JP5332322B2 - Optical rotator and optical head device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a polarizer capable of easily controlling the polarization state of incident light, and controlling the polarization state so as to reduce the wavelength dependency of light and use temperature dependency. <P>SOLUTION: The polarizer is constituted by arranging alignment film 13a, 13b, respectively, on transparent substrates 11a, 11b, and is composed of a liquid crystal layer 14 consisting of 90&deg; twisted nematic liquid crystal or polymer liquid crystal. The polarizer 10b consisting of the nematic liquid crystal and in which transparent electrodes 12a, 12b are arranged, respectively, can change the alignment directions of the liquid crystal molecules. For example, the thickness of the liquid crystal layer 14 can be set at a value greater than the intermediate value between the thickness which is the secondary Morgan minimum conditions of wavelength of incident light &lambda;<SB>1</SB>and the thickness of the primary Morgan minimum conditions of a wavelength &lambda;2(&lambda;<SB>1</SB>&lt;&lambda;<SB>2</SB>). <P>COPYRIGHT: (C)2010,JPO&amp;INPIT

Description

  The present invention, for example, can rotate linearly polarized light that is incident in an optical system such as optical storage, optical communication, or optical imaging, and can control the polarization state of the emitted light by controlling the applied voltage. Related to optical rotators.

  In recent years, optical head devices are used not only for CDs, DVDs, and magneto-optical disks, but also for high-density optical recording media (hereinafter referred to as “optical disks”) such as “Blu-ray” (registered trademark: BD) and HD-DVD. Corresponding technology has been developed. In particular, in an optical head device compatible with BD, HD-DVD, DVD, and CD, optical components having different performances with respect to three wavelengths of blue laser, red laser, and near infrared laser, and these three wavelengths. In addition, optical components having less wavelength dependency and equivalent performance are appropriately combined. Among them, a function is also required to convert the polarization states of these three wavelengths equally. For example, in the case where the linear polarization direction of the light emitted from each light source is orthogonal to the axis that is the linear transmission direction of the polarization beam splitter, the linearly polarized light of three wavelengths is the linear transmission direction of the polarization beam splitter. An optical rotator (multi-wavelength optical rotator) that functions with respect to multiple wavelengths is required so that the polarization direction is rotated 90 ° parallel to the axis.

  For example, a polarization converter described in Patent Document 1 has been proposed as a multiwavelength rotator that converts light incident at three different wavelengths into orthogonal polarization states. By disposing this polarization converter in an optical path in which three different wavelengths of light are shared, light of any wavelength can be converted by 90 ° with respect to incident light and emitted. Further, by using the polarization converter in the optical head device, the polarization state can be efficiently converted with a small number of parts.

JP 2007-304572 A

  However, the polarization converter described in Patent Document 1 has a characteristic that conversion efficiency is likely to change due to variations in the wavelength of incident light. For example, if the oscillation wavelength of the semiconductor laser that is the light source varies, or if the oscillation wavelength of the semiconductor laser changes due to different operating temperatures, the conversion efficiency will decrease, so the wavelength of the incident light will be accurately controlled. There was a problem that had to be. Further, when a polarization converter is applied to the optical head device, the polarization conversion function is fixed, and thus there is a problem that it is difficult to switch the polarization conversion function depending on the situation.

The present invention has been made to solve the above-described problem, and includes a pair of transparent substrates arranged in parallel and a liquid crystal layer filled with liquid crystal in a space opposed to the pair of transparent substrates, and has at least a wavelength λ. ₁ and an optical rotator in which light that is linearly polarized light having a wavelength λ ₂ larger than the wavelength λ ₁ is incident, wherein the liquid crystal is made of nematic liquid crystal or polymer liquid crystal, and the liquid crystal has a thickness of the liquid crystal layer. in a twist alignment state is twisted in a spiral direction, the thickness d _.lambda.21 as the primary Morgan minimum conditions of the wavelength lambda ₂ at room temperature is, the thickness d _Ramuda1n more than the n-th order Morgan minimum conditions of the wavelength lambda ₁ in and the wavelength lambda ₁ of the (n + 1) with a thickness of less than that the next Morgan minimum condition _{d λ1 (n + 1),} (d λ1n + d λ1 (n + 1)) / 2 or more der Ri (n is Natural number), the thickness of the liquid crystal layer provides polarization rotator is d _.lambda.21 greater than.

In addition, a transparent electrode for applying a voltage to the liquid crystal layer is provided, and the twist alignment state and a vertical alignment state in which the liquid crystal is aligned in the thickness direction of the liquid crystal layer are switched according to the voltage applied to the liquid crystal layer. An optical rotator as described above is provided. The thickness of the liquid crystal layer provides polarization rotator according to the intermediate value greater than der Ru above said the front KiAtsu of d _.lambda.21 thickness d λ1 _{(n + 1).}

  With this configuration, when expressing a specific polarization state, such as conversion to linear polarization orthogonal to linearly polarized light incident at two or more different wavelengths, changes in the output polarization state due to wavelength dependence and use temperature dependence Can be reduced. In addition, by using an active optical rotator by voltage control, it is possible to easily change the polarization state of incident light and emit it without changing the polarization state. By utilizing this, for example, when this optical rotator is used in an optical head device using light of three different wavelengths, information recording and / or reproduction (hereinafter referred to as “recording / reproduction”) on a stable optical disk is performed. ) And the quality of the optical head device is improved. Furthermore, it is possible to realize an optical head device that switches the traveling direction of light by controlling the polarization state of light by voltage control.

  The present invention can efficiently convert the polarization state of a plurality of lights having different wavelengths over a wide band, and can easily switch between a conversion state and a non-conversion state, and also converts a change in temperature. An optical rotator with little variation in efficiency can be provided.

(First embodiment)
FIG. 1A and FIG. 1B are diagrams showing a conceptual configuration of optical rotators 10a and 10b according to the present embodiment, respectively. The optical rotator 10a includes a substrate in which alignment films 13a and 13b are stacked on transparent substrates 11a and 11b, and a liquid crystal layer 14 in which liquid crystal is sandwiched between the substrates. The optical rotator 10b is composed of a substrate in which transparent electrodes 12a and 12b are stacked on the transparent substrates 11a and 11b, respectively, and a liquid crystal layer 14 in which liquid crystal is sandwiched between the substrates. It is an active optical rotator. The transparent electrodes 12a and 12b are connected to the voltage control device by electric wiring (not shown). Here, the optical rotator 10a without the transparent electrodes 12a and 12b has the same characteristics as when the voltage of the optical rotator 10b is not applied, and the optical rotator 10a is explained by explaining the function of the optical rotator 10b when no voltage is applied. The description of the function of is assumed to be substituted. When no voltage is applied to the optical rotator 10b, the liquid crystal molecules on the surface in contact with the alignment film 13a are aligned in the X direction and the liquid crystal molecules on the surface in contact with the alignment film 13b are aligned in the Y direction as shown in FIG. The major axis direction of the liquid crystal molecules is twisted and distributed in the thickness direction (Z direction) of the liquid crystal layer 14.

  As long as the transparent substrate is transparent to incident light, various materials such as a resin can be used. However, if an optically isotropic material such as glass or quartz glass is used, the influence of birefringence on the transmitted light can be used. Is preferable. As a transparent electrode, an oxide transparent conductive film such as an ITO (tin oxide doped indium oxide) film, an AZO (aluminum doped zinc oxide) film, or a GZO (gallium doped zinc oxide) film can provide high transparency and conductivity. Therefore, it is preferably used. The alignment film may be formed by rubbing a polyimide film or may be formed by oblique deposition of an inorganic material such as SiO.

  The liquid crystal layer 14 is composed of a twisted nematic liquid crystal cell (hereinafter referred to as a TN liquid crystal cell) in which the alignment direction of the liquid crystal molecules 15a is twisted in a thickness direction of, for example, 90 ° when no voltage is applied. The description will be made assuming that all the light incident on the TN liquid crystal cell travels in the Z direction. For example, the following Jones matrix can be used for polarization conversion in which linearly polarized light oscillating in the X direction is incident and emitted in the Y direction (a direction orthogonal to the incident direction) as in the TN liquid crystal cell. . Note that the twist angle (hereinafter, twist angle) of the liquid crystal molecules in the thickness direction is not limited to 90 °, and the Jones matrix relationship is established even at different angles.

The ordinary refractive index of the liquid crystal in the TN liquid crystal cell is n _o , the extraordinary refractive index is _ne , the refractive index anisotropy is Δn (= | n _e −n _o |), the twist angle is Φ _t , When the thickness is d, the angle of the liquid crystal director on the substrate surface on which light is incident (hereinafter, the pretwist angle) is ψ _i , and the wavelength of incident light is λ, the Jones matrix W _TN of the TN liquid crystal cell is It is expressed as equation (1).

  Note that R (Ψ) represents a rotation matrix, as shown in Equation (2).

  Here, sin θ = 0, that is, m when the expression (3) is established is referred to as m-th order Morgan limit (Morgan minimum condition).

The J _TN of the formula (1) when the Morgan limit of the formula (3) is satisfied is represented by the formula (4), and the TN liquid crystal cell functions as an optical rotator with a twist angle Φ _t .

Here, assuming that the pre-twist angle Ψ _i = 0 ° and the twist angle Φ _t = 90 ° of the TN liquid crystal cell and linearly polarized light of 0 ° is incident, a polarization direction (90 °) component orthogonal to the polarization direction of the incident light When the rate of conversion to is defined as the polarization conversion efficiency I, it is expressed as in equation (5).

  As described above, the thickness d of the TN liquid crystal cell satisfying the Morgan minimum condition of the formula (3) varies depending on the wavelength λ of the incident light, and there exists an optical rotator having incident light of any wavelength and all having the same optical rotation function. do not do. Furthermore, since the refractive index of the liquid crystal changes depending on the operating temperature, there is a limit to the realization of an optical rotator having a constant optical rotatory function in a wide temperature range, but the optical rotator of the present embodiment has a thickness of the liquid crystal layer 14. By adjusting, the same optical rotation characteristic can be developed for two or more different wavelengths.

Here, the wavelengths of two different incident lights will be specifically described as λ ₁ = 405 nm and λ ₂ = 660 nm (λ ₁ <λ ₂ ). As a design method, the thickness (= d _λ12 ) of the liquid crystal layer that becomes the limit of Morgan with m = 2 with respect to the wavelength λ ₁ (hereinafter referred to as the secondary Morgan minimum condition) under the Morgan minimum condition of the formula (3). Think. Note that the two numbers following 'λ' in the notation of thickness (for example, d _λ12 ) are the numbers indicated by the wavelengths, such as wavelength λ ₁ , wavelength λ _{2 ,.} It is represented by a number corresponding to m of the Morgan minimum condition.

Next, consider the thickness (= d _λ21 ) of the liquid crystal layer that becomes the limit of Morgan with m = 1 with respect to the wavelength λ ₂ (hereinafter referred to as the primary Morgan minimum condition). FIG. 2A shows a perspective view when no voltage is applied to the optical rotator 10b. As shown in this figure, the liquid crystal molecules 15a are distributed between the transparent substrates 11a and 11b at a twist angle Φ _t = 90 °. Yes. Here, FIG. 2B shows an example of the polarization conversion efficiency when the thickness of the liquid crystal layer is changed in the case where X-direction linearly polarized light traveling in the Z direction is incident from the transparent substrate 11a side. Here, the polarization conversion efficiency means the ratio of light that is orthogonal to the polarization direction of incident light, that is, converted from linearly polarized light in the X direction to the Y direction.

As shown in FIG. 2B, the polarization conversion efficiency becomes the highest under the condition of the thickness of the liquid crystal layer that satisfies the Morgan minimum condition. For example, when 405 nm light is incident, the thickness of the liquid crystal layer is about 3.7 μm, and the primary Morgan minimum condition is about 8.3 μm, and the secondary Morgan minimum condition is about 8.3 μm. Further, when the polarization state of the incident light is the same and the wavelengths are different, the thickness of the liquid crystal layer that becomes the primary Morgan minimum condition takes a larger value as the wavelength becomes longer. The thickness of lambda ₃ as 780nm wavelengths _{(λ 1 <λ 2 <λ} 3) also lambda ₁ when light is incident, the liquid crystal layer to be the primary Morgan minimum conditions for lambda ₂ is large. Therefore, when light having wavelengths of λ ₁ and λ ₂ is incident, in order to increase the polarization conversion efficiency of light of both wavelengths, the thickness of the liquid crystal layer that satisfies the first order Morgan minimum condition of λ ₂ is exceeded. I just need it.

  Next, changes in polarization conversion efficiency due to temperature changes will be described. Usually, the liquid crystal tends to have a smaller refractive index anisotropy Δn as the temperature rises. From equation (3), when Δn is reduced, the TN liquid crystal cell thickness d needs to be increased to satisfy the Morgan minimum condition. As an example, FIG. 3 shows a change in polarization conversion efficiency with respect to the thickness of the liquid crystal layer when only 405 nm light is incident and only the temperature condition is changed. As a result, as the temperature increases, the thickness of the liquid crystal layer that satisfies the primary and secondary Morgan minimum conditions increases, so this characteristic must be considered in order to maintain high polarization conversion efficiency under a wide temperature range. There is a need to.

Thus, in order to realize an optical rotator in which light of two or more different wavelengths is incident and a wider use temperature range is required, the thickness of the liquid crystal layer may be set as follows. At room temperature, lambda ₂ in the primary Morgan minimum conditions to become a thickness d _.lambda.21 is, for example, the primary Morgan minimum condition d _{[lambda] 11} in lambda _1, located between the secondary Morgan minimum condition d _{[lambda] 12} in lambda _1, d _.lambda.21 There consider the case where _{(d λ11 + d λ12) /} 2 or more. At this time, when d _λ12 is larger than a value between d _λ21 and d _λ12 , (d _λ21 + d _λ12 ) / 2, high polarization conversion efficiency is obtained for incident light having wavelengths of at least λ ₁ and λ _2. Can do. In addition, normal temperature here shall be the temperature between 20-30 degrees. In addition, although the thickness of the liquid crystal layer may be set to be thicker, the response speed at the time of voltage application is slowed in proportion to the square of the thickness, or the amount of transmitted light is reduced by light absorption. More preferably, the thickness of the liquid crystal layer is as thin as possible while satisfying the above conditions. The upper limit of the thickness is about the lower limit +4 μm, and it is preferable to set the thickness within this range.

In addition, although the wavelength λ ₁ and the wavelength λ ₂ have been described as 405 nm and 660 nm in the above example as incident light of two different wavelengths, the present invention is not limited to this. _{Depending on} the combination of the wavelength λ ₁ and the wavelength λ ₂ , as shown in FIG. 4A or FIG. 4B, the thickness d _{λ21 that} is the primary Morgan minimum condition of the wavelength λ _{2 is} the secondary Morgan of the wavelength λ _1. The thickness d _λ12 may be larger than the minimum condition. Next, in consideration of these, the lower limit of the thickness of the liquid crystal layer that provides high polarization conversion efficiency will be generalized and described. Incidentally, the solid line lambda ₁ characteristic shown in FIG. 4 (a) and 4 (b), together with the dotted line represents a lambda ₂ properties, both working temperature to room temperature.

4 (a) is a thickness d _.lambda.21 is located between the thickness d _Ramuda13 as a tertiary Morgan minimum conditions thickness d _{[lambda] 12} and the wavelength lambda ₁ to be secondary Morgan minimum at a wavelength of lambda _1, further , D _λ21 ≧ (d _λ12 + d _λ13 ) / 2. The lower limit of the thickness of the liquid crystal layer at this time is preferably as long as d _.lambda.21 greater than, and more preferably it is in consideration of the temperature change _{(d λ21 + d λ13) /} 2 greater than.

FIG. 4 (b), the thickness d _.lambda.21 is located between the thickness d _Ramuda13 as a tertiary Morgan minimum conditions thickness d _{[lambda] 12} and the wavelength lambda ₁ to be secondary Morgan minimum at a wavelength of lambda _1, further , D _λ21 <(d _λ12 + d _λ13) ) / 2. The lower limit of the thickness of the liquid crystal layer at this time is preferably as long as d _{[lambda] 12} greater than, and more preferably it is in consideration of the temperature change _{(d λ12 + d λ21) /} 2 greater than.

Further, the combination of the wavelength lambda ₁ and wavelength lambda _2, these generally represent the thickness d _.lambda.21 is, the thickness of which becomes n-th order Morgan minimum at a wavelength of _{λ 1} d λ1n the wavelength lambda ₁ of the (n + 1) Consider a case where the thickness is between the thicknesses d _{λ1 (n + 1)} which is the next Morgan minimum condition (n is a natural number) and d _λ21 ≥ (d _λ1n + d _{λ1 (n + 1)} ) / 2. The lower limit of the thickness of the liquid crystal layer at this time is preferably a value larger than d _λ21 , and more preferably a value larger than (d _λ21 + d _{λ1 (n + 1)} ) / 2 in consideration of a temperature change. Similarly, the thickness d _.lambda.21 is located between the wavelength lambda ₁ of the n-th Morgan the minimum conditions thickness d _Ramuda1n the wavelength lambda ₁ of the (n + 1) becomes the following Morgan minimum conditions thickness d _{.lambda.1 (n + 1),} Further, consider a case where d _λ21 <(d _λ1n + d _{λ1 (n + 1)} ) / 2. The lower limit of the thickness of the liquid crystal layer at this time is preferably as long as d _Ramuda1n greater than, and more preferably it is in consideration of the temperature change _{(d λ1n + d λ21) /} 2 greater than.

  Up to now, as shown in FIG. 2A, it has been described that linearly polarized light in the same direction as the alignment direction of the liquid crystal molecules on the transparent substrate 11a side is incident. When linearly polarized light is incident on the optical rotators 10a and 10b (when no voltage is applied) according to this embodiment, the thickness of the liquid crystal layer is adjusted to adjust the incident light in any direction. The light is emitted as linearly polarized light rotated about 90 ° with respect to the polarization direction.

  As an example, FIG. 5 shows the polarization conversion efficiency with respect to the direction of incident linearly polarized light when linearly polarized light having a wavelength of 405 nm is incident on the optical rotator 10b. That is, the polarization conversion efficiency when linearly polarized light in a direction different from the alignment direction (X direction) of the liquid crystal molecules on the transparent substrate 11a side of the optical rotator 11b in FIG. At this time, the linearly polarized light in the X direction is defined as 0 °, the counterclockwise direction is defined as the plus (+) direction, and the clockwise direction is defined as the minus (−) direction. Based on this, the solid line in FIG. 5 indicates the polarization conversion efficiency that is linearly polarized in the 90 ° direction, that is, the Y direction and becomes linearly polarized in the X direction (180 °), and the dotted line in FIG. 5 indicates the linearly polarized light in the 45 ° direction. Shows the polarization conversion efficiency when is incident and exits as linearly polarized light in the 135 ° direction. FIG. 2 (b) shows the polarization conversion efficiency in the Y direction (90 °) when any wavelength is incident as linearly polarized light in the X direction (0 °). Although not shown, any light incident in other polarization directions falls between the characteristics of the solid line and the dotted line in FIG. As a result, when the thickness of the liquid crystal layer is near the m-th order Morgan limit (m is a natural number), high polarization conversion efficiency can be obtained without depending on the polarization direction, and the liquid crystal layer is parallel to the alignment direction of the liquid crystal molecules on the transparent substrate 11a side. Incident incidence in the direction of orthogonal linearly polarized light is more preferable because the change in polarization conversion efficiency due to variations in the thickness of the liquid crystal layer is small.

  Further, when a voltage is applied to the liquid crystal layer, the liquid crystal molecules are aligned in a substantially vertical direction (Z direction) with respect to the transparent substrate surface. At this time, the light incident in the Z direction is emitted as it is without changing the polarization state. Therefore, by controlling the voltage applied to the liquid crystal layer, the polarization state can be easily changed with respect to light incident over a wide band.

(Second Embodiment)
Next, an embodiment for realizing the effect of increasing the polarization conversion efficiency as compared with the first embodiment will be described below. FIG. 6 is a schematic cross-sectional view showing a conceptual configuration of the optical rotators 20a and 20b according to the present embodiment. An optical rotator 20a shown in FIG. 6 (a) is obtained by laminating a wave plate 22 on an optical rotator 10. An optical rotator 20b shown in FIG. 6 (b) has an optical rotator 10 with wave plates 24a and 24b on both sides. Each is laminated. Further, in FIGS. 6A and 6B, the same parts as those in the first embodiment are denoted by the same reference numerals to avoid duplication of description. In FIG. 6A, when light enters from the transparent substrate 21a side parallel to the Z axis, two wave plates 22a and 22b are laminated on the light incident side. The arrangement of the wave plates is not limited to the light incident side, and may be stacked on the light emission side. The number of wave plates is not limited to two, and may be one or three or more. Good.

  FIGS. 7A and 7B are perspective views of the optical rotator 20a in which the wave plates 22 are stacked, and in the case where linearly polarized light in the X direction that travels in the Z direction is incident, It shows how light is modulated when no voltage is applied and when a voltage is applied. FIG. 7C is a perspective view showing the wave plates 22a and 22b, and the direction of the arrow indicates the direction to be the optical axis (fast axis or slow axis). As described above, the wave plate 22 is formed by laminating the two wave plates 22a and 22b, and the optical axis of each wave plate is set so as to have an angle shifted from the X direction (Y direction).

Below, the specific design method of the wave plate to laminate | stack is demonstrated. In the following description, it is assumed that linearly polarized light is incident from the wave plate 22 and is emitted from the substrate 11b, but the same principle is established even if the light is incident from the substrate 11b and emitted from the wave plate 22 due to the reciprocal nature of light. To do. The Jones matrix of the wave plate 22a as a function of the wavelength λ of the incident light and the temperature T is A (λ, T), and the Jones matrix of the wave plate 22b is B (λ, T). Here, the Jones matrix WP of the wave plate is expressed by the following equation (6), where Ψ _WP is the slow axis direction of the wave plate, Δn _WP is the refractive index anisotropy of the wave plate, and d _WP is the thickness of the wave plate. Can be expressed as

Since the Jones matrix of the liquid crystal layer also depends on λ and T, assuming that this is W _TN (λ, T), the Jones matrix of the optical rotator having this configuration is W _TN (λ, T) × B (λ , T) × A (λ, T). Therefore, B (λ, T) × A (λ, T) cancels changes due to the incident wavelength dependency and temperature dependency of W _TN (λ, T). That is, by changing the polarization state of the incident light and the polarization state of the emitted light slightly, the polarization conversion efficiency that changes depending on the incident wavelength and temperature can be adjusted to be small.

For example, the wavelength dependence and temperature dependence of polarization conversion efficiency can be reduced by designing as follows. A description will be given assuming that the wavelengths of two different incident lights are specifically λ ₁ = 405 nm and λ ₂ = 660 nm. First, the thickness of the liquid crystal layer is slightly thicker than that of the first embodiment, the thickness d near _λ12 as a secondary Morgan minimum at a wavelength of lambda _1. Thus, for light of a wavelength lambda ₁ as shown in FIG. 2 (b), the polarization conversion efficiency of the liquid crystal layer itself can be set at a value close to 100%. On the other hand, if it is near the thickness d _{[lambda] 12,} with respect to the wavelength lambda ₂ of light, although the polarization conversion efficiency by the wavelength dependence of the liquid crystal layer is reduced, the wavelength plate so that emitted as Y direction linearly polarized light 22 is laminated and corrected. For example, when the wave plate 22 is composed of a single wave plate whose optical axes are aligned in the thickness direction, the wave plate 22 is caused to function as a λ plate for light of wavelength λ ₁ . In other words, the wave plate 22 has a phase difference of 360 ° × Δn · d / λ ₁ of 360 ° × m (m is a natural number) so that the polarization state does not change greatly between incident light and outgoing light. Δn (refractive index anisotropy) and d (thickness) may be designed so as to be in the vicinity. From this, even if transmitted through the wavelength plate 22, lambda ₁ of the polarization state does not substantially change the polarization state is converted only in the liquid crystal layer.

In addition, as shown in FIG. 7C, when the wave plate 22 is formed by laminating two wave plates 22a and 22b, the retardation value (Δn · d) of the two wave plates, the slow axis (fast phase) The angle between the axis) and the polarization direction of the incident light (values other than 0 ° and 90 °) are adjusted. In other words, for the light of wavelength λ ₂ incident on the two wave plates, the decrease in polarization conversion efficiency due to the wavelength dependence of the liquid crystal layer is corrected, and the polarization state is not changed for the light of wavelength λ _1. The above conditions should be set.

The optical rotator 20b shown in FIG. 6B can be designed by the same method. When the Jones matrix of the wave plate 24a is C (λ, T), the Jones matrix of the wave plate 24b is D (λ, T), and the Jones matrix of the liquid crystal layer is W _TN (λ, T), the Jones matrix of the 20b configuration is , D (λ, T) × W _TN (λ, T) × C (λ, T). Therefore, the wavelength dependence and temperature dependence of W _TN (λ, T) are changed by slightly changing the incident polarization state at C (λ, T) and the outgoing polarization state at D (λ, T). Correct to cancel. Thereby, the wavelength dependence and temperature dependence of polarization conversion efficiency can be reduced.

(Third embodiment)
In the third embodiment, a configuration of an optical rotator having a layer made of a polymer liquid crystal that realizes an effect of further reducing temperature dependence will be described. FIG. 8 is a schematic cross-sectional view showing the configuration of the optical rotator 30 according to the present embodiment. The optical rotator 30 has a configuration in which a TN type polymer liquid crystal layer 32 formed by twisting polymer liquid crystal by 90 ° in the thickness direction is stacked on the optical rotator 10b of the first embodiment. Although omitted in FIG. 8, an alignment film (not shown) that may be used in the manufacturing process may be provided so as to sandwich the TN type polymer liquid crystal layer 32. Further, the liquid crystal (nematic liquid crystal) constituting the liquid crystal layer 14 when no voltage is applied and the polymer liquid crystal of the TN type polymer liquid crystal layer 32 are each twisted and aligned by about 90 ° in the thickness direction. In the optical rotator 30, light may be from the transparent substrate 11b side or the transparent substrate 31 side in parallel with the Z axis.

  In addition, when rotating incident light using 90 ° twisted liquid crystal, if it is composed of polymer liquid crystal compared to nematic liquid crystal, the temperature expressed by the change in polarization conversion efficiency with respect to the change in use temperature due to its material characteristics It is characterized by small dependence and stable characteristics. Thus, an optical rotator having small temperature dependence can be realized by realizing a function of rotating incident light by 90 ° with a polymer liquid crystal. In the configuration of the optical rotator 30 of FIG. 8, when a voltage is applied to the liquid crystal layer 14, the light transmitted through the liquid crystal layer 14 is rotated by 90 ° in the TN type polymer liquid crystal layer 32 without changing the polarization state.

  On the other hand, when no voltage is applied to the liquid crystal layer 14, the light rotated 90 ° by the TN type polymer liquid crystal layer 32 is rotated 90 ° again by the liquid crystal layer 14, so that the incident light is polarized. It passes through the optical rotator 30. Here, the twist direction of the liquid crystal molecules of the liquid crystal layer 14 and the twist direction of the liquid crystal molecules of the TN type polymer liquid crystal layer 32 may be opposite to each other or the same direction. Note that the pretwist directions of the liquid crystal layer 14 and the TN polymer liquid crystal 32 (the alignment direction of the liquid crystal molecules on the incident side of the layer with respect to the polarization direction of the incident light) are substantially parallel or substantially orthogonal to each other in the XY plane. I just need it.

  Further, as the above-described configuration, in particular, the optical rotator 30 is configured such that the liquid crystal molecule twist direction of the liquid crystal layer 14 and the liquid crystal molecule twist direction of the TN type polymer liquid crystal layer 32 are opposite to each other, and the pre-twist angle Are substantially parallel to each other, the path following the change in the polarization state in the TN type polymer liquid crystal layer 32 and the path following the polarization state in the liquid crystal layer 14 on a Poincare sphere (not shown) substantially coincide with each other in opposite directions. The polarization state of the emitted light is particularly preferable because it returns to the position of the polarization state of the incident light.

  Next, a change in the polarization state of incident light will be specifically described. FIG. 9 is a perspective view of the optical rotator 30 in which the TN type polymer liquid crystal layer 32 is laminated. In each case, when the linearly polarized light in the X direction traveling in the Z direction is incident, It shows how the light is modulated during application. When no voltage is applied as shown in FIG. 9A, the light is rotated by the TN type polymer liquid crystal layer 32 to be linearly polarized in the Y direction, but is rotated by the liquid crystal layer 14 to be linearly polarized in the X direction. When the voltage shown in FIG. 9B is applied, the TN-type polymer liquid crystal layer 32 rotates to become linearly polarized light in the Y direction, and the liquid crystal layer 14 remains linearly polarized in the Y direction without changing the polarization state. To Penetrate. Thus, with respect to the optical rotator according to the first embodiment and the second embodiment, the polarization state of the light emitted when the voltage is applied and when the voltage is not applied is reversed. Moreover, it is good also as a structure which laminates | stacks a wavelength plate on the optical rotator 30 of this embodiment like 2nd Embodiment, and stabilizes wavelength dependency and temperature dependency further.

(Fourth embodiment)
The present embodiment is an optical head device provided with an optical rotator, and a schematic diagram is shown in FIG. The optical head device 50 can reproduce and record BD, HD-DVD, DVD, and CD, respectively. Note that a BD or HD-DVD high-density optical recording medium uses laser light in the 405 nm wavelength band (385 to 420 nm), DVD uses 660 nm wavelength band (640 to 675 nm), and CD uses 780 nm wavelength band (770 to 800 nm).

  If the optical head device 50 is configured to realize these three different wavelength laser beams using a single polarizing beam splitter, a single quarter-wave plate, and a single objective lens, It can be expected that the score will decrease. However, it is difficult to control the polarization state for all the laser beams over a wide band and to achieve high light utilization efficiency. If a polarizing beam splitter, a quarter-wave plate and an objective lens are individually provided for each of the three wavelengths, it becomes possible to obtain controllability of polarization state and high utilization efficiency, but the number of parts increases. Therefore, miniaturization is difficult. In the present embodiment, as will be described later, an example in which three laser beams are separated into two optical paths and polarization state control, high light use efficiency, and miniaturization can be realized. Note that if the three different wavelengths are all in the same optical path and the objective lens is shared, effective condensing characteristics cannot be obtained for light of all these wavelengths due to differences in numerical aperture, etc. An optical system in which an optical path and an optical path sharing a 660 nm wavelength band and an optical path sharing a 780 nm wavelength band are separated into two and an objective lens is arranged on each of them can be considered.

  The light source 41 may be configured to emit linearly polarized light with two or three types of wavelengths. As a light source having such a configuration, a so-called hybrid two-wavelength laser light source or three-wavelength laser light source in which two or three semiconductor laser chips are mounted on the same substrate, or two light sources emitting different wavelengths of light are used. Alternatively, a monolithic type two-wavelength laser light source or three-wavelength laser light source having three light emitting points may be used. Here, all the light emitted from the light source travels in the Z-axis direction and will be described as linearly polarized light in the X-axis direction.

  The light emitted from the light source 41 becomes parallel light by the collimator lens 42 and enters the optical rotator 40. The optical rotator 40 may have any configuration of the optical rotators 10b, 20a, 20b, and 30 described in the first to third embodiments. Here, description will be made on the assumption that the optical rotator 10b, 20a, and 20b rotates about 90 ° when no voltage is applied and does not change the polarization state when the voltage is applied.

  In the optical head device 50, the optical disk 47a is a BD, the optical disk 47b is an HD-DVD / DVD / CD, and these two optical paths are different depending on the standard of the optical disk. First, when recording / reproducing a BD (optical disc 47a), a voltage is applied to the optical rotator 40 so as not to change the polarization state of incident light in the 405 nm wavelength band. In addition, the optical path at this time is indicated by a solid line, the optical path from the light source 41 to the optical disc (BD) 47a is defined as the forward path 51a, and the optical path from the optical disc 47a to the photodetector 48 is indicated as the return path 51b.

  When recording / reproducing the HD-DVD / DVD / CD to be the optical disc 47b, no voltage is applied to the optical rotator 40, and the polarization state of incident light in the 405 nm wavelength band, 660 nm wavelength band, and 780 nm wavelength band is changed to 90. ° Rotate and exit. Further, the optical path at this time is indicated by a dotted line, the optical path from the light source 41 to the optical disk 47b is indicated as the forward path 52a, and the optical path from the optical disk 47b to the photodetector 48 is indicated as the return path 52b.

  In FIG. 10, when recording / reproducing BD with light in the 405 nm wavelength band that is linearly polarized light in the X direction, the light rotates through the optical rotator 40 without changing the polarization state and enters the polarization beam splitter 43. The polarizing beam splitter deflects light oscillating in the X direction in the direction of the optical disk 47a, passes through the quarter-wave plate 45a and the objective lens 46a, and focuses the light on the information recording surface of the optical disk 47a. The reflected return light 51b passes through the objective lens 46a and the quarter-wave plate 45a, becomes linearly polarized light in the Z direction, passes straight through the polarization beam splitter 43, and reaches the photodetector 48.

  On the other hand, when light of each wavelength band emitted by linearly polarized light in the X direction records / reproduces HD-DVD / DVD / CD, the light incident on the optical rotator 40 changes the polarization state by about 90 ° and changes in the Y direction. Since it is emitted as linearly polarized light, it passes through the polarization beam splitter 43 in a straight line. The light transmitted through the polarization beam splitter 43 is reflected by the mirror 44 in the direction of the optical disc 47b, passes through the (broadband) quarter-wave plate 45b and the objective lens 46b, and is condensed on the information recording surface of the optical disc 47b. The reflected light passes through the objective lens 46b and the (broadband) quarter-wave plate 45b and travels in the direction of the polarization beam splitter 43 by the mirror. The light on the return path is reflected by the polarization beam splitter 43 and reaches the photodetector 48.

  Thus, by using the optical rotator of the present invention in an optical head device using laser beams of three different wavelengths, it is possible to control the polarization state of forward light until reaching each optical disc of different standards, and the number of parts And an optical head device with a high degree of design freedom can be realized. The configuration of the optical head device of the present embodiment is an example, and the position of the optical component is not limited to this. The optical rotator 40 uses the optical rotator 30 according to the third embodiment to reverse the switching of the polarization state by voltage application / voltage non-application, and has stable characteristics with less wavelength dependency and temperature dependency. May be obtained.

Example 1
A method for producing the optical rotator 10b of FIG. 1 according to the first embodiment will be described. First, a quartz glass substrate is used as the transparent substrates 11a and 11b. Transparent electrodes 12a and 12b are formed by depositing ITO on a quartz glass substrate by sputtering to a film thickness that provides a sheet resistance value of about 300Ω / □. After applying and baking polyimide on the formed transparent electrodes 12a and 12b, the alignment films 13a and 13b are formed by performing a rubbing process in one direction, respectively. A sealing material such as an epoxy resin (not shown) is annularly applied to the periphery of one substrate. In the seal material, a spacer having a diameter of about 8.1 μm and conductive fine particles serving as a conductive path for applying a voltage are mixed in advance. The transparent substrates 11a and 11b are stacked so that the alignment films 13a and 13b face each other so that the rubbing directions thereof are orthogonal to each other, and then the sealing material is solidified by thermocompression bonding so that the cells having the same gap as the spacer are formed. Form.

A liquid crystal having an ordinary refractive index (n _o ) of 1.510 and an extraordinary refractive index (n _e ) of 1.605 at 25 ° C. with light having a wavelength of 405 nm from an injection port (not shown) is filled in the gap. The optical rotator 10b. FIG. 2B shows the optical characteristics of the optical rotator 10b when no voltage is applied when the thickness of the liquid crystal layer filled with the liquid crystal having the above characteristics is changed. The thickness of 8.1 μm is based on the characteristics shown in FIG. 2B, and the limit of the secondary Morgan thickness of 8.3 μm in the light of wavelength 405 nm and the thickness of the primary Morgan minimum condition in the light of wavelength 660 nm of 6.4 μm. And an intermediate value of 7.35 μm.

  Next, FIG. 11 shows the polarization conversion efficiency when no voltage is applied to the liquid crystal layer when the temperature ranges are changed to −10 ° C., 25 ° C., and 70 ° C. for light of 405 nm, 660 nm, and 780 nm, respectively. Note that the polarization conversion efficiency is the ratio at which P-polarized light that is linearly polarized light in the X direction in FIG. 1 is converted to S-polarized light (linearly polarized light in the Y direction). Even if the operating temperature changes greatly, the polarization conversion efficiency I becomes a high value of 90% or more at any wavelength and at any temperature. When an alternating voltage of 9 Vrms is applied between the liquid crystal layers (when a voltage is applied), light incident as P-polarized light is emitted with approximately 100% P-polarized light, and S-polarized light is approximately 0%.

(Comparative Example 1)
As Comparative Example 1, in the configuration of the optical rotator 10b described in Example 1, the thickness of the liquid crystal layer 14 was set to 3.75 μm, which is the thickness of the light having a wavelength of 405 nm and the primary Morgan minimum condition at 25 ° C. Other than that, it shall have the same configuration. The characteristics of this optical rotator are shown in FIG. Thus, although the polarization conversion efficiency at 405 nm shows a value close to 100% with respect to the temperature change, it is 80% or less at 660 nm and 60% or less at 780 nm, which is a high value even for light of different wavelengths and temperature change. Can't get.

(Example 2)
A method for producing the optical rotator 20a of FIG. 6A according to the second embodiment of the present invention will be described. Of these, the optical rotator 10 is the same material and the same manufacturing method as in Example 1, and the liquid crystal layer 14 The twist angle is 90 °, but the thickness of the liquid crystal layer is 8.7 μm. The wavelength plate 22a is manufactured by first forming a polyimide film (not shown) on a quartz glass substrate 21a, forming a horizontal alignment film by rubbing, and spraying spacers having a diameter of 8.8 μm on the alignment film. Thereafter, a quartz glass substrate having a horizontal orientation (not shown) is opposed so as to be parallel to the orientation direction on the quartz glass substrate 21a side, and a UV curable liquid crystalline monomer composition is injected into a constant gap of 8.8 μm. Light is irradiated to solidify the liquid crystal. A wave plate 22a made of polymer liquid crystal is produced by releasing a glass substrate (not shown).

In the same manner, a wavelength plate 22b made of a polymer liquid crystal having a thickness of 2 μm is prepared on a quartz glass substrate 21b, the wavelength plate 22a and the wavelength plate 22b are bonded with an adhesive (not shown), and the quartz glass substrate 11a and the quartz plate are bonded. A glass substrate 21b is bonded to obtain an optical rotator 20a. Further, the wave plates 24a and 24b of FIG. 6B use the same method, and are bonded to the surfaces of the quartz glass substrate 11a and the quartz glass substrate 11b with an adhesive (not shown) to produce the optical rotator 20b. At this time, the ordinary refractive index (n _o ) of the polymer liquid crystal with respect to 405 nm light at room temperature (25 ° C.) is 1.544, and the extraordinary refractive index (n _e ) is 1.589.

  In FIG. 7C, the counterclockwise direction is defined as the plus (+) direction and the clockwise direction as the minus (−) direction with reference to the X direction that coincides with the polarization direction of the incident light. In addition, as shown in FIG. 7A, the liquid crystal layer 14 when no voltage is applied is twisted and aligned from 0 ° to + 90 ° in the light traveling direction. Then, the optical axis 20a is configured by overlapping the slow axis of the wave plate 22a in the + 93 ° direction and the slow axis of the wave plate 22b in the 89 ° direction.

  Here, when 405 nm light is incident, the retardation value (360 ° · Δn · d / λ) of the wave plate 22a at room temperature (25 ° C) is about 352 °. Further, the retardation value of the wave plate 22b at room temperature is about 80 °. Furthermore, the liquid crystal layer 14 has a pre-twist angle of 0 ° with respect to the X direction of the polarization direction of incident light, and when 405 nm light is incident, the retardation value at room temperature is about 735 °. FIG. 13A shows the polarization conversion efficiency when no voltage is applied when P-polarized light enters the optical rotator 20a from the Z direction. From this result, even if the use temperature changes greatly, the polarization conversion efficiency I becomes a high value of 95% or more at any wavelength and any temperature, and is stabilized with higher polarization conversion efficiency by laminating wave plates. be able to.

  Further, when an AC voltage of 9 Vrms is applied between the liquid crystal layers (when voltage is applied), the major axis direction of the liquid crystal molecules in the liquid crystal layer is aligned parallel to the electric field direction, and the polarization state of incident light does not change in the liquid crystal layer. The polarization state is changed by the laminated wave plates. FIG. 13B shows the polarization conversion efficiency when a voltage is applied when P-polarized light is incident on the optical rotator 20a from the Z direction. From this result, even if the operating temperature changes greatly, the polarization conversion efficiency I can be suppressed to a value of 1% or less at any wavelength and at any temperature, and the light is transmitted almost as P-polarized light. From these results, the extinction ratio of the polarization conversion efficiency I by voltage non-application / voltage application can be increased.

(Example 3)
A specific configuration of the optical rotator 20b of FIG. 6B according to the second embodiment of the present invention will be described. Of these, the optical rotator 10 is made of the same material and the same manufacturing method as in Example 1, and the twist angle of the liquid crystal layer 14 is 90 °, but the thickness of the liquid crystal layer is 8.2 μm. The wave plates 24a and 24b are made of the same material and the same manufacturing method as in the second embodiment, but the thickness of 24a is set to 8.5 μm, and the slow axis is set to 92 ° with respect to the X direction. The thickness of 24b is 2.3 μm, and the slow axis is set to 2 ° with respect to the X direction.

  Here, when 405 nm light is incident, the retardation value of the wave plate 24a at room temperature (25 ° C.) is about 340 °. The liquid crystal layer 14 is twisted 90 ° in the thickness direction with a pre-twist angle of 2 ° with respect to the X direction of the polarization direction of incident light, and when 405 nm light is incident, the retardation value at room temperature is about 692 °. Further, the retardation value of the wave plate 24b at room temperature is about 92 °. FIG. 14A shows the polarization conversion efficiency when no voltage is applied when P-polarized light enters the optical rotator 20b from the Z direction. From this result, even if the operating temperature changes greatly, the polarization conversion efficiency I becomes a high value of 94% or more at any wavelength and at any temperature, and is stabilized with higher polarization conversion efficiency by laminating wave plates. be able to.

  When an AC voltage of 9 Vrms is applied between the liquid crystal layers (when voltage is applied), the major axis direction of the liquid crystal molecules of the liquid crystal layer is aligned parallel to the electric field direction, and the polarization state of incident light in the liquid crystal layer is Although it does not change, the polarization state is changed by the laminated wave plate. FIG. 14B shows the polarization conversion efficiency when a voltage is applied when P-polarized light is incident on the optical rotator 20b from the Z direction. From this result, even if the operating temperature changes greatly, the polarization conversion efficiency I can be suppressed to a value of 1% or less at any wavelength and at any temperature, and the light is transmitted almost as P-polarized light. From these results, the extinction ratio of the polarization conversion efficiency I by voltage non-application / voltage application can be increased.

Example 4
A specific configuration of the optical rotator 30 of FIG. 8 according to the third embodiment of the present invention will be described. Of these, the optical rotator 10b has a twist angle of 91 °, and the twist direction is twisted in the clockwise direction (− direction) as shown in FIG. Is the same as in Example 1. Moreover, the ordinary light refractive index and the extraordinary light refractive index of the liquid crystal are the same as those in Example 1. At this time, the thickness of the liquid crystal layer 14 is 8.6 μm.

  Next, a method for producing a TN type polymer liquid crystal cell will be described. A polyimide film is formed on the quartz glass substrate 31 and rubbed to form a horizontal alignment film (not shown), and spacers having a diameter of 18.2 μm are scattered on the alignment film. Thereafter, a glass substrate on which another horizontal alignment film (not shown) is formed is opposed to the quartz glass substrate 31 so that the alignment direction forms an angle of 94 °, and a UV curable liquid crystal is provided in a constant gap of 18.2 μm. The monomer composition is injected. At this time, the liquid crystal molecules are twisted and aligned in the thickness direction. Next, UV light is irradiated to solidify the liquid crystal. Here, when 405 nm light is incident, the retardation value at 25 ° C. of the liquid crystal layer 14 is about 726 °, and the TN-type polymer liquid crystal layer 32 under the same conditions is about 728 °. Then, a TN type polymer liquid crystal layer 32 is formed by releasing a glass substrate (not shown), and the quartz glass substrate 11a and the TN type polymer liquid crystal layer 32 are bonded with an adhesive (not shown) to form an optical rotator 30. Make it.

  In the TN-type polymer liquid crystal layer 32, the alignment direction of the liquid crystal molecules 33 on the light incident side coincides with the X direction, and the pretwist angle is 0 °. The liquid crystal molecules 33 are aligned in the + 94 ° direction (counterclockwise twist) on the light emission side of the TN type polymer liquid crystal layer 32. In addition, when no voltage is applied, the liquid crystal molecules 15b on the light incident side of the liquid crystal layer 14 are aligned with a pretwist angle of 1 ° with respect to the X direction, and on the light emitting side of the liquid crystal layer 14, the liquid crystal molecules 15b are 90 °. Oriented in the direction (clockwise twist). Further, when a voltage is applied, the liquid crystal of the liquid crystal layer 14 is aligned in the direction of the electric field as shown in FIG. 9B, so that the light emitted from the TN type polymer liquid crystal layer is emitted in the polarization state.

  FIG. 15A shows the polarization conversion efficiency when no voltage is applied when P-polarized light is incident on the optical rotator 30 from the Z direction. From this result, the polarization conversion efficiency I becomes a value of 5% or less at any wavelength and at any temperature even if the operating temperature changes greatly. When an AC voltage of 9 Vrms is applied between the liquid crystal layers (when voltage is applied), the major axis direction of the liquid crystal molecules in the liquid crystal layer is aligned parallel to the electric field direction, and emitted from the TN type polymer liquid crystal layer. The light is emitted in the polarization state. FIG. 15B shows the polarization conversion efficiency when a voltage is applied when P-polarized light is incident on the optical rotator 30 from the Z direction. From this result, the polarization conversion efficiency I becomes a high value of 97% or more at any wavelength and at any temperature even if the operating temperature changes greatly. Therefore, when no voltage is applied to the optical rotator, the polarization direction of the incident linearly polarized light is not changed, and when the voltage is applied, the incident linearly polarized light can be emitted so that the polarization direction is substantially orthogonal.

  As described above, the optical rotator according to the present invention can easily change the polarization state of the emitted light by switching the voltage between the applied state and the non-applied state when specific linearly polarized light is incident with light of different wavelengths. . Furthermore, there is little change in characteristics due to changes in use temperature, high quality in which the polarization state of emitted light is stable can be obtained, and application to an optical head device having stable characteristics can be realized.

The cross-sectional schematic diagram of the optical rotator which concerns on 1st Embodiment. The perspective schematic diagram of the optical rotator according to the first embodiment and the polarization conversion efficiency characteristics with respect to the liquid crystal layer thickness. The polarization conversion efficiency characteristic by the use temperature change at the time of 405 nm incidence of the optical rotator according to the first embodiment. Polarization conversion efficiency characteristics with respect to the liquid crystal layer thickness. Polarization conversion efficiency characteristics with respect to the polarization direction of incident light at 405 nm. The cross-sectional schematic diagram of the optical rotator which concerns on 2nd Embodiment. The perspective schematic diagram of the optical rotator which concerns on 2nd Embodiment. The cross-sectional schematic diagram of the optical rotator which concerns on 3rd Embodiment. The perspective schematic diagram of the optical rotator which concerns on 3rd Embodiment. The schematic diagram which shows an example of the optical head apparatus using an optical rotator. The polarization conversion efficiency characteristic of the optical rotator according to the first embodiment. Polarization conversion efficiency characteristics of an active rotator with a conventional design. The polarization conversion efficiency characteristic of the optical rotator according to the second embodiment (laminated on the light incident side with respect to the two liquid crystal layers of the wave plate). The polarization conversion efficiency characteristic of the optical rotator according to the second embodiment (one wave plate is laminated before and after the light incident side of the liquid crystal layer). The polarization conversion efficiency characteristic of the optical rotator according to the third embodiment.

Explanation of symbols

10a, 10b, 20a, 20b, 30, 40 Optical rotators 11a, 11b, 21a, 21b, 25, 31 Transparent substrates 12a, 12b Transparent electrodes 13a, 13b Alignment film 14 Liquid crystal layers 15a, 15b Liquid crystal molecules 22, 22a, 22b, 24, 24a, 24b Wave plate 32 TN type polymer liquid crystal layer 33 Polymer liquid crystal molecule 41 Light source 42 Collimator lens 43 Polarizing beam splitter 44 Mirror 46a, 46b Objective lens 45a, 45b 1/4 wavelength plate 47a, 47b Optical disc 48 Optical detection 50 Optical Head Device 51a Optical Path for Recording / Reproducing BD (Outward Path)
51b Optical path for recording / reproducing BD (return path)
52a Optical path for recording / reproducing HD-DVD / DVD / CD (outward)
52b Optical path for recording / reproducing HD-DVD / DVD / CD (return path)

Claims (5)

A pair of transparent substrates arranged in parallel;
A liquid crystal layer filled with liquid crystal in a space facing the pair of transparent substrates;
An optical rotator on which light that is linearly polarized light having a wavelength λ ₁ and a wavelength λ ₂ larger than the wavelength λ ₁ is incident;
The liquid crystal is a nematic liquid crystal or a polymer liquid crystal,
In the twist orientation state which the liquid crystal is twisted helically in the thickness direction of the liquid crystal layer, the thickness d _.lambda.21 to be the primary Morgan minimum at a wavelength of lambda ₂ at room temperature is, n order Morgan of the wavelength lambda ₁ The thickness d _λ1n that is the minimum condition is less than or _{equal to the} thickness d _{λ1 (n + 1)} that is the (n + 1) th order Morgan minimum condition of the wavelength λ ₁ , and (d _λ1n + d _{λ1 (n + 1)} ) / 2 or more der Ri (n is a natural number),
The thickness of the liquid crystal layer is a d _.lambda.21 greater than
An optical rotator. A transparent electrode for applying a voltage to the liquid crystal layer;
The optical rotator according to claim 1, wherein the twist alignment state and a vertical alignment state in which the liquid crystal is aligned in a thickness direction of the liquid crystal layer can be switched by a voltage applied to the liquid crystal layer. The thickness of the liquid crystal layer, before KiAtsu of d _.lambda.21 and the thickness d λ1 _{(n + 1)} intermediate value greater than der the Ru claim 1 or claim 2 polarization rotator according to. A light source that emits at least the linearly polarized light having the wavelength λ ₁ and the wavelength λ ₂ ;
A polarizing beam splitter that separates the optical path of the light emitted from the light source;
An objective lens for condensing the light emitted from the polarizing beam splitter onto an optical recording medium;
A light detector for detecting light reflected by the optical recording medium, and an optical head device comprising:
An optical head device in which the optical rotator according to any one of claims 1 to 3 is disposed in an optical path between the light source and the polarization beam splitter. A light source that emits light that is linearly polarized light having a wavelength λ ₃ greater than the wavelength λ ₁ , the wavelength λ _2, and the wavelength λ ₂ ;
The optical head device according to claim 4 , wherein the wavelength λ ₁ is included in a 405 nm wavelength band, the wavelength λ ₂ is included in a 660 nm wavelength band, and the wavelength λ ₃ is included in a 780 nm wavelength band.

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