JP5056059B2 - Broadband wave plate - Google Patents

Description

  The present invention relates to a broadband wave plate.

  The broadband wavelength plate is used as a quarter-wave plate, a half-wave plate, or the like corresponding to a plurality of wavelengths in an optical storage optical pickup or a lighting device such as a projector. Conventional broadband wave plates have a structure in which a quarter wave plate and a half wave plate are bonded together (for example, see Patent Documents 1 and 2), or have a refractive index periodic structure (for example, a patent). In the case of using as a quarter-wave plate, the retardation value is about 90 °, and when used as a half-wave plate, the retardation value is about 180 °. It is configured as follows.

In an optical pickup, for example, it is used as a broadband wavelength plate that can be applied to linearly polarized light in two wavelength bands, that is, linearly polarized light in a wavelength band used for CDs and linearly polarized light in a wavelength band used for DVDs. This optical pickup is composed of a plurality of optical devices, and some of these optical devices have different optical rotation depending on the wavelength of the transmitted light wave. Some light-emitting elements used as two-wavelength light sources have different polarization directions of two wavelengths when emitted. Accordingly, when the two-wavelength linearly polarized light incident on the pickup passes through the optical device having such optical rotation before being incident on the broadband wave plate, the polarization direction of the two-wavelength light wave incident on the broadband wave plate Are different from each other.
Japanese Patent Laid-Open No. 11-149015 JP-A-10-68816 JP 2004-170623 A

  However, in the conventional broadband wave plate, in order to obtain the same retardation value for two-wavelength linearly polarized light, the polarization planes (polarization directions) of the two-wavelength linearly polarized light must be incident on each other. There wasn't. Therefore, when a polarizer is added immediately before the broadband wavelength plate in order to align the polarization direction, there is a problem that light incident on the broadband wavelength plate is attenuated by the polarizer. Further, when two-wavelength linearly polarized light is incident on a conventional broadband wavelength plate in a state where the polarization directions are different from each other, there is a problem that desired polarization characteristics cannot be obtained.

  The present invention has been made to solve the above-described problems, and gives substantially the same retardation value (hereinafter simply referred to as Rd value) to a plurality of incident lights having different wavelengths and polarization directions. It is an object of the present invention to provide a broadband wave plate that can give desired polarization characteristics to linearly polarized light of any wavelength different in direction.

The broadband wave plate of the present invention is configured such that first and second phase plates formed of a birefringent material are stacked on each other, and at least a first polarized light having a wavelength λ ₁ and a wavelength longer than λ ₁ The second polarized light having λ _{2 is used} to enter the first phase plate as linearly polarized light in different directions, change into a predetermined polarization state, and exit from the second phase plate. A broadband wave plate, wherein an angle between the optical axis of the first phase plate and the polarization direction of the first polarization is θ, and the polarization direction of the first polarization and the polarization of the second polarization The angle formed by the direction is α, the angle formed by the optical axis of the first phase plate and the optical axis of the second phase plate is β, and the retardation value at an arbitrary wavelength λ of the first phase plate was the Rd 1 _(λ), any wavelength of the second phase plate When the retardation value is Rd ₂ (lambda) at, for a wavelength lambda satisfying λ ₁ ≦ λ ≦ λ ₂ relations, the Rd ₁ (lambda) is _{80 ° ≦ Rd 1 (λ)} ≦ 100 ° relationship And β satisfies the relational expression of 30 ° ≦ β ≦ 60 °, and Rd ₂ (λ ₁ ), Rd ₂ (λ ₂ ), and α are | {Rd ₂ (λ ₁ ) −Rd ₂ (Λ ₂ )} / 2−α | ≦ 10 ° is satisfied, and θ is the difference Rd ₂ (λ _{1) of the} phase difference given to the two polarized lights by passing through the second phase plate. ) −Rd ₂ (λ ₂ ) cancels the latitude difference between the two polarizations after exiting the first phase plate on the Poincare sphere, and the polarizations of the two polarizations after passing through the second phase plate broadband wave plate der, characterized in that state are both set so that ellipticity of 90% or more elliptically polarized .

Also, broadband wave plate of the present invention is configured such that the first and second phase plates are formed by a birefringent material are laminated to one another, at least, than the lambda ₁ and the first polarized light having a wavelength lambda ₁ The second polarized light having the long wavelength λ ₂ is incident on the first phase plate as linearly polarized light in different directions, changed to a predetermined polarization state, and emitted from the second phase plate. A broadband wave plate used, wherein an angle between the optical axis of the first phase plate and the polarization direction of the first polarization is θ, and the polarization direction of the first polarization and the second polarization Is defined as α, the angle between the optical axis of the first phase plate and the optical axis of the second phase plate is β, and at an arbitrary wavelength λ of the first phase plate. the retardation value was Rd 1 and _(lambda), any of the second phase plate When the retardation value at the wavelength lambda, and Rd ₂ (lambda), with respect to the wavelength lambda satisfying λ ₁ ≦ λ ≦ λ ₂ relations, the Rd ₁ (lambda) is _{80 ° ≦ Rd 1 (λ)} ≦ 100 ° Where β satisfies the relationship of 30 ° ≦ β ≦ 60 °, and Rd ₂ (λ ₁ ), Rd ₂ (λ ₂ ), and α are | {Rd ₂ (λ ₁ ) − Rd ₂ (λ ₂ )} / 2−α | ≦ 10 ° is satisfied, and θ is the difference Rd ₂ ( Rd ₂ (the phase difference given to the two polarized lights by passing through the second phase plate ). λ ₁ ) −Rd ₂ (λ ₂ ) cancels out the difference in latitude on the Poincare sphere of the two polarizations after exiting the first phase plate, and the two polarizations after passing through the second phase plate Broadband wavelength characterized in that both polarization states are set to elliptical polarization with an ellipticity of 10% or less It is.

In the broadband wavelength plate of the present invention, the ratio Rd ₂ / Rd ₁ of the retardation value Rd ₂ of the second phase plate to the retardation value Rd ₁ of the first phase plate is substantially equal to the increase of α. It is characterized the increase to Rukoto in manner.

In the broadband wavelength plate of the present invention, the birefringent material is preferably composed of a thin film layer of polymer liquid crystal .

In the broadband wavelength plate of the present invention, it is preferable that the θ is set so as to substantially increase as the α increases .

  Furthermore, the optical head device of the present invention includes a light source that emits light of a plurality of wavelengths, an objective lens that condenses the light emitted from the light source on the optical recording medium, and the light reflected by the optical recording medium. In the optical head device including the photodetector for detecting the above, the broadband wave plate described above is disposed in the optical path between the light source and the objective lens.

  In the present invention, the directions of the polarization planes are different from each other by laminating the first phase plate and the second phase plate formed of a birefringent material so that their optical axes intersect at a specific angle. It is possible to provide a broadband wavelength plate having an effect that the same retardation value can be obtained for linearly polarized light having a plurality of wavelengths.

  The present invention relates to a broadband wavelength plate having a birefringence and having a first thin film layer (phase plate) positioned on the incident side and a second thin film layer (phase plate) positioned on the output side. It is.

  In the present invention, when two linearly polarized light having different wavelengths and polarization directions are incident on the broadband wavelength plate, the first phase plate once changes the polarization state of the two linearly polarized light on the Poincare sphere. And then using the refractive index dispersion of the second phase plate to offset the difference in latitude on the Poincare sphere of the polarization state of the two polarizations, and after passing through the second phase plate, the two polarizations Both of the polarization states move to substantially the same location on the Poincare sphere. Finally, if the two polarized lights are moved to the vicinity of the poles on the Poincare sphere, they become a broadband quarter-wave plate, and if they are moved near the equator, they become a broadband half-wave plate.

  In the present invention, “almost” in the case of “substantially on the same meridian on the Poincare sphere”, “almost along the meridian on the Poincare sphere” or “substantially the same location on the Poincare sphere” This means that it only needs to be matched to such a degree that it can be converted into a substantially equal polarization state after passing through the second phase plate. When the broadband wave plate of the present invention is used as a quarter wave plate, it is preferable that the two polarized lights after passing through the second phase plate have an ellipticity of 90% or more. Further, when the broadband wave plate of the present invention is used as a half-wave plate, it is preferable that both of the two polarized lights after passing through the second phase plate have an ellipticity of 10% or less.

  By adjusting the orientation and retardation of the second phase plate, the two polarizations are finally removed by slightly removing the positions of the two polarizations on the Poincare sphere after passing through the first phase plate from the same meridian. May have the same polarization state, and can be adjusted as appropriate.

Hereinafter, the broadband wave plate of the present invention will be described with reference to FIGS.
FIG. 1 is a schematic diagram showing a schematic configuration of a broadband wavelength plate according to the present invention. A broadband wavelength plate 101 includes an incident side phase plate 1 and an output side phase plate 2.

  FIG. 2 is a schematic diagram showing the configuration of the broadband wavelength plate of the present invention in more detail. The broadband wave plate 102 includes a first phase plate 103 which is a phase plate on the incident side of linearly polarized light and a second phase plate 104 which is a phase plate on the output side through an adhesive layer such as a UV adhesive 9. Are stacked on each other. The procedure for stacking will be described in Examples.

  The first phase plate 103 includes a thin film layer of polymer liquid crystal 3 constituting a birefringent material, a glass substrate 5, and a polyimide alignment film 7.

  On the other hand, the second phase plate 104 includes a thin film layer of a polymer liquid crystal 4 constituting a birefringent material, a glass substrate 6, and a polyimide alignment film 8.

  As shown in FIG. 3, the first phase plate 103 and the second phase plate 104 respectively have slow axes 201 and 202 that constitute an optical axis.

The broadband wavelength plate according to the present invention may be laminated on one or more transparent substrates. In this case, the transparent substrate is preferable because distortion due to thermal expansion and wavefront aberration of transmitted light are improved.
Further, it is preferable that the transparent substrate is subjected to an AR (Anti Reflection) process because the transmittance of incident light is improved.

  Further, in the broadband wave plate according to the present invention, the birefringent material may be composed of either an organic material or an inorganic material. Further, the first phase plate and the second phase plate may be made of the same material or may be made of different materials.

  Here, an embodiment in the case of applying the broadband wave plate according to the present invention to a quarter wave plate will be described.

Hereinafter, it is assumed that the wavelength of the first linearly polarized light incident on the broadband wave plate 102 is λ ₁ , the wavelength of the second linearly polarized light is λ _2, and λ ₁ <λ ₂ is satisfied.

The Rd dispersion of the material forming the thin film layers of the polymer liquid crystals 3 and 4 may be any, but if Rd (λ ₁ ) / Rd (λ ₂ ) is in the range of 1.15 to 1.67, This is preferable because the range of angles formed by the polarization directions of two-wavelength linearly polarized light to which the broadband wave plate of the present invention can be applied is widened. Furthermore, if Rd (λ ₁ ) / Rd (λ ₂ ) is in the range of 1.15 to 1.25, the range of angles formed by the polarization directions of the two-wavelength linearly polarized light can be further increased. preferable.

  Depending on the configuration of the optical head device, when the difference in polarization direction of two linearly polarized light beams having different wavelengths is larger than the Rd value of the first phase plate, the Rd value is effectively larger. It is preferable to use a second phase plate. That is, the angle between the polarization directions 203 and 204 of the first and second linearly polarized light is α, and the angle between the polarization direction 203 of the first linearly polarized light and the slow axis 201 of the first phase plate 103 is θ. Then, in order to convert two linearly polarized light having a large α into the same polarization state, it is preferable to select a large θ. In this sense, it can be said that α and θ usually have a linear relationship.

  In order to move the polarization state of two incident polarized lights to substantially the same meridian on the Poincare sphere, a phase plate that functions as a substantially quarter-wave plate at the two polarized wavelengths is used as the first phase plate. Although it is preferable to use, a high-order wave plate or the like having a similar function may be used. In particular, the first phase plate preferably has an Rd value corresponding to a quarter-wave plate in the wavelength range of incident polarized light and in the vicinity thereof.

Specifically, the retardation value Rd ₁ of the thin film layer of the polymer liquid crystal 3 constituting the first phase plate 103 is λ satisfying λ ₁ <λ <λ ₂ .
80 ° ≦ Rd ₁ (λ) ≦ 100 ° (1)
It is preferable to have the following relationship. Although it is also preferable to use a material having a large dispersion, the first phase plate 103 functions as a quarter-wave plate for λ of any one wavelength among λ satisfying λ ₁ <λ <λ _2. You may make it do.

In the present invention, after exiting the first phase plate 103, the two polarized lights move on the same meridian on the Poincare sphere, but when the polarized light passes through the second phase plate 104, The angle β formed by the slow axis 201 of the first phase plate 103 and the slow axis 202 of the second phase plate 104 is set so that the polarization state moves substantially along the meridian on the Poincare sphere. Specifically, 30 ° ≦ β ≦ 60 ° (3)
It is preferable to have the following relationship. In general, it is preferable that the value of β is around 45 °. However, since the most preferable angle varies depending on the retardation of the two phase plates, their dispersion, etc., it is preferable to adjust appropriately within the above range.

In the present invention, θ is determined by the difference Rd ₂ (λ ₁ ) −Rd ₂ (λ ₂ ) of the phase difference given to the light of the two wavelengths by passing through the second phase plate 104. The difference in latitude on the Poincare sphere of the two polarizations after exiting the first phase plate 103 is canceled, and the polarization states of the two wavelengths of light after passing through the second phase plate 104 are almost the same on the Poincare sphere. It is set to move to a location. The difference in latitude on the Poincare sphere after the two wavelengths of light are emitted from the first phase plate 103 is basically the polarization plane of the first linear polarization and the polarization plane of the second linear polarization. Is generated based on the angle α formed by Therefore, if α is increased, it is necessary to increase Rd ₂ (λ ₁ ) −Rd ₂ (λ ₂ ) in order to match the polarization states of the two polarized lights. On the other hand, the point where the polarization states of the two polarizations coincide with each other is near the pole of the Poincare sphere in the quarter wavelength plate, and near the equator of the Poincare sphere in the half wavelength plate. In order to make the point where the polarization states of the two polarizations coincide with each other as the pole or equator, the pole or equator as the target point and the polarization states of the two polarizations after passing through the first phase plate 103 are shown on the Poincare sphere. It is necessary to adjust the distance to the point, which can be done mainly by appropriately adjusting θ.

Thus, by appropriately setting θ, the retardation value Rd ₂ of the thin film layer of the polymer liquid crystal 4 constituting the second phase plate 104 is
| {Rd ₂ (λ ₁ ) −Rd ₂ (λ ₂ )} / 2−α | ≦ 10 ° (2)
It is preferable to have the following relational expression. Further, a material having a large dispersion may be used. Here, the value on the right side of Equation (2) may be configured to be close to zero.

  Next, an optical pickup using the broadband wavelength plate 102 of the present invention having the relationship between the polarization direction and the optical axis direction shown in FIG. 5 as a quarter wavelength plate (hereinafter referred to as a broadband quarter wavelength plate). An example of the optical system of the optical head device) is shown in FIG.

  The linearly polarized light emitted from the first semiconductor laser 11 has a polarization direction 207, and the linearly polarized light emitted from the second semiconductor laser 12 has a polarization direction 208.

  The linearly polarized light emitted from the first semiconductor laser 11 passes through the first and second half mirrors 21 and 22, is collimated by the collimator lens 31, and is circularly polarized by the broadband quarter-wave plate according to the present invention. Converted to. Circularly polarized light emitted from the broadband quarter-wave plate is condensed by the objective lens 32 onto a signal track on the optical disk 33.

  The condensed light is reflected by the optical disc 33, passes through the objective lens 32 and the broadband quarter-wave plate, and is converted into linearly polarized light having a polarization direction orthogonal to the polarization direction of the incident light. Thereafter, the linearly polarized light reflected by the second half mirror 22 is detected by the photodetector 34. On the other hand, the linearly polarized light transmitted through the second half mirror 22 and the first half mirror 21 is incident on the first semiconductor laser 11. At this time, the polarization direction of the linearly polarized light incident on the first semiconductor laser 11 is orthogonal to the polarization direction of the emitted light, and does not interfere with the emitted light inside the first semiconductor laser 11. Therefore, noise due to interference does not occur and the operation of the first semiconductor laser 11 is not adversely affected. The same applies to linearly polarized light emitted from the second semiconductor laser 12.

  At this time, even when the wavelength and the polarization direction of the emitted light from the first semiconductor laser 11 and the second semiconductor laser 12 are different from each other, the broadband wavelength plate of the present invention is used as a broadband quarter-wave plate. The polarization direction of the return light from the optical disk 33 and the polarization direction of the linearly polarized light emitted from each semiconductor laser can be made orthogonal to the linearly polarized light of the respective wavelengths. Therefore, no noise is generated in any of the semiconductor lasers, and as a result, the signal component of the return light corresponding to the signal track can be detected well by the photodetector 34.

  On the other hand, in an optical pickup using a conventional broadband quarter-wave plate as a broadband wavelength plate, the two outgoing lights from the first and second semiconductor lasers are in a wide-band quarter with different polarization directions. When incident on one wavelength plate, it does not function sufficiently as a quarter-wave plate for outgoing light of one wavelength, and as a result, the polarization direction of at least one linearly polarized light returning to the semiconductor laser is It has a component that is not orthogonal to the polarization direction of the emitted light of the semiconductor laser. For this reason, noise is generated in the semiconductor laser, and noise components are included in the light emitted from the semiconductor laser. Therefore, even if the return light having a signal component corresponding to the signal track is detected by the photodetector, there is a problem that the signal component cannot be detected because the noise component is large.

  In order to solve this problem, in the conventional optical pickup, a polarizing beam splitter is used instead of the second half mirror 22, or a wavelength plate or a polarizing filter is installed in the vicinity of the semiconductor laser, It was necessary to reduce the influence of the return light on the semiconductor laser. However, in the optical pickup using the broadband wave plate according to the present invention, noise caused by the return light is not generated in any semiconductor laser, so it is possible to reduce the number of components conventionally required for reducing or preventing the noise. As a result, it is possible to realize an optical pickup having a simpler structure at a lower cost than the conventional one.

  In the above description, the case where light of one wavelength is emitted from one light source has been described, but light of two wavelengths or light of three wavelengths may be emitted from one light source.

[Example 1]
An embodiment in which the broadband wave plate according to the present invention is applied to a quarter wave plate will be described with reference to FIGS.

  A glass substrate 5 having a thickness of 0.5 mm is prepared, and a polyimide film is formed on one surface of the glass substrate 5. The polyimide alignment film 7 is formed by performing a horizontal alignment process on the polyimide film.

Next, an SiO ₂ bead (not shown) having a diameter of 3 μm is provided as a spacer on the polyimide alignment film 7 subjected to the alignment treatment so that the gap between the glass substrates is kept constant when the liquid crystal cell is formed in the subsequent process. Scatter at a density of 5000 / cm ² .

Then, the horizontal alignment glass substrate (not shown) that has been subjected to the mold release treatment is opposed to the glass substrate 5 that has been subjected to the alignment treatment. At this time, a thermosetting epoxy seal material (not shown) is printed on the outer periphery of the substrate so that the two glass substrates face each other through the epoxy seal material.
The gap between the two glass substrates is set to 3.0 μm.

  A liquid liquid crystal material is injected between the two glass substrates formed in this manner, and sandwiched between the two glass substrates. Here, the liquid crystal material has a difference Δn≈0.061 between the ordinary refractive index and the extraordinary refractive index at 660 nm, which is the central wavelength of the wavelength region used for DVD, and the central wavelength of the wavelength region used for CD. The difference Δn≈0.058 between the ordinary light refractive index and the extraordinary light refractive index at 785 nm. That is, it is formed of a liquid crystalline monomer in which the ratio of Δn in the wavelength region used for DVD to Δn in the wavelength region used for CD is about 0.95.

  The liquid crystalline monomer is added with 1% benzoin isopropyl ether as a photopolymerization initiator so as to constitute a UV curable liquid crystalline monomer composition.

  Thereafter, the entire liquid crystal material was irradiated with UV light having a wavelength of 365 nm so that the entire liquid crystalline monomer composition was polymerized and solidified on the glass substrate in a horizontally aligned state.

  Next, after heat-treating the above glass substrate at 140 ° C. for 30 minutes, the horizontal alignment glass substrate (not shown) is removed from the mold, whereby the horizontally aligned organic thin film of the polymer liquid crystal 3 having a thickness of 3 μm becomes a polyimide alignment film. 7 formed.

  Moreover, the organic thin film of the polymer liquid crystal 4 was formed on the polyimide alignment film 8 of the glass substrate 6 by said process.

  For the first and second phase plates 103 and 104 produced as described above, the Rd value and the azimuth of the optical axis were measured using semiconductor laser light having a wavelength of 660 nm. As a result, the Rd value of each of the first and second phase plates 103 and 104 was 184 nm. Further, the Rd value was similarly measured using a semiconductor laser beam having a wavelength of 785 nm. As a result, the Rd value of each of the first and second phase plates 103 and 104 was 175 nm.

  Next, among the optical axes obtained by the above measurement, the slow axis 201 of the first phase plate 103 intersects with the slow axis 202 of the second phase plate 104 at about 45 °. A first phase plate 103 and a second phase plate 104 are arranged. Here, the angle is counterclockwise in the state of FIG. 3 in which the first phase plate 103 is disposed on the second phase plate 104 and the second phase plate 104 is viewed from the first phase plate 103. The direction of is positive (+).

  Next, the UV adhesive 9 is dropped between the first and second phase plates 103 and 104, and the rotational speed of 5000 rpm is 20 seconds at a rotational speed of 1000 rpm so that the thickness of the UV adhesive 9 becomes 5 μm. And rotated for 100 seconds.

  After correcting the axial displacement of the first and second phase plates 103 and 104 during rotation, the UV adhesive 9 was cured by irradiating with 5000 mJ UV light, and the broadband wavelength plate 102 was produced.

  The ellipticity of the broadband wavelength plate 102 was measured using linearly polarized light of semiconductor laser light having wavelengths of 660 nm and 785 nm. The polarization direction 204 of linearly polarized light having a wavelength of 785 nm was arranged to be 10 ° with respect to the polarization direction 203 of linearly polarized light having a wavelength of 660 nm. Further, linearly polarized light having a wavelength of 660 nm was incident from the glass substrate 5 side so that the polarization direction 203 was about −95 ° with respect to the direction of the slow axis 201 of the first phase plate 103. As a result, in the case of linearly polarized light having a wavelength of 660 nm, the measured ellipticity was 0.98 or more. Further, an ellipticity of 0.98 or more was obtained for linearly polarized light having a wavelength of 785 nm, and practically sufficient characteristics were obtained.

  Here, the ellipticity with respect to the wavelength when linearly polarized light around the wavelength of 660 nm is incident on the broadband wavelength plate 102 in accordance with the polarization direction 203 is shown by a solid line in FIG. For comparison, the ellipticity with respect to the wavelength when the linearly polarized light having the same wavelength is incident on the conventional broadband quarter-wave plate is indicated by a broken line. On the other hand, the ellipticity with respect to the wavelength when linearly polarized light around the wavelength of 785 nm is incident on the broadband wave plate 102 in accordance with the polarization direction 204 is shown by a solid line in FIG. The ellipticity with respect to the wavelength when the linearly polarized light having the same wavelength is incident on the wave plate is indicated by a broken line.

  4 (a) and 4 (b), in the conventional broadband quarter-wave plate, the ellipticity becomes 0.9 or less when the polarization directions of the incident two-wavelength linearly polarized light are different from each other. It can be seen that it does not function as a half-wave plate. On the other hand, in the broadband wavelength plate 102 according to the present invention, a good ellipticity is obtained for two-wavelength linearly polarized light having different polarization directions.

  Next, with respect to the direction of the slow axis 201 of the broadband wave plate 102, the outer phase is cut to 5 mm × 5 mm with a dicing saw with reference to the direction of −95 ° to produce a broadband phase element.

  The transmitted wavefront aberration of the broadband phase element fabricated as described above was measured using a He—Ne laser beam having a wavelength of 633 nm. As a result, the value of the root mean square was 25 mλrms or less, and it was confirmed that the broadband phase element could be used as an optical element.

  In addition, when the Rd values for linearly polarized light with wavelengths of 660 nm and 785 nm of the broadband wave plate 102 are indicated by arc degrees, they are about 100 ° and 80 °, respectively, and the difference is about 20 °. Therefore, the incident direction of the two-wavelength linearly polarized light is approximately twice the angle formed by each other.

[Example 2]
In the first embodiment, the angle formed by the polarization directions of the two-wavelength linearly polarized light incident on the broadband wave plate 102 is α = 10 °, but in this embodiment, 0 ° ≦ α ≦ 90 °. The characteristic of the preferable broadband waveplate calculated | required as a result of convergence calculation in is shown.

  In practice, a broadband wavelength plate can be produced by substantially the same production procedure and configuration as in Example 1. The difference from the manufacturing procedure and configuration of Example 1 is the spacer diameter and the bonding angle between the first phase plate 103 and the second phase plate 104.

  Here, the definition of the relationship between the polarization direction of incident linearly polarized light and the direction of the slow axis of the laminated first and second phase plates 103 and 104 will be described with reference to FIG.

  The polarization direction of the linearly polarized light with a wavelength of 660 nm as the first incident light is 207, the polarization direction of the linearly polarized light with a wavelength of 785 nm as the second incident light is 208, and the slow axis of the first phase plate 103 on the incident side is The direction is 205, and the direction of the slow axis of the second phase plate 104 on the emission side is 206.

  In addition, the angle formed by the polarization direction 207 of the linearly polarized light with a wavelength of 660 nm and the polarization direction 208 of the linearly polarized light with a wavelength of 785 nm is α, the polarization direction 207 of the linearly polarized light with a wavelength of 660 nm, and the incident-side first phase plate 103 The angle formed by the slow axis direction 205 is θ, and the angle formed by the slow axis direction 205 of the first phase plate 103 and the slow axis direction 206 of the second phase plate 104 is β.

  FIG. 6A shows the relationship of the ellipticity with respect to α when linearly polarized light having a wavelength of 660 nm is incident, and FIG. 6B shows the relationship of the ellipticity with respect to α when linearly polarized light having a wavelength of 785 nm is incident. Show. In any case, the ellipticity is in the vicinity of 1 for α in the range of 0 ° ≦ α ≦ 90 °. Therefore, it can be seen that the broadband wavelength plate according to this example functions as a quarter-wave plate for incident light of two wavelengths having different polarization directions.

  Next, the relationship between α and θ is shown in FIG. It can be seen that α and θ have a linear relationship in α of 5 ° ≦ α ≦ 85 °.

Next, FIG. 8 shows the relationship between α and the ratio of the retardation value Rd ₂ of the _second phase plate to the retardation value Rd ₁ of the first phase plate. The retardation ratio Rd ₂ / Rd ₁ indicates that the retardation ratio Rd ₂ / Rd ₁ substantially increases with an increase in α with a constant amplitude.

  FIG. 9 shows the relationship between α and β. It can be seen that β takes a value of 30 ° ≦ β ≦ 60 ° with respect to α of 0 ° ≦ α ≦ 90 °.

Next, the value of the retardation value Rd ₁ of the first phase plate with respect to α is shown in FIG. Here, the solid line is Rd ₁ (λ ₁ ) when linearly polarized light with a wavelength λ ₁ = 660 nm is used for incident light, and the broken line is Rd ₁ when linearly polarized light with a wavelength λ ₂ = 785 nm is used for incident light. (Λ ₂ ). From the solid line and broken line in FIG. 10, at a wavelength λ satisfying λ ₁ ≦ λ ≦ λ ₂ , 80 ° ≦ Rd ₁ (λ) ≦ 100 ° with respect to α of 7 ° ≦ α ≦ 83 °, which is approximately 4 minutes. It can be seen that it functions as a single wavelength plate.

Further, in order to show the retardation dispersion characteristic of the second phase plate, the difference Rd ₂ (λ ₁ ) −Rd ₂ (λ ₂ ) due to the wavelength λ of the retardation value Rd ₂ of the second phase plate is halved. FIG. 11 shows the relationship between α and the value obtained by subtracting α. It can be seen that the retardation dispersion value is 10 or less in the range of 0 ° ≦ α <90 °.
That is, Rd ₂ and α satisfy the relationship of the above formula (2).

  Note that the angle θ, the angle β, the film thickness of the polymer liquid crystals 3 and 4, the Rd value of the first phase plate 103 with respect to the wavelength of 660 nm, the Rd value of the first phase plate 103 with respect to the wavelength of 785 nm, and the second phase plate 104 FIG. 12 shows values of the Rd value for the wavelength 660 nm and the Rd value for the wavelength 785 nm of the second phase plate 104 with respect to the angle α.

[Example 3]
In the second embodiment, an example of calculating the characteristics when the broadband wave plate 102 according to the present invention is applied to a quarter wave plate has been shown. However, in this embodiment, the broadband wave plate 102 is divided into two minutes. An example of calculation of characteristics when applied to a single wavelength plate is shown.

  Also in this case, the broadband wavelength plate can be manufactured by the manufacturing procedure and configuration almost the same as those of the first embodiment. The difference from the manufacturing procedure and configuration of Example 1 is that a liquid crystal material is a polymer liquid crystal in which the ratio of the retardation value of linearly polarized light with a wavelength of 660 nm and the retardation value of linearly polarized light with a wavelength of 785 nm is about 0.821. It was a point. The diameters of the spacers used for manufacturing the first phase plate and the second phase plate are 0.9 μm and 4.2 μm, respectively. The accuracy of the spacer diameter was 0.1 μm, and the accuracy of the bonding angle between the first phase plate and the second phase plate was 1 °.

  Here, the definition of the relationship between the polarization direction of incident linearly polarized light and the direction of the slow axis of the laminated first and second phase plates will be described with reference to FIG.

  The polarization direction of linearly polarized light with a wavelength of 660 nm as the first incident light is 211, the polarization direction of linearly polarized light with a wavelength of 785 nm as the second incident light is 212, and the direction of the slow axis of the first phase plate on the incident side Is 209, and the direction of the slow axis of the second phase plate on the emission side is 210.

  In this embodiment, the angle formed by the polarization direction 211 of the linearly polarized light with a wavelength of 660 nm and the polarization direction 212 of the linearly polarized light with the second wavelength of 785 nm is 45 °, the polarization direction 211 of the linearly polarized light with a wavelength of 660 nm, The angle formed by the slow axis direction 209 of the phase plate is 74 °, and the angle formed by the slow axis direction 209 of the first phase plate and the slow axis direction 210 of the second phase plate is 49 °. It comprised so that it might become.

  First, in a state before the first phase plate and the second phase plate constituting the broadband wave plate of the present embodiment are laminated, the retardation value when linearly polarized light having a wavelength of 660 nm is incident on these phase plates is 198 nm and 925 nm, respectively.

  Next, the first phase plate and the second phase plate were laminated as described above, and the ellipticity of the broadband wavelength plate was measured using linearly polarized light of semiconductor laser light having wavelengths of 660 nm and 785 nm. As a result, when linearly polarized light with a wavelength of 660 nm is used, the ellipticity is 0.05 or less and the azimuth angle is 71.3 °. When linearly polarized light with a wavelength of 785 nm is used, the ellipticity is 0.05 or less, The angle was 71.6 °. That is, it was confirmed that when two linearly polarized light having different wavelengths and polarization directions are incident, they can be converted into linearly polarized light having the same polarization direction. In addition, this broadband wavelength plate was sufficiently practically used as a broadband half-wave plate.

[Example 4]
In the third embodiment, the angle formed by the polarization directions of the two-wavelength linearly polarized light incident on the broadband wave plate 102 is α = 45 °, but in this embodiment, 0 ° ≦ α ≦ 90 °. Fig. 5 shows the characteristics of a preferable broadband wave plate obtained as a result of convergence calculation.

  The difference from the manufacturing procedure and configuration of Example 3 is that the diameter of the spacer, the bonding angle between the first phase plate 103 and the second phase plate 104, and the wavelength region in which the liquid crystal material to be used is used for a DVD. The difference between the ordinary light refractive index and the extraordinary light refractive index at 660 nm, which is the central wavelength, is Δn≈0.035, and the difference between the ordinary light refractive index and the extraordinary light refractive index, at 785 nm, which is the central wavelength of the wavelength region used for CD. The point is 0.034.

  When linearly polarized light with a wavelength of 660 nm is incident and when linearly polarized light with a wavelength of 785 nm is incident, the ellipticity with respect to α is almost 0 (zero) with respect to α in the range of 0 ° ≦ α ≦ 90 °. ) Therefore, it can be seen that the broadband wavelength plate according to the present example functions as a half-wave plate for incident light of two wavelengths having different polarization directions.

  Next, the relationship between α and θ is shown in FIG. It can be seen that α and θ are approximately in a linear relationship when α is 5 ° ≦ α ≦ 85 °.

Next, the ratio of the retardation values Rd ₁ second of the first phase plate 103 and the retardation values Rd ₂ phase plate 104, the relationship between α in Figure 16. The retardation ratio Rd ₂ / Rd ₁ indicates that the retardation ratio increases substantially as α increases.

  FIG. 17 shows the relationship between α and β. It can be seen that β generally takes a value of 30 ° ≦ β ≦ 60 ° for α of 20 ° ≦ α ≦ 90 °.

Next, the value of the retardation value Rd ₁ of the first phase plate 103 with respect to α is shown in FIG. Here, the solid line is Rd ₁ (λ ₁ ) when linearly polarized light with a wavelength λ ₁ = 660 nm is used for incident light, and the broken line is Rd ₁ when linearly polarized light with a wavelength λ ₂ = 785 nm is used for incident light. (Λ ₂ ). From the solid line and broken line in FIG. 18, at a wavelength λ satisfying λ ₁ ≦ λ ≦ λ ₂ , approximately 80 ° ≦ Rd ₁ (λ) ≦ 100 ° with respect to α of 10 ° ≦ α ≦ 80 °, which is approximately 4 minutes. It can be seen that it functions as a single wavelength plate.

Further, in order to show the retardation dispersion characteristic of the second phase plate 104, the difference Rd ₂ (λ ₁ ) −Rd ₂ (λ ₂ ) due to the wavelength λ of the retardation value Rd ₂ of the second phase plate 104 is divided into two. FIG. 19 shows the relationship between the value obtained by subtracting α from 1 and α. It can be seen that the retardation dispersion value is generally 10 or less in the range of 10 ° ≦ α ≦ 80 °.
That is, Rd ₂ and α satisfy the relationship of the above formula (2).

  Note that the angle θ, the angle β, the film thickness of the polymer liquid crystals 3 and 4, the Rd value of the first phase plate 103 with respect to the wavelength of 660 nm, the Rd value of the first phase plate 103 with respect to the wavelength of 785 nm, and the second phase plate 104 FIG. 20 shows values of the Rd value for the wavelength 660 nm and the Rd value for the wavelength 785 nm of the second phase plate 104 with respect to the angle α.

  As described above, the broadband wavelength plate according to the present invention has an effect that the same retardation value can be obtained for linearly polarized light having a plurality of wavelengths whose directions of polarization are different from each other. It is useful as a broadband wavelength plate that does not require a polarizer for aligning the polarization plane of the linearly polarized light.

Sectional drawing which shows typically the structure of the broadband wavelength plate in this invention Sectional drawing which shows typically the structure of the broadband wavelength plate in this invention The schematic diagram which shows the relationship between the slow axis of the phase plate which concerns on Example 1 of this invention, and the polarization direction of two linearly polarized light which injects. (A) Characteristic diagram showing the relationship between wavelength and ellipticity when linearly polarized light in the vicinity of a wavelength of 660 nm is incident on the broadband wavelength plate according to Example 1 and the conventional broadband quarter-wave plate (b) wavelength 785 nm FIG. 5 is a characteristic diagram showing the relationship between the wavelength and the ellipticity when near linearly polarized light is incident on the broadband wavelength plate according to the first embodiment and the conventional broadband quarter-wave plate. The schematic diagram which shows the relationship between the slow axis of the broadband wavelength plate which concerns on Example 2, and the polarization direction of a linearly polarized light (A) Characteristic diagram showing the ellipticity of linearly polarized light with a wavelength of 660 nm with respect to the polarization direction difference α when two wavelengths of linearly polarized light are incident on the broadband wavelength plate. (B) Two wavelengths of linearly polarized light are incident on the broadband wavelength plate. Characteristic diagram showing the ellipticity of linearly polarized light with a wavelength of 785 nm with respect to the difference α in the polarization direction A characteristic diagram showing an angle θ formed by the polarization direction of the first linearly polarized light and the slow axis of the first phase plate with respect to the difference α of the polarization direction of the linearly polarized light of two wavelengths. Characteristic diagram showing the relationship between the difference α in the polarization direction of two-wavelength linearly polarized light and Rd ₂ / Rd ₁ A characteristic diagram showing the relationship between the polarization direction difference α of two-wavelength linearly polarized light and the angle β formed by the slow axes of the first and second phase plates. The characteristic diagram which shows the relationship between the difference (alpha) of the polarization direction of 2 wavelength linearly polarized light, and the retardation value Rd1 of a _1st phase plate. Characteristic diagram showing the relationship between the difference α between the polarization directions of two wavelengths of linearly polarized light and the value on the left side of equation (2) The characteristic diagram which shows the value with respect to angle (alpha) of angle (theta), angle (beta), the film thickness and retardation value of the polymer liquid crystal which the 1st and 2nd phase plate has The schematic diagram which shows the relationship between the slow axis of the broadband wavelength plate which concerns on Example 3, and the polarization direction of a linearly polarized light. Schematic diagram showing the configuration of an optical pickup according to the present invention. FIG. 6 is a characteristic diagram showing an angle θ formed by the polarization direction of the first linearly polarized light and the slow axis of the first phase plate with respect to the difference α between the polarization directions of the two-wavelength linearly polarized light in Example 4. FIG. 6 is a characteristic diagram showing the relationship between the polarization direction difference α of two-wavelength linearly polarized light in Example 4 and Rd ₂ / Rd ₁ . FIG. 6 is a characteristic diagram showing the relationship between the difference α between the polarization directions of two-wavelength linearly polarized light in Example 4 and the angle β formed by the slow axes of the first and second phase plates. The characteristic view which shows the relationship between the difference (alpha) of the polarization direction of the linearly polarized light of 2 wavelengths in Example 4, and the retardation value Rd1 of a _1st phase plate. The characteristic view which shows the relationship between the difference (alpha) of the polarization direction of 2 wavelength linearly polarized light in Example 4, and the value of the left side of Formula (2). Characteristic diagram showing values of angle θ, angle β, film thickness and retardation value of polymer liquid crystal included in first and second phase plates with respect to angle α in Example 4

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Incident side phase plate 2 Outgoing side phase plate 3, 4 Polymer liquid crystal 5, 6 Glass substrate 7, 8 Polyimide orientation film 9 UV adhesive 11 1st semiconductor laser (light source)
12 Second semiconductor laser (light source)
21 First half mirror 22 Second half mirror 31 Collimator lens 32 Objective lens 33 Optical disc (optical recording medium)
34 Photodetector 101 Broadband wave plate 102 Broadband wave plate 103 First phase plate 104 Second phase plate 201 Slow axis 202 of first phase plate Slow axis 203 of second phase plate Polarization direction (polarization plane) )
204 Polarization direction (polarization plane)
205 Slow axis 206 of the first phase plate 206 Slow axis 207 of the second phase plate Polarization direction (polarization plane)
208 Polarization direction (polarization plane)

Claims (6)

A first phase plate and a second phase plate formed of a birefringent material are stacked on each other;
At least a first polarized light having a wavelength λ ₁ and a second polarized light having a wavelength λ ₂ longer than λ ₁ are incident on the first phase plate as linearly polarized light in different directions, and a predetermined polarization state A broadband wave plate used to be emitted from the second phase plate,
The angle at which the optical axis of the first phase plate and the polarization direction of the first polarization intersect is θ,
The angle formed by the polarization direction of the first polarization and the polarization direction of the second polarization is α,
The angle formed by the optical axis of the first phase plate and the optical axis of the second phase plate is β,
The retardation value at an arbitrary wavelength λ of the first phase plate is Rd ₁ (λ),
When the retardation value at an arbitrary wavelength λ of the second phase plate is Rd ₂ (λ),
Rd ₁ (λ) satisfies the relationship of 80 ° ≦ Rd ₁ (λ) ≦ 100 ° with respect to the wavelength λ satisfying the relationship of λ ₁ ≦ λ ≦ λ ₂ .
Β satisfies the relational expression of 30 ° ≦ β ≦ 60 °,
Rd ₂ (λ ₁ ), Rd ₂ (λ ₂ ), and α satisfy the relational expression | {Rd ₂ (λ ₁ ) −Rd ₂ (λ ₂ )} / 2−α | ≦ 10 °,
The θ is emitted from the first phase plate by the difference Rd ₂ (λ ₁ ) −Rd ₂ (λ ₂ ) of the phase difference given to the two polarized lights by passing through the second phase plate. The latitude difference between the latter two polarizations on the Poincare sphere is canceled and the polarization state of the two polarizations is set to be elliptically polarized light having an ellipticity of 90% or more after passing through the second phase plate. A broadband wave plate characterized by that. A first phase plate and a second phase plate formed of a birefringent material are stacked on each other;
At least the wavelength λ ₁ A first polarization having λ and the λ ₁ Longer wavelength λ ₂ A broadband wavelength that is used so as to be incident on the first phase plate as linearly polarized light in directions different from each other, changed to a predetermined polarization state, and emitted from the second phase plate. A board,
The angle at which the optical axis of the first phase plate and the polarization direction of the first polarization intersect is θ,
The angle formed by the polarization direction of the first polarization and the polarization direction of the second polarization is α,
The angle formed by the optical axis of the first phase plate and the optical axis of the second phase plate is β,
The retardation value at an arbitrary wavelength λ of the first phase plate is Rd. ₁ (Λ)
The retardation value at an arbitrary wavelength λ of the second phase plate is Rd. ₂ (Λ)
λ ₁ ≦ λ ≦ λ ₂ For a wavelength λ satisfying the relational expression ₁ (Λ) is 80 ° ≦ Rd ₁ Satisfies the relational expression (λ) ≦ 100 °,
Β satisfies the relational expression of 30 ° ≦ β ≦ 60 °,
Rd ₂ (Λ ₁ ), Rd ₂ (Λ ₂ ) And α are | {Rd ₂ (Λ ₁ -Rd ₂ (Λ ₂ )} / 2−α | ≦ 10 ° is satisfied,
The θ is a difference Rd between phase differences given to the two polarized lights by passing through the second phase plate. ₂ (Λ ₁ -Rd ₂ (Λ ₂ ) Cancels the difference in latitude on the Poincare sphere of the two polarizations after exiting the first phase plate, and the polarization state of the two polarizations after passing through the second phase plate is 10% ellipticity. A broadband wave plate, which is set to have the following elliptically polarized light. The retardation value Rd of the first phase plate according to the increase of α ₁ The retardation value Rd of the second phase plate with respect to ₂ Ratio Rd ₂ / Rd ₁ The broadband wave plate according to claim 1 or 2, wherein substantially increases. The broadband wave plate according to any one of claims 1 to 3, wherein the birefringent material is composed of a thin film layer of polymer liquid crystal. 5. The broadband wave plate according to claim 1, wherein θ is set so as to substantially increase in accordance with an increase in α. A light source that emits light of a plurality of wavelengths;
An objective lens for condensing the light emitted from the light source onto an optical recording medium;
In an optical head device comprising a photodetector for detecting light reflected by the optical recording medium,
An optical head and wherein the broadband wave plate according is disposed in an optical path from claim 1 to any one of 5 between the light source and the objective lens.

Images