of the wave plate 13a and the wave plate 13b, respectively. It is expressed using equation (3).
Γ = Δn × d × 2π / λ [rad] (3)
Here, Δn represents the refractive index anisotropy of the wave plate, d represents the thickness of the wave plate, and λ represents the wavelength.

  The transformation matrix G (θ, Γ) is further divided into three transformation matrices as shown below.

  G (θ, Γ) = T (θ) C (Γ) T (−θ) (4)

  Here, T (θ) in Equation (5) is an optical rotator matrix, and the optical rotator matrix T (θ) rotates the Stokes vector by 2θ around the S3 axis of the Poincare sphere as shown in Equation (5). This is the transformation matrix Further, C (Γ) is a phaser matrix, and the phaser matrix C (Γ) is a transformation matrix for rotating the Stokes vector by Γ around the S1 axis of the Poincare sphere, as shown in Expression (6).

Here, Δn, which is the refractive index anisotropy of a birefringent material, generally has a wavelength dependency, and when A, B, and C are dispersion coefficients depending on the material, it is approximately expressed by the following Cauchy equation: The
Δn (λ) = A + B / λ ² + C / λ ⁴ (7)
Here, λ is the wavelength, and Tables 1 and 2 show the values of Δn with respect to the wavelength of incident light when the birefringent material is quartz or polymer liquid crystal, respectively.

From the characteristics at each wavelength in Table 1, for quartz, values of A = 0.00849, B = 0.002555, and C = −1.46E−05 are obtained. On the other hand, in the polymer liquid crystal, values of A = 0.0360, B = 0.000751, and C = 0.000140 are obtained from the characteristics at each wavelength in Table 2, respectively. Further, assuming a birefringent material that does not have wavelength dispersion, that is, does not depend on wavelength, the respective dispersion coefficients are obtained as values of A = constant, B = 0, and C = 0. FIG. 3 shows the results of the ellipticity and azimuth calculated using the dispersion coefficients of the above birefringent materials. 3 (a) and 3 (b) show the characteristics when each wave plate is made of each birefringent material, and FIGS. 3 (c) and 3 (d) show two wave plates stacked. The characteristics when each birefringent material is used. These are the characteristics when standardized as a λ plate having a retardation value of 360 degrees when light having a wavelength of 405 nm is incident. In the case of a single wave plate, the optical axis is at an angle of 45 ° with respect to the polarization direction of incident light. In the case of two wave plates, the optical axis is relative to the polarization direction of incident light. The optical axis of the wave plate that is incident first forms an angle of θ ₁ = 22.5 °, and the optical axis of the wave plate that is incident next forms an angle of θ ₂ = 90 °. The azimuth angle (of the polarization direction) is an angle formed by the polarization direction of incident light and the polarization direction of outgoing light.

In this manner, the two-plate configuration functions as a λ plate in the 405 nm wavelength band and both the 660 nm wavelength band for DVD and the 785 nm wavelength band for CD compared to the configuration of one wavelength plate. As a result, the ellipticity becomes a high value, and the intended effect of the present invention is easily obtained. Each wavelength band has a wavelength range as described above. For example, at a wavelength that changes due to variations in individual elements of the semiconductor laser itself serving as a light source or temperature changes, the two-element configuration can obtain stable characteristics without greatly reducing the ellipticity. 3 (a) and 3 (c) show the ellipticity with respect to the wavelength of the incident light. In order to have a high ellipticity in both the 660 nm wavelength band and the 785 nm wavelength band, it is between 660 nm and 785 nm. The ratio of the retardation values is λ _c
R ₁ (λ ₁ ) / R ₁ (λ _c ) = 2 (8)
R ₁ (λ _c ) / R ₂ (λ _c ) = 2 (9)
It is preferable to satisfy. Particularly crystal and polymer liquid crystal is preferable as this condition is satisfied birefringent material, in yet medium wavelength 723nm with values 660nm and 785nm of the lambda _c, more preferably it satisfies the above conditions. The above conditions (A) to (E) are conditions for light of wavelengths λ ₁ and λ ₂ , but light of λ ₁ , λ ₂ and λ ₃ (λ ₁ <λ ₂ <λ ₃ ) is incident. Then, the parameters (A) to (E) related to λ ₂ may be replaced with the parameters of λ ₃ .

Next, the ellipticity and the azimuth angle of the polarized light that enters and exits the broadband wavelength plate 10 with the light having the wavelength λ ₁ , the first wavelength plate 13 a and the second wavelength plate constituting the broadband wavelength plate 10. Each setting condition 13b will be described. First, the angle theta ₁ formed between the polarization direction of the optical axis and the incident light of the first wave plate 13a is 22.5 [degrees], the angle formed between the polarization direction of the incident light and the optical axis of the second wave plate 13b is Let θ ₂ be 90 degrees. The ratio of retardation values of the wave plates, R ₁ (λ ₁ ) / R ₂ (λ ₁ ), is 2.

FIG. 4A and FIG. 4B respectively show the ellipticity with respect to the retardation value (R ₁ (λ ₁ )) of the first wave plate 13a and the azimuth angle of the polarization direction with respect to R ₁ (λ ₁ ). Indicates. These all have continuously changing characteristics. In addition, since it functions as a λ wavelength plate, when the absolute value of the ellipticity is 0.1 or less and the azimuth angle of the polarization direction is in the range of 0 ± 10 [degrees], the light incident as linearly polarized light This is preferable because the polarization state is not greatly changed. Here, when R ₁ (λ ₁ ) is in the range of 360 ± 5 [degrees], the function as the λ wavelength plate is satisfied. In addition, when R ₁ (λ ₁ ) deviates significantly from the above range, the absolute value of the ellipticity exceeds 0.1 from the characteristics shown in FIG.

FIG. 5 is a diagram showing the characteristics of the broadband wave plate 10 based on the relationship between R ₁ (λ ₁ ) / R ₂ (λ ₁ ) and R ₁ (λ ₁ ), and FIG. FIG. 5 (b) shows the azimuth angle of the polarization direction obtained by a combination of these. Further, in FIG. 5A, the ellipticity range is shown, and a region composed of the above two combinations from which the respective ellipticities are obtained is indicated by a bold frame. From FIG. 5 (a), when R ₁ (λ ₁ ) is in the range of 360 ± 5 [degrees] and R ₁ (λ ₁ ) / R ₂ (λ ₁ ) is in the range of 2 ± 0.1, the ellipse The ratio is in the range of −0.05 to 0.05, and the function as a λ wavelength plate is satisfied. And when R ₁ (λ ₁ ) deviates significantly from the above range, the absolute value of the ellipticity exceeds 0.1. Further, in the combination of the ranges R ₁ (λ ₁ ) / R ₂ (λ ₁ ) and R ₁ (λ ₁ ) shown in FIG. It becomes a range. Although the ellipticity and the azimuth angle of the polarization direction are divided into regions, they have characteristics that change continuously.

FIG. 6 is a diagram showing the characteristics of the broadband wave plate 10 based on the relationship between θ ₁ and θ ₂ , FIG. 6 (a) shows the ellipticity obtained by the combination thereof, and FIG. 6 (b) shows these characteristics. The azimuth angle of the polarization direction obtained by the combination is shown. Further, in FIG. 6B, the range of the azimuth angle in the polarization direction is shown, and a region composed of a combination of θ ₁ and θ ₂ from which the azimuth angle in each polarization direction is obtained is indicated by a bold frame. In FIG. 6B, when θ ₁ is in the range of 22.5 ± 2.5 [degrees] and θ ₂ is in the range of 90 ± 5 [degrees], the azimuth angle of the polarization direction is 0 ± 10 [degrees]. It satisfies the function as a λ wavelength plate. Further, when θ ₂ is out of the range of 90 ± 5 [degrees], the azimuth angle of the polarization direction is also out of the range of 0 ± 10 [degrees].

FIG. 7 shows that the light of wavelength λ ₁ is incident and the upper limit values ((B): 2.1, (D): 95 [degrees]) within the ranges of the conditions (B) and (D), respectively. , Fixed at the intermediate value ((B): 2.0, (D): 90 [degrees]), lower limit value ((B): 1.9, (D): 85 [degrees]), and other above conditions The values are changed within the ranges of (A) and (C) to show the ellipticity of the emitted light and the azimuth angle of the polarization direction. FIG. 7A shows the ellipticity with respect to θ _{1. The} dotted line indicates the maximum ellipticity at the upper limit value, the alternate long and short dash line indicates the minimum ellipticity at the lower limit value, and the solid line indicates the intermediate value. This is an intermediate ellipticity characteristic. As a result, the ellipticity that fluctuates by changing the conditions (A) and (C) is within the range of 0 ± 0.1 in the range of θ _{1 in} FIG.

FIG. 7B also shows the azimuth angle of the polarization direction with respect to θ _{1 under the} same conditions as in FIG. 7A. The dotted line indicates the maximum azimuth angle of the polarization direction at the above upper limit value, which is a one-dot chain line. Is the characteristic of the azimuth angle of the polarization direction that is minimum at the lower limit value, and the solid line is the characteristic of the azimuth angle of the polarization direction that is intermediate at the intermediate value. Thus the azimuth angle of the polarization direction varies by varying the above conditions (A) and (C), in theta ₁ range of FIG. 7 (b), it becomes substantially 0 ± 10 range [degrees]. 7 (a) and 7 (b) show that it functions as a λ wavelength plate for light having a wavelength of λ ₁ by setting within the range of (A) to (D) above. Has been.

Next, the ellipticity of light that enters and exits the broadband wavelength plate 10 with light having a wavelength λ ₂ will be described. Each of the above conditions (A) to (E) is an intermediate value, that is, R ₁ (λ ₂ ) / R ₂ (λ ₂ ) is 2 as (A) and R ₁ (λ ₁ ) / R ₁ (B is as (B). (λ ₂ ) is 2, (C) is R ₁ (λ ₁ ) is 360 [degrees], (D) is θ ₂ is 90 [degrees], and (E) is θ ₁ is 22.5 [degrees]. .

FIG. 8 shows the relationship between R ₁ (λ ₂ ) / R ₂ (λ ₂ ) and the ellipticity under the conditions at this time. It is desirable to be emitted by the circularly polarized light when light of wavelength lambda ₂ enters, and most preferred if the 1 in ellipticity. Thus to function as a lambda / 4 plate with respect to light having a wavelength of lambda _2, the absolute value of ellipticity is preferably 0.7 or more, and more preferably equal to 0.85 or more. From FIG. 8, when R ₁ (λ ₂ ) / R ₂ (λ ₂ ) = 2, the ellipticity is the maximum value (= 1.0), and R ₁ (λ ₂ ) / R ₂ (λ ₂ ) is 2 ±. meets the function of lambda / 4 plate at the wavelength lambda ₂ is also within the scope of 0.1. In addition, the absolute value of the ellipticity falls below 0.7 because this condition is greatly deviated.

Further, FIG. 9 shows the relationship between R ₁ (λ ₁ ) / R ₁ (λ ₂ ) and the ellipticity, with the above conditions (A) to (E) as intermediate values. When R ₁ (λ ₁ ) / R ₁ (λ ₂ ) = 2, the ellipticity is the maximum value (= 1.0), and R ₁ (λ ₁ ) / R ₁ (λ ₂ ) is in a range of 2 ± 0.1 meets the function of lambda / 4 plate at the wavelength lambda ₂ is also at the inner. In addition, the absolute value of the ellipticity falls below 0.7 because this condition is greatly deviated.

Next, FIG. 10 the relationship between theta ₂ and ellipticity. With the above conditions (A) to (C) and (E) as intermediate values, the ellipticity = 1 when θ ₂ = 90 [degrees] under the condition (D), and θ ₂ is 90 ± 5 [degrees]. meets the function of lambda / 4 plate at the wavelength lambda ₂ is also within the scope. In addition, the absolute value of the ellipticity falls below 0.7 because this condition is greatly deviated.

Similarly, illustrating the relationship between theta ₁ and ellipticity in Figure 11. With the above conditions (A) to (D) as intermediate values, when the above condition (E), that is, θ ₂ is 90 [degrees], θ ₁ is in the range of 22.5 ± 2.5 [degrees]. The wavelength λ ₂ satisfies the function as a λ / 4 plate. In addition, the absolute value of the ellipticity falls below 0.7 because this condition is greatly deviated.

Next, FIG. 12 shows the relationship between the ellipticities obtained by combining R ₁ (λ ₁ ) / R ₁ (λ ₂ ) and R ₁ (λ ₂ ) / R ₂ (λ ₂ ). In addition, the above-described combination area that is the range of each ellipticity is surrounded by a bold line to represent the area. From this, under the condition that R ₁ (λ ₁ ) / R ₁ (λ ₂ ) is 2 ± 0.1 and R ₁ (λ ₂ ) / R ₂ (λ ₂ ) is 2 ± 0.1, 0.9 or more ellipticity is obtained, ellipticity meets function of lambda / 4 plate is 0.7 or more at a wavelength of lambda _2. Although the ellipticity is divided into regions, it has a characteristic of continuously changing.

Similarly, FIG. 13 shows the relationship of ellipticity obtained by the combination of θ ₁ and θ ₂ . In addition, the above-described combination area that is the range of each ellipticity is surrounded by a bold line to represent the area. From this characteristic, the relationship between θ ₁ and θ ₂ for the ellipticity to have the same value is
θ ₁ = 22.5 − {(90−θ ₂ ) / 2} [degrees]
Can be shown. At this time, theta ₂ is 90 ± 5 [degrees], and theta ₁ 22.5 - in the range of _{{(90-θ 2) /} 2} ± 2.5 [ degrees], the wavelength lambda ₂ lambda / Satisfies the function of 4 plates. If it is significantly out of the above range, the absolute value of the ellipticity is below 0.7. Similarly, the ellipticity is divided into regions but has a characteristic of continuously changing.

FIG. 14 shows intermediate values ((A): 2.0, (B): 2.0, (C): 360 [degrees]), conditions within the ranges of the above conditions (A) to (C), respectively. 1 ((A): 1.9, (B): 2.1, (C): 355 [degrees]),
Fixed under condition 2 ((A): 2.1, (B): 1.9, (C): 365 [degrees]), and the value changed within the range of the other conditions (D) and (E) The ellipticity of the emitted light is shown. FIG. 14A shows the ellipticity with respect to θ ₁ with θ ₂ fixed at 95 degrees, the dotted line indicates the ellipticity that is the maximum at the intermediate value, and the alternate long and short dash line indicates the condition 1 above. The minimum ellipticity and the solid line are the characteristics of the minimum ellipticity in the above condition 2.

Similarly, FIG. 14 (b), when fixing the theta ₂ and 90 [degrees], FIG. 14 (c), ellipticity when the theta ₂ fixed with 85 [degrees] changing the theta ₁ Is shown. Note that the ellipticity under other conditions is between the ellipticity at the intermediate value shown in FIGS. 14A to 14C and the ellipticity under condition 1. FIG. 15A summarizes the characteristics of ellipticity when θ ₁ and θ ₂ are changed in the intermediate values of FIGS. 14A to 14C. 15 (b) and 15 (c) respectively show θ ₁ and Condition 1 in FIGS. 14 (a) to 14 (c) and Condition 2 in FIGS. 14 (a) to 14 (c), respectively. The characteristics of ellipticity when θ ₂ is changed are summarized. These characteristics, within the range of the above condition (A) ~ (E), the ellipticity becomes 0.7 or more at a wavelength of lambda _2, it is shown that functions as lambda / 4 plate.

(Second Embodiment)
The first embodiment has a function as a λ plate for light of wavelength λ ₁ and a function as a λ / 4 plate having an ellipticity of 0.7 or more for light of wavelength λ _2. The broadband wave plate having the above has been described. In the second embodiment, a broadband wavelength that functions as a λ plate with respect to light of wavelength λ ₁ and becomes a λ / 4 plate with an ellipticity of 0.85 or more with respect to light of wavelength λ _2. The structural conditions of the plate will be described.

As in the first embodiment, the following (A ′) and (B) are used by using the retardation values and the lamination conditions θ ₁ and θ ₂ of the first wave plate 13a and the second wave plate 13b. The broadband wave plate may be configured to satisfy all of the conditions (D) and (E ′). In addition, (B)-(D) are the same conditions as 1st Embodiment.
(A ′) R ₁ (λ ₂ ) [degree]> R ₂ (λ ₂ ) [degree], and R ₁ (λ ₂ ) / R ₂ (λ ₂ ) is in the range of 2 ± 0.05. .
(B) R ₁ (λ ₁ ) [degree]> R ₁ (λ ₂ ) [degree], and R ₁ (λ ₁ ) / R ₁ (λ ₂ ) is in the range of 2 ± 0.1.
(C) R ₁ (λ ₁ ) [degree] is in the range of (360 × k) ± 5 [degree] (k is an odd number: 1, 3, 5,...).
(D) θ ₂ is in a range of 90 ± 5 [degrees].
(E ′) θ ₁ is in the range of 22.5 − {(90−θ ₂ ) / 2} ± 1.5 [degrees].

FIG. 16 shows the intermediate values ((A ′): 2.0, (B): 2.0, (C): 360 [) within the ranges of the above conditions (A ′), (B), and (C), respectively. Degree)), condition 3 ((A ′): 1.95, (B): 2.1, (C): 355 [degree]), condition 4 ((A ′): 2.05, (B): 1.9, (C): 365 [degrees]), and the value is changed within the range of the other conditions (D) and (E ′) to show the ellipticity of the emitted light. FIG. 16A shows the ellipticity with respect to θ ₁ with θ ₂ fixed at 95 [degrees]. The dotted line indicates the ellipticity that is maximum at the intermediate value, and the alternate long and short dash line indicates the condition 3 above. The minimum ellipticity and the solid line are the characteristics of the minimum ellipticity in the above condition 4.

Similarly, FIG. 16 (b), when the theta ₂ is fixed at 90 [degrees], FIG. 16 (c), ellipticity when the theta ₂ fixed with 85 [degrees] changing the theta ₁ Is shown. Note that the ellipticity under the other conditions is between the ellipticity at the intermediate value and the ellipticity under condition 4 shown in FIGS. FIG. 17A summarizes the characteristics of ellipticity when θ ₁ and θ ₂ are changed in the intermediate values of FIGS. 16A to 16C. 17 (b) and 17 (c) respectively show θ ₁ and Condition _{1 in} conditions 3 in FIGS. 16 (a) to 16 (c) and conditions 4 in FIGS. 16 (a) to 16 (c). The characteristics of ellipticity when θ ₂ is changed are summarized. From these characteristics, within the range of the above conditions (A ′), (B) to (C), and (E ′), the ellipticity is 0.85 or more at the wavelength λ ₂ and functions as a λ / 4 plate. It has been shown that Note that the ellipticity and the azimuth angle of the polarization direction when the light with the wavelength λ ₁ is incident are within the range shown in FIG. 7, and the light has a function as a λ plate with respect to the light with the wavelength λ ₁ . The above conditions (A ′), (B) to (C) and (E ′) are conditions for light having wavelengths of λ ₁ and λ ₂ , but λ ₁ , λ ₂ and λ ₃ (λ ₁ < When light of λ ₂ <λ ₃ ) enters, the parameters (A) to (E) related to λ ₂ may be replaced with the parameters of λ ₃ .

In addition, although common also in 1st Embodiment, (theta) ₁ and (theta) ₂ are materialized even if a code | symbol is reversed. That is, even when θ _{1 is set} to −θ ₁ and θ _{2 is set} to −θ ₂ , the above relationship is satisfied. In the description so far, the retardation value (R ₁ (λ ₁ )) [degree] of the first wave plate 13a having the wavelength λ ₁ is a wave plate of 360 ± 5 degrees, but (360 × k) ± It is also established as 5 degrees (k is an odd number: 1, 3, 5...). 360 ± 5 degrees is an example when k = 1.

(Third embodiment)
FIG. 18 is a schematic cross-sectional view of a polarizing diffraction grating 40 with a broadband wavelength plate. The polarization diffraction grating 40 with a broadband wavelength plate is obtained by laminating a polarization hologram 30 on the broadband wavelength plate 10 according to the first or second embodiment. The polarization hologram 30 includes a diffraction grating 31 having a periodic uneven shape on a transparent substrate 11d and having a stripe shape parallel to the Y direction, and a filler 32 that is optically transparent. Further, the diffraction grating 31 with showing a birefringence, as isotropic refractive index n _s of the filler 32 is substantially equal to either the ordinary refractive index n _o and extraordinary refractive index n _e of the diffraction grating 31 And

For example, the direction in which the diffraction grating 31 showing the ordinary refractive index n _o is an X direction in FIG. 18, when the refractive index n _s of the filler 32 is substantially equal to n _o, X direction of linearly polarized light traveling in the Z-direction The light passes through the polarizing diffraction grating 40 with the broadband wavelength plate in a straight line without changing the polarization state. Further, linearly polarized light in the Y direction is diffracted so feel the difference between the refractive index n _s with the refractive index n _e. Thus, by laminating the hologram element on the broadband wavelength plate 10, the action of the emitted light can be changed with respect to the polarization direction of the incident light. The cross-sectional shape of the diffraction grating is shown as an example of a rectangular shape, but is not limited thereto, and may be a blazed shape or a pseudo-blazed shape that approximates a blazed shape to a staircase shape.

(Fourth embodiment)
Next, as a fourth embodiment of the present invention, a case where the broadband wavelength plate 10 based on the design of the first or second embodiment is arranged as an optical component of the optical head device will be described. FIG. 19 shows a configuration of the optical head device 100 using light of three different wavelengths, a 405 nm wavelength band, a 660 nm wavelength band, and a 785 nm wavelength band. Further, the same optical parts as those of the conventional optical head apparatus 400 are denoted by the same reference numerals as the optical parts constituting the optical head apparatus 100 to avoid duplicate description. In the optical head device 100 according to the third embodiment, the broadband wavelength plate 10 according to the first embodiment is disposed as the broadband wavelength plate 111. In addition, a λ / 4 plate 118 corresponding to light in the 405 nm wavelength band is disposed in the optical path between the trichromatic prism 112 and the objective lens 114.

  Here, in FIG. 19, light in the 405 nm wavelength band is emitted from the light source 101 and reflected by the dichroic prism 108. The light reflected at this time is incident on the broadband wavelength plate 111 as X-direction linearly polarized light (referred to as P-polarized light) traveling in the Z direction. Since the broadband wave plate 111 emits the light without changing the polarization state, the P-polarized light in the 405 nm wavelength band is reflected by the trichroic prism 112 and travels in the X direction while changing the traveling direction. The light is polarized and condensed on the information recording surface of the optical disk 116. As described above, light in the 405 nm wavelength band transmitted through the broadband wavelength plate 111 is reflected by the trichromatic prism 112 in the state of linear polarization having a specific polarization component (P-polarized light) and is incident on the λ / 4 plate. It can be condensed with circularly polarized light.

  Further, the signal light including the information of the pits formed on the information recording surface of the optical disk 116 is transmitted through the λ / 4 plate 118 again and becomes linearly polarized light, and is reflected by the trichroic prism 112. The light is linearly polarized light in the Y direction (referred to as S-polarized light) and is similarly emitted from the broadband wavelength plate 111 without changing the polarization state. Then, the light is reflected by the polarization beam splitter 109 and collected on the photodetector 117.

  On the other hand, the light in the 660 nm wavelength band and the light in the 785 nm wavelength band emitted from the light source 102 and the light source 103 are respectively P-polarized linearly polarized light and enter the broadband wavelength plate 111 to become circularly polarized light, and travel straight through the trichromatic prism 112. The light is transmitted, reflected by the mirror 113, and collected on the optical disk 116. The signal light reflected by the optical disk 116 is reflected again by the mirror 113, travels straight through the trichroic prism 112, and changes from circularly polarized light to S polarized light by the broadband wavelength plate 111. Then, the light is reflected by the polarization beam splitter 109 and collected on the photodetector 117. By including the broadband wavelength plate 111 and the λ / 4 plate 118 in this way, recording / reproduction can be performed by condensing on the optical disc with good circular polarization with respect to light of three different wavelengths. A good optical head device can be realized.

(Fifth embodiment)
Next, as a fifth embodiment of the present invention, a case where a polarizing diffraction grating 40 with a broadband wavelength plate based on the design of the third embodiment is arranged as an optical component of an optical head device will be described. FIG. 20 shows a configuration of an optical head device 200 different from the optical head device 100 of the third embodiment using light of three different wavelengths, a 405 nm wavelength band, a 660 nm wavelength band, and a 785 nm wavelength band. Further, the same optical components as those of the optical head device 100 according to the third embodiment are denoted by the same reference numerals as the optical components constituting the optical head device 200 to avoid duplicate description. In the optical head device 200 according to the fourth embodiment, the polarizing diffraction grating with a broadband wavelength plate according to the second embodiment is disposed as the polarizing diffraction grating with a broadband wavelength plate 211. Further, the photodetector 217 is disposed at a position where the light diffracted by the polarization diffraction grating 211 with the broadband wavelength plate reaches.

  The polarization type diffraction grating 211 with the broadband wave plate is laminated and integrated with other optical elements used in the optical head device as in the second embodiment, thereby reducing the number of parts, simplifying the assembly of the optical head device, and Miniaturization can be realized. By adopting such a configuration, it is possible to avoid arranging the diffraction gratings respectively arranged in the optical paths immediately after the emission of the light sources of the respective wavelengths, and further downsizing can be realized. Thus, it is more preferable that the broadband wave plate is integrated with at least one optical element that changes the optical properties of the laser light.

  Here, in FIG. 20, light in the 405 nm wavelength band is emitted from the light source 101 and reflected by the dichroic prism 108. The light reflected at this time becomes P-polarized light traveling in the Z direction and enters the polarizing diffraction grating 211 with a broadband wavelength plate. Further, the refractive index of the hologram element 30 included in the polarizing diffraction grating 211 with the broadband wavelength plate is adjusted so as not to be diffracted with respect to P-polarized light. Therefore, since the polarization type diffraction grating 211 with the broadband wavelength plate emits the light without changing the polarization state, the P-polarized light in the 405 nm wavelength band is reflected by the trichroic prism 112, changes the traveling direction, and proceeds in the X direction. It becomes circularly polarized light by the λ / 4 plate 118 and is condensed on the information recording surface of the optical disk 116.

  Further, the signal light including the information of the pits formed on the information recording surface of the optical disk 116 is transmitted again through the λ / 4 plate 118 to be linearly polarized and reflected by the trichroic prism 112. Then, the light enters the polarizing diffraction grating 211 with the broadband wave plate. Of the polarizing diffraction grating 211 with a broadband wavelength plate, the polarization state is not changed by the broadband wavelength plate 10, and the hologram element 30 diffracts because of the difference in refractive index between the diffraction grating and the filler, and is detected by the collimator lens 110. The light is condensed on the container 217.

  On the other hand, the light in the 660 nm wavelength band and the light in the 785 nm wavelength band emitted from the light source 102 and the light source 103 are respectively P-polarized linearly polarized light and enter the polarizing diffraction grating 211 with the broadband wavelength plate to become circularly polarized light. The light travels straight through the ic prism 112, is reflected by the mirror 113, and is collected on the optical disk 116. The signal light reflected by the optical disk 116 is reflected by the mirror 113 again, travels straight through the trichroic prism 112, and enters the polarization diffraction grating 211 with a broadband wavelength plate. Of the polarizing diffraction grating 211 with a broadband wave plate, the broadband wave plate 10 changes from circularly polarized light to linearly polarized light (S-polarized light), and the hologram element 30 diffracts because it feels the difference in refractive index between the diffraction grating and the filler, The light is condensed on the photodetector 217 by the objective lens 110.

  In addition, the hologram element 30 included in the polarization diffraction grating 211 with the broadband wavelength plate is optimized for any one of the three different wavelengths of incident light, or has three wavelengths. If the design is optimized for light of an intermediate value, the forward path has high straight transmission characteristics for light of any wavelength, and the signal light is sent to the photodetector 217 without greatly reducing the diffraction efficiency in the return path. It can be condensed. Although depending on the arrangement and configuration of the photodetector 217, the diffraction grating structure of the hologram element 30 may be rectangular. However, if it is a blazed shape, the diffraction efficiency in one direction can be increased and the light utilization efficiency can be increased. This is preferable. By including the polarization diffraction grating 211 with the broadband wavelength plate in this way, an optical component that can be recorded and reproduced by condensing on an optical disk with good circular polarization with respect to light of three different wavelengths. Since the number of points can be reduced, it is possible to realize a miniaturized optical head device with high quality.

  In addition, as an example of the optical element that can be integrated in the optical head device, the broadband wavelength plate 10 and the hologram element 30 are given, but the invention is not limited thereto. Other examples of optical elements that can be integrated include phase correction elements using liquid crystals that are used to improve the light condensing characteristics on the optical disk, and signal light reflected by the optical disk to the photodetector by diffraction. A guiding diffraction grating is mentioned. The polarizing diffraction grating 211 with a broadband wavelength plate according to the present invention is particularly effective when used in an optical head device having an optical element that utilizes the difference in characteristics depending on the polarization state of light, and further requires miniaturization and weight reduction. It is suitable for parts for optical head devices used for recording / reproducing optical information.

Example 1
A method for manufacturing the broadband wavelength plate 10 shown in FIG. 1 will be described. On the quartz glass substrate to be the transparent substrates 11a and 11b, a polyimide film to be the alignment films 12a and 12b aligned horizontally with respect to the surface is formed and sealed, and the other substrate (not shown) is overlapped, and the substrate interval is increased. Two sets of 8.9 μm and 4.5 μm empty cells are formed. As for the rubbing direction, for an empty cell with a substrate spacing of 8.9 μm, the angle θ ₁ formed with one side of the substrate as the reference direction 14 (X direction) when the substrates face each other is 22.5 degrees, angle theta ₂ which form an empty cell in the substrate spacing 4.5μm is performed such that the direction of 90 degrees. Next, a photopolymerizable liquid crystal in which the liquid crystal compounds (10), (11), (12) and (13) represented by [Chemical Formula 1] are mixed at a weight ratio of 15.6: 16.9: 32.1: 35.4 A compound is prepared, injected into each cell, and photopolymerized to form a polymer liquid crystal layer that becomes the wave plates 13a and 13b. In the polymer liquid crystal layer thus formed, the refractive index anisotropy Δn that is the difference between the ordinary light refractive index and the extraordinary light refractive index at wavelengths of 405 nm, 660 nm, and 785 nm is 0.0457, 0.0384,. 0375.

Next, one substrate (not shown) of the liquid crystal cell sandwiching the polymer liquid crystal layer is released and separated, and the polymer liquid crystal layer is overlapped so that the reference directions 14 coincide with each other to form a transparent material in the three different wavelength bands. Adhering using the adhesive 11c, a broadband wavelength plate having a structure in which two polymer liquid crystal layers of this example are laminated is obtained. The ellipticity of the light that enters and exits the linearly polarized light having the polarization direction 15 parallel to the reference direction 14 with respect to the broadband wavelength plate is obtained. At this time, θ ₂ is 90 degrees, R ₁ (λ ₂ ) / R ₂ (λ ₂ ) is 2 and R ₁ (λ ₁ ) is 360 degrees when λ ₂ = 660 nm, and θ ₁ FIG. 21A shows the ellipticity for incident light with wavelengths of 660 nm and 785 nm. Thus, the ellipticity at θ ₁ = 22.5 [degrees] is 0.00, 0.98, and 0.90 for light with wavelengths of 405 nm, 660 nm, and 785 nm, respectively. In addition, the azimuth angle of the polarization direction with a wavelength of 405 nm is 0 [degree] in the same direction as the incident polarization direction 15, and it can be confirmed that it functions as a λ wavelength plate. In addition, even if θ ₁ = 22.5 [degrees], the ellipticity in the range of 0 ± 0.1 and the azimuth angle of the polarization direction in the range of ± 10 [degrees] with respect to the light having a wavelength of 405 nm Yes, a high ellipticity can be obtained for light with wavelengths of 660 nm and 785 nm.

(Example 2)
Example 1 in FIG. 21B except that R ₁ (λ ₂ ) / R ₂ (λ ₂ ) is 2.05 and R ₁ (λ ₁ ) is 365 degrees when λ ₂ = 660 nm. The ellipticity for incident light of 660 nm and 785 nm with respect to θ ₁ is shown. Accordingly, the ellipticity at θ ₁ = 22.5 [degrees] is −0.03, 0.93, and 0.93 with respect to light having wavelengths of 405 nm, 660 nm, and 785 nm, respectively. In addition, the azimuth angle of the polarization direction at a wavelength of 405 nm is 0.3 [degrees], and it can be confirmed that it functions as a λ wavelength plate. Further, even if θ ₁ = 22.5 [degrees], the ellipticity is in the range of 0 ± 0.1 and the azimuth angle of the polarization direction is in the range of ± 10 [degrees] with respect to the light having a wavelength of 405 nm. Yes, a high ellipticity can be obtained for light with wavelengths of 660 nm and 785 nm.

(Example 3)
FIG. 21C shows the ellipticity for incident light at 660 nm and 785 nm with respect to θ _{1 under the} same conditions as in Example 1 except that θ ₂ = 85 [degrees]. Accordingly, the ellipticity at θ ₁ = 20 [degrees] is 0.00, 0.99, and 0.92 for light with wavelengths of 405 nm, 660 nm, and 785 nm, respectively. In addition, the azimuth angle of the polarization direction at a wavelength of 405 nm is -10 [degrees], and it can be confirmed that it functions as a λ wavelength plate. Further, even if it changes from θ ₁ = 20 [degrees], the ellipticity in the range of 0 ± 0.1 and the azimuth angle of the polarization direction in the range of ± 10 [degrees] with respect to the light of wavelength 405 nm High ellipticity is obtained for light at 660 nm and 785 nm.

Example 4
FIG. 21D shows the ellipticity for incident light of 660 nm and 785 nm with respect to θ _{1 under the} same conditions as in Example 2 except that θ ₂ = 85 [degrees]. Accordingly, the ellipticity at θ ₁ = 20 [degrees] is −0.05, 0.94, and 0.95 with respect to light having wavelengths of 405 nm, 660 nm, and 785 nm, respectively. In addition, the azimuth angle of the polarization direction at a wavelength of 405 nm is −9.6 [degrees], and it can be confirmed that the film functions as a λ wavelength plate. Further, even if it changes from θ ₁ = 20 [degrees], the ellipticity in the range of 0 ± 0.1 and the azimuth angle of the polarization direction in the range of ± 10 [degrees] with respect to the light of wavelength 405 nm High ellipticity is obtained for light at 660 nm and 785 nm.

(Example 5)
A manufacturing method of the polarizing diffraction grating 40 with the broadband wavelength plate shown in FIG. 18 will be described. The broadband wavelength plate 10 is omitted because it uses the same manufacturing method as in the first embodiment, and the hologram element 30 will be described. A non-illustrated horizontal alignment film made of a polyimide film whose surface is rubbed is formed on a quartz glass substrate as the transparent substrate 11d and sealed to form an empty cell with a substrate spacing of 4.4 μm. The rubbing is performed so as to be in a direction orthogonal to the polarization direction of incident light. Next, a photopolymerizable liquid crystal in which the liquid crystal compounds (10), (11), (12) and (13) represented by [Chemical Formula 1] are mixed at a weight ratio of 15.6: 16.9: 32.1: 35.4 A compound is prepared, injected into each cell, and photopolymerized to form a polymer liquid crystal layer. In the polymer liquid crystal layer thus formed, the refractive index anisotropy Δn that is the difference between the ordinary light refractive index and the extraordinary light refractive index at wavelengths of 405 nm, 660 nm, and 785 nm is 0.0457, 0.0384,. 0375. The substrate spacing of empty cells is set in advance so that the first-order diffraction efficiency is maximized at 405 nm. Therefore, the retardation value of the polymer liquid crystal layer was 179 [degrees] at a wavelength of 405 nm.

  Next, one substrate (not shown) of the liquid crystal cell sandwiching the polymer liquid crystal layer is peeled off and a striped polymer liquid crystal having a width of 5 μm is formed by using a photolithography technique and an etching technique. At that time, the stripe width of the polymer liquid crystal and the groove width without the polymer liquid crystal were set to 1: 1 so that the first-order diffraction efficiency was maximized. Thereby, the diffraction grating 31 having a grating pitch of 10 μm can be formed.

  Next, an adhesive that becomes transparent as a filler 32 in three different wavelength bands is applied on the diffraction grating 31 made of polymer liquid crystal, and the broadband wavelength plate 10 is laminated. Make it. At this time, an adhesive having the same ordinary refractive index and refractive index of the polymer liquid crystal was used. As a result of incidence of linearly polarized light in the same direction as rubbing on the produced polarizing diffraction grating 40 with a broadband wavelength plate, a value of 39% is obtained for each of the + 1st order and -1st order diffraction efficiencies for a laser beam having a wavelength of 405 nm. . On the other hand, as a result of incidence of linearly polarized light in the direction orthogonal to the rubbing, a value with a zero-order transmittance of 97% is obtained.

(Example 6)
The broadband wavelength plate 10 described in Example 1 is arranged as the broadband wavelength plate 111 of the optical head device 100 of FIG. The wavelength of each semiconductor laser is 405 nm for the light source 101, 658 nm for the light source 102, and 789 nm for the light source 103. All of these lights are incident on the broadband wave plate 111 in the same linear polarization direction. As a result, the polarization state does not change at the three wavelengths of 405 nm, and the linear polarization is emitted, and satisfactory circular polarization is obtained for 658 nm and 789 nm. And signal light with high light utilization efficiency can be obtained.

As described above, as a broadband wave plate formed by laminating two wave plates into which a plurality of light beams having different wavelengths are incident, by combining the optical axis of the wave plates to be laminated and the retardation value at each wavelength, λ ₁ Linearly polarized light having a wavelength is emitted without changing the polarization state, and light having wavelengths of λ ₂ and λ ₃ (λ ₁ <λ ₂ <λ ₃ ) can be emitted as circularly polarized light. Furthermore, an optical head device that is reduced in size and weight can be realized by arranging this broadband wavelength plate in an optical head device that records and reproduces optical disks of different standards, which is useful.

The cross-sectional schematic diagram of the broadband wavelength plate of this invention, and the plane schematic diagram which show relationships, such as the angle of each optical axis of a wavelength plate. The Poincare sphere representing the polarization state of the broadband wave plate of the present invention. The wavelength dependence of the ellipticity and azimuth of the transmitted light of the broadband wave plate of the present invention. Characteristics of relative retardation value R ₁ of the first wave plate 13a _{(lambda 1)} at the wavelength lambda _1, ellipticity and azimuth of the polarization direction. Characteristics of ellipticity and azimuth angle of polarization direction with respect to a combination of R ₁ (λ ₁ ) / R ₂ (λ ₁ ) and R ₁ (λ ₁ ). Characteristics of ellipticity and azimuth angle with respect to a polarization direction with respect to a combination of angles θ ₁ and θ ₂ formed by the polarization direction of incident light and the optical axis of each wave plate. The ellipticity and orientation of the polarization direction with respect to θ ₁ under effective conditions (upper limit value, intermediate value, lower limit value) of θ ₂ , R ₁ (λ ₁ ) and R ₁ (λ ₁ ) / R ₂ (λ ₁ ) Corner characteristics. The ellipticity characteristic for R ₁ (λ ₂ ) / R ₂ (λ ₂ ). The ellipticity characteristic for R ₁ (λ ₁ ) / R ₁ (λ ₂ ). for theta _2, the characteristics of ellipticity. The characteristic of ellipticity with respect to θ ₁ . The ellipticity characteristic for the combination of R ₁ (λ ₁ ) / R ₁ (λ ₂ ) and R ₁ (λ ₂ ) / R ₂ (λ ₂ ). The ellipticity characteristic for the combination of θ ₁ and θ ₂ . Effective conditions (intermediate value, condition 1, condition 2) of θ ₂ , R ₁ (λ ₂ ) / R ₂ (λ ₂ ), R ₁ (λ ₁ ) / R ₂ (λ ₁ ) and R ₁ (λ ₁ ) ) Characteristics of ellipticity with respect to θ ₁ . Effective conditions (intermediate value, condition 1, condition 2) of θ ₂ , R ₁ (λ ₂ ) / R ₂ (λ ₂ ), R ₁ (λ ₁ ) / R ₂ (λ ₁ ) and R ₁ (λ ₁ ) ) Characteristics of ellipticity with respect to θ ₁ . Effective conditions (intermediate value, condition 3 and condition 4) of θ ₂ , R ₁ (λ ₂ ) / R ₂ (λ ₂ ), R ₁ (λ ₁ ) / R ₂ (λ ₁ ) and R ₁ (λ ₁ ) ) Characteristics of ellipticity with respect to θ ₁ . Effective conditions (intermediate value, condition 3 and condition 4) of θ ₂ , R ₁ (λ ₂ ) / R ₂ (λ ₂ ), R ₁ (λ ₁ ) / R ₂ (λ ₁ ) and R ₁ (λ ₁ ) ) Characteristics of ellipticity with respect to θ ₁ . The cross-sectional schematic diagram of the polarizing diffraction grating with a broadband wavelength plate in this invention. The block diagram of the optical head apparatus concerning the 3rd Embodiment of this invention. The block diagram of the optical head apparatus concerning the 4th Embodiment of this invention. The ellipticity characteristic of the broadband wavelength plate concerning Examples 1-4. FIG. 1 is a configuration diagram of a conventional optical head device. FIG. 2 is a configuration diagram of a conventional optical head device.

Explanation of symbols

10, 111 Broadband wave plate (present invention)
11a, 11b, 11d Transparent substrate 11c Adhesives 12a, 12b Alignment film 13a First wave plate 13b Second wave plate 14 Reference direction 15 Polarization direction of incident light 16 Optical axis direction 17 of first wave plate Second Optical axis direction 21 of the wavelength plate 21 Incident polarization state 22 on the Poincare sphere Rotation axis 23 corresponding to the first wavelength plate Rotation axis 24 corresponding to the second wavelength plate First wavelength plate (R ₁ (λ ₂ )) Polarization state 25 on the Poincare sphere converted by <180 [degrees]) Polarization state 26 on the Poincare sphere converted by the first wave plate (R ₁ (λ ₂ )> 180 [degrees]) By the broadband wave plate Trajectory 30 of the polarization state on the converted Poincare sphere 30 Hologram element 31 Diffraction grating 32 Filling materials 40, 211 Polarization type diffraction grating with broadband wave plate 100, 200 Optical head device (present invention)
101, 301 Light source (405 nm wavelength band)
102, 302 Light source (660 nm wavelength band)
103, 303 Light source (785 nm wavelength band)
104, 105, 106, 304, 305, 306 Diffraction element 107, 308 Dichroic prism 108, 112 Trichromatic prism 109, 307, 309 Polarizing beam splitter 110, 310, 311 Collimator lens 113, 314, 315 Mirror 114, 115, 316 317 Objective lens 116, 318 Optical disc 117, 217, 319, 320 Optical detector 118, 312, 313 λ / 4 plate 300, 400 Optical head device (conventional)
411 Broadband wave plate (conventional)

Claims (6)

A first wave plate and a second wave plate arranged in parallel;
A broadband wave plate on which light of m wavelengths (m is an integer of 2 or more) is incident,
The optical axis of the first wave plate and the optical axis of the second wave plate intersect,
The incident wavelength is expressed as satisfying the relationship of λ _j <λ _{j + 1} (j = _{1 to} m−1), and the wavelength of the i-th wave plate with respect to light is represented as λ _n (n = _{1 to} m). When the retardation value is R _i (λ _n ) [degree] (i is 1 or 2),
For at least one set of wavelength λ ₁ and wavelength λ _{j + 1}
R ₁ (λ _j +1) / R ₂ (λ _{j + 1} ) is in the range of 2 ± 0.1, and R ₁ (λ ₁ ) / R ₁ (λ _{j + 1} ) is in the range of 2 ± 0.1. There,
k is an odd number, and R ₁ (λ ₁ ) is within a range of (360 × k) ± 5 [degrees],
The angle θ ₂ formed by the polarization direction in which the linearly polarized light of the wavelength λ ₁ and the wavelength λ _{j + 1} is incident in the same polarization direction and the optical axis of the second wavelength plate is within a range of 90 ± 5 [degrees]. I Oh,
The angle θ ₁ formed by the polarization direction in which linearly polarized light having the wavelength λ ₁ and the wavelength λ _{j + 1} is incident in the same polarization direction and the optical axis of the first wave plate is 22.5 − {(90−θ ₂ ) / 2} A broadband wave plate within a range of ± 2.5 [degrees] . R ₁ (λ _{j + 1} ) / R ₂ (λ _{j + 1} ) is in the range of 2 ± 0.05, and the angle θ ₁ is 22.5 − {(90−θ ₂ ) / 2} ± 1.5 [ The broadband wave plate according to claim ₁ , wherein linearly polarized light having at least one wavelength λ _{j + 1} is in the range of [degree]. A broadband wave plate on which linearly polarized light of the wavelength λ ₁ , the wavelength λ ₂ and the wavelength λ ₃ (λ ₁ <λ ₂ <λ ₃ ) is incident;
The broadband wavelength plate according to claim 1 or 2 , wherein the λ ₁ is a 405 nm wavelength band, the λ ₂ is a 660 nm wavelength band, and the λ ₃ is a 785 nm wavelength band. A light source that emits light of at least the wavelength λ ₁ and the wavelength λ ₂ ;
First separation means for deflecting and separating light emitted from the light source;
Second separation means for deflecting and separating the light having the wavelength λ _{1 and} the light having the wavelength λ ₂ into different optical paths from the light emitted from the first separation means;
An objective lens for condensing the light emitted from the second separating means on the optical disc;
An optical head device that detects light reflected by the optical disc, and λ / 4 in the optical path of the light having the wavelength λ ₁ between the second separating means and the objective lens. As the board is placed,
An optical head device broadband wave plate is arranged according to any one of claims 1-3 in an optical path between said first separating means and said second separating means. The light source emits the wavelength λ ₁ , the wavelength λ ₂ and the wavelength λ ₃ ,
It said second separation means comprises a light emitting direction of the wavelength of the wavelength lambda _1, claim a Trichroic prism to the emission direction of the wavelength lambda ₂ of the light and the wavelength lambda ₃ of the light differently 5. The optical head device according to 4 . The first separating means is a hologram element in which a diffraction grating having birefringence is filled and flattened with a filler that is an isotropic material,
The refractive index n _s of the filler, the optical head apparatus according to a substantially equal claim 4 or claim 5 in the ordinary refractive index n _o or the extraordinary refractive index n _e of the diffraction grating.

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