Polarization modulation device

Abstract

The polarization modulation device includes a polarization modulation element including a plurality of liquid crystal cells, and a controller that applies a voltage to each of the plurality of liquid crystal cells. In a case where an average value of an angle of an alignment axis of the light incidence side substrate and an angle of an alignment axis of the light emission side substrate in each of the plurality of liquid crystal cells is defined as an average alignment angle, the controller applies a voltage higher than a voltage applied to other liquid crystal cells to a liquid crystal cell whose absolute value of a difference between 45 DEG and the average alignment angle is minimum.

Description

[0001] Cross - reference to related applications

[0002] This application claims the priority of Japanese Patent Application No. 2023 - 217616 filed on December 25, 2023, the entire disclosure of which is incorporated herein by reference. Technical field

[0003] The present disclosure generally relates to polarization modulation devices. Background art

[0004] Polarization modulation elements including a plurality of liquid crystal cells are known in the related art. For example, Japanese Patent Application Laid - Open No. 2003 - 98503 describes a liquid crystal optical switch (polarization modulation element) including a liquid crystal polarization rotator formed of a plurality of liquid crystal cells having antiparallel or parallel alignment.

[0005] In the plurality of liquid crystal cells of Japanese Patent Application Laid - Open No. 2003 - 98503, the ratio between the cell thickness of each liquid crystal cell and the retardation of the liquid crystal layer of each liquid crystal cell is substantially equal. Further, in adjacent liquid crystal cells, the light - emitting side substrate and the light - incident side substrate face each other, and the liquid crystal director angles of the liquid crystal layers in contact with the light - emitting side substrate and the light - incident side substrate are substantially equal.

[0006] In a state where no voltage is applied to the plurality of liquid crystal cells, the liquid crystal optical switch of Japanese Patent Application Laid - Open No. 2003 - 98503 functions as a half - wave plate for linearly polarized incident light. Meanwhile, in a state where a voltage is applied to the plurality of liquid crystal cells, the liquid crystal optical switch of Japanese Patent Application Laid - Open No. 2003 - 98503 emits without rotating the polarization direction of the linearly polarized incident light and does not function as a half - wave plate. In Japanese Patent Application Laid - Open No. 2003 - 98503, the response time of the liquid crystal optical switch is shortened by reducing the cell thickness of each liquid crystal cell.

[0007] In liquid crystal cells having antiparallel or parallel alignment, the liquid crystal molecules near the interface (alignment film) between the liquid crystal layer and the substrate are less likely to respond to the applied voltage. Therefore, birefringence caused by the liquid crystal molecules still exists near the interface. Thus, when linearly polarized light enters in a state where a voltage is applied, due to the residual birefringence, the emitted light from the liquid crystal optical switch of Japanese Patent Application Laid - Open No. 2003 - 98503 becomes elliptically polarized light. Therefore, for the liquid crystal optical switch of Japanese Patent Application Laid - Open No. 2003 - 98503, the intensity of the polarized light component having a desired polarization direction is reduced. Further, when a high voltage is applied to the liquid crystal cell such that the liquid crystal molecules near the interface respond, the power consumption increases and the liquid crystal cell may short - circuit.

[0008] The present disclosure has been made in view of the above circumstances, and an object of the present disclosure is to provide a polarization modulation device that can emit a polarized light component having a desired polarization direction with a low voltage and high intensity. Summary of the Invention

[0009] The polarization modulation device according to the present disclosure includes:

[0010] A polarization modulation element including a plurality of liquid crystal cells, each liquid crystal cell including a light incident side substrate, a light emitting side substrate opposite to the light incident side substrate, and a liquid crystal layer sandwiched between the light incident side substrate and the light emitting side substrate, and linearly polarized light enters the light incident side substrate of the first liquid crystal cell among the plurality of liquid crystal cells; and

[0011] A controller that applies a voltage to each of the plurality of liquid crystal cells to switch the polarization direction of the emitted light emitted from the polarization modulation element, where

[0012] The plurality of liquid crystal cells are sequentially stacked, where the light incident side substrate of one liquid crystal cell is opposite to the light emitting side substrate of another liquid crystal cell, and

[0013] In a case where each of the angle of the alignment axis of the light incident side substrate and the angle of the alignment axis of the light emitting side substrate in each of the plurality of liquid crystal cells is set to an angle related to the polarization direction of the linearly polarized light and the average value of the angle of the alignment axis of the light incident side substrate and the angle of the alignment axis of the light emitting side substrate in each of the plurality of liquid crystal cells is defined as the average alignment angle,

[0014] The average alignment angles of each of the plurality of liquid crystal cells are different from each other,

[0015] The plurality of liquid crystal cells include at least one liquid crystal cell in which the absolute value of the difference between 45° and the average alignment angle is different from the absolute value of another liquid crystal cell, and

[0016] The voltage applied by the controller to the liquid crystal cell having the smallest absolute value is higher than the voltage applied to other liquid crystal cells.

[0017] It should be understood that the above general description and the following detailed description are both exemplary and explanatory, and do not limit the present disclosure.

[0018] According to the present disclosure, in a case where the average value of the angle of the alignment axis of the light incident side substrate and the angle of the alignment axis of the light emitting side substrate in each of the plurality of liquid crystal cells is defined as the average alignment angle, a voltage is applied to the liquid crystal cell having the smallest absolute value of the difference between 45° and the average alignment angle, and this voltage is higher than the voltage applied to other liquid crystal cells. Therefore, polarized light having a desired polarization direction can be emitted with a low voltage and high light intensity. Brief Description of the Drawings

[0019] The present application can be more fully understood when the following detailed description is considered in conjunction with the following drawings, in which:

[0020] Figure 1 is a schematic diagram showing a polarization modulation device according to Embodiment 1;

[0021] Figure 2 is a schematic diagram showing a cross-section of a polarization modulation element according to Embodiment 1;

[0022] Figure 3 is a schematic diagram showing a cross-section of a liquid crystal cell according to Embodiment 1;

[0023] Figure 4 is a schematic diagram showing the alignment axis and alignment of liquid crystal according to Embodiment 1;

[0024] Figure 5 is a schematic diagram showing the angle of the alignment axis of the light incident side substrate according to Embodiment 1;

[0025] Figure 6 is a schematic diagram showing the angle of the alignment axis of the light emission side substrate according to Embodiment 1;

[0026] Figure 7 is a schematic diagram showing the alignment axis and alignment state of the liquid crystal of the polarization modulation element in the initial alignment state according to Embodiment 1;

[0027] Figure 8 is a diagram showing the alignment axes of the light incident side substrate and the light emission side substrate of the first liquid crystal cell according to Embodiment 1;

[0028] Figure 9 is a diagram showing the alignment axes of the light incident side substrate and the light emission side substrate of the second liquid crystal cell according to Embodiment 1;

[0029] Figure 10 is a diagram showing the alignment axes of the light incident side substrate and the light emission side substrate of the third liquid crystal cell according to Embodiment 1;

[0030] Figure 11 is a diagram showing an example of the polarization contrast and the voltage applied to each liquid crystal cell according to Embodiment 1;

[0031] Figure 12 is a diagram showing the relationship between the polarization contrast and the average voltage value according to Embodiment 1;

[0032] Figure 13 is a diagram showing the relationship between the wavelength of linearly polarized light and the polarization contrast in the ON state according to Embodiment 1;

[0033] Figure 14It is a graph showing the relationship between the wavelength of linearly polarized light and the polarization contrast in the initial alignment state according to Embodiment 1;

[0034] Figure 15 It is a schematic diagram showing a birefringent plate for modeling the first liquid crystal cell according to Embodiment 1;

[0035] Figure 16 It is a schematic diagram showing a birefringent plate for modeling the second liquid crystal cell according to Embodiment 1;

[0036] Figure 17 It is a schematic diagram showing a birefringent plate for modeling the third liquid crystal cell according to Embodiment 1;

[0037] Figure 18 It is a schematic diagram showing an alignment model according to Embodiment 1;

[0038] Figure 19 It is a graph showing the relationship between the voltage applied to the liquid crystal cell and the residual birefringence in the liquid crystal cell in the ON state according to Embodiment 1;

[0039] Figure 20 It is a graph showing an example of the angle of the alignment axis and the voltage applied to the liquid crystal cell according to Embodiment 2;

[0040] Figure 21 It is a graph showing the relationship between the wavelength of linearly polarized light and the polarization contrast in the initial alignment state according to Embodiment 2;

[0041] Figure 22 It is a graph showing the relationship between the wavelength of linearly polarized light and the polarization contrast in the initial alignment state according to Embodiment 2;

[0042] Figure 23 It is a schematic diagram showing a cross-section of a polarization modulation element according to Embodiment 3;

[0043] Figure 24 It is a graph showing the alignment axes of the light incident side substrate and the light emitting side substrate of the first liquid crystal cell according to Embodiment 3;

[0044] Figure 25 It is a graph showing the alignment axes of the light incident side substrate and the light emitting side substrate of the second liquid crystal cell according to Embodiment 3;

[0045] Figure 26 It is a graph showing the relationship between the wavelength of linearly polarized light and the polarization contrast in the initial alignment state according to Embodiment 3;

[0046] Figure 27 It is a graph showing an example of the angle of the alignment axis and the voltage applied to the liquid crystal cell according to Embodiment 4;

[0047] Figure 28 is a schematic diagram showing a cross-section of a polarization modulation element according to Embodiment 5;

[0048] Figure 29 is a diagram showing an example of the polarization contrast and the voltage applied to each liquid crystal cell according to Embodiment 5;

[0049] Figure 30 is a diagram showing the relationship between the polarization contrast and the average voltage value according to Embodiment 5;

[0050] Figure 31 is a diagram showing the alignment axes of the light incident side substrate and the light emitting side substrate of the first liquid crystal cell according to Embodiment 6;

[0051] Figure 32 is a diagram showing the alignment axes of the light incident side substrate and the light emitting side substrate of the second liquid crystal cell according to Embodiment 6;

[0052] Figure 33 is a diagram showing the alignment axes of the light incident side substrate and the light emitting side substrate of the third liquid crystal cell according to Embodiment 6;

[0053] Figure 34 is a diagram showing the alignment axes of the light incident side substrate and the light emitting side substrate of the fourth liquid crystal cell according to Embodiment 6;

[0054] Figure 35 is a diagram showing an example of the polarization contrast and the voltage applied to each liquid crystal cell according to Embodiment 6; and

[0055] Figure 36 is a diagram showing the relationship between the polarization contrast and the average voltage value according to Embodiment 6. DETAILED DESCRIPTION

[0056] Hereinafter, a polarization modulation device according to various embodiments will be described with reference to the accompanying drawings.

[0057] Embodiment 1

[0058] Reference Figures 1 - 19 is made to the polarization modulation device 10 according to the present embodiment. As Figure 1 shown, the polarization modulation device 10 includes a polarization modulation element 20 and a controller 30. As Figure 1 and Figure 2As shown, the polarization modulation element 20 includes three liquid crystal cells 110 - 130. The polarization modulation element 20 is formed by sequentially stacking the liquid crystal cells 110 - 130, with an adhesive layer (not shown) disposed therebetween. In a state where no voltage is applied to the liquid crystal cells 110 - 130 (initial alignment state), the polarization modulation element 20 functions as a half-wave plate for linearly polarized light (incident light) L1 incident on the liquid crystal cell 110, and emits emitted light L2, in which a phase difference of a half wavelength is imparted to the linearly polarized light L1. Further, in a state where voltage is applied to the liquid crystal cells 110 - 130 (ON state), the polarization modulation element 20 maintains the polarization direction of the linearly polarized light L1 and emits emitted light L2. The controller 30 applies voltage to the liquid crystal cells 110 - 130 to switch the polarization direction of the emitted light L2 emitted from the polarization modulation element 20.

[0059] For ease of understanding, the following description is given in the specification. In Figure 2 , the rightward direction (rightward direction on the paper surface) of the polarization modulation element 20 is referred to as the “+X direction”, the upward direction (upward direction on the paper surface) is referred to as the “+Z direction”, and the direction perpendicular to the +X direction and the +Z direction (depth direction on the paper surface) is referred to as the “+Y direction”. The polarization direction of the linearly polarized light L1 incident on the polarization modulation element 20 is set to the “X direction”. The liquid crystal cell may be referred to as the m-th (where m is an integer of 1 or greater) liquid crystal cell from the incident side of the linearly polarized light L1.

[0060] The polarization modulation element 20 of the polarization modulation device 10 includes liquid crystal cells 110 - 130 that are sequentially stacked. Hereinafter, the liquid crystal cells may be collectively referred to as “liquid crystal cell 100”.

[0061] As Figure 3 shown, each liquid crystal cell 100 includes a light incident side substrate 102, a light emitting side substrate 104, and a nematic liquid crystal 106. As Figure 2 shown, the liquid crystal cells 100 are sequentially stacked, where the light incident side substrate 102 of one liquid crystal cell 100 and the light emitting side substrate 104 of another liquid crystal cell 100 face each other. In a state where no voltage is applied (initial alignment state), each of the liquid crystal cells 100 in the present embodiment functions as a half-wave plate.

[0062] As Figure 2 shown, the light incident side substrate 102 is the substrate located on the incident side of the linearly polarized light L1. As Figure 3As shown, the light-incident-side substrate 102 faces the light-emitting-side substrate 104. The light-incident-side substrate 102 and the light-emitting-side substrate 104 sandwich the nematic liquid crystal 106. In one example, the light-incident-side substrate 102 is implemented as a glass substrate. The light-incident-side substrate 102 includes a light-transmissive electrode 102b and an alignment film 102c on the main surface 102a facing the light-emitting-side substrate 104.

[0063] The light-transmissive electrode 102b of the light-incident-side substrate 102 is formed on the entire surface of the main surface 102a. The light-transmissive electrode 102b is formed of indium tin oxide (ITO). The alignment film 102c of the light-incident-side substrate 102 aligns the nematic liquid crystal 106 in a predetermined direction. In one example, the alignment film 102c of the light-incident-side substrate 102 is implemented as a polyimide alignment film. The alignment of the nematic liquid crystal 106 will be described later.

[0064] As Figure 2 shown, the light-emitting-side substrate 104 is the substrate on the side where the emitted light L2 is emitted. As Figure 3 shown, the light-incident-side substrate 102 and the light-emitting-side substrate 104 are attached to each other by a sealing material 108. In one example, the light-emitting-side substrate 104 is implemented as a glass substrate. The light-emitting-side substrate 104 includes a light-transmissive electrode 104b and an alignment film 104c on the main surface 104a facing the light-incident-side substrate 102.

[0065] The light-transmissive electrode 104b of the light-emitting-side substrate 104 is formed on the entire surface of the main surface 104a. The light-transmissive electrode 104b is formed of indium tin oxide (ITO). The alignment film 104c of the light-emitting-side substrate 104 aligns the nematic liquid crystal 106 in a predetermined direction. In one example, the alignment film 104c of the light-emitting-side substrate 104 is implemented as a polyimide alignment film.

[0066] The nematic liquid crystal 106 is sandwiched between the light-incident-side substrate 102 and the light-emitting-side substrate 104. In this embodiment, the refractive index anisotropy Δn of the nematic liquid crystal 106 is 0.1948 at a wavelength of 380 nm and 0.1403 at a wavelength of 535 nm. In addition, the dielectric anisotropy Δε of the nematic liquid crystal 106 is 4.8 at 20 °C. The thickness d of the nematic liquid crystal 106 (the cell thickness of each liquid crystal cell 100) is 1.5 μm. The refractive index anisotropy Δn and the thickness d of the nematic liquid crystal 106 are set to give a value of half-wavelength birefringence Δnd to light having a predetermined wavelength.

[0067] Next, the alignment of the nematic liquid crystal 106 (i.e., liquid crystal molecules 106M), the angle θ1 of the alignment axis 200A of the light incident side substrate 102, and the angle θ2 of the alignment axis 200B of the light emission side substrate 104 are described. Note that the alignment axes of the light incident side substrates 102 of the various liquid crystal cells 110 - 130 are collectively referred to as the alignment axis 200A, and the alignment axes of the light emission side substrates 104 of the various liquid crystal cells 110 - 130 are collectively referred to as the alignment axis 200B.

[0068] In this embodiment, as Figure 4 shown, the nematic liquid crystal 106 is aligned antiparallel by the alignment film 102c of the light incident side substrate 102 and the alignment film 104c of the light emission side substrate 104. In this specification, as Figure 4 and Figure 5 shown, the alignment axis of the light incident side substrate 102 (alignment film 102c) at the initial alignment of the liquid crystal molecules 106M is defined as the alignment axis 200A of the light incident side substrate 102, and the angle of the alignment axis 200A with respect to the polarization direction (X direction) of the linearly polarized light L1 incident on the polarization modulation element 20 is defined as the angle θ1 of the alignment axis 200A of the light incident side substrate 102. In addition, as Figure 4 and Figure 6 shown, the alignment axis of the light emission side substrate 104 (alignment film 104c) at the initial alignment of the liquid crystal molecules 106M is defined as the alignment axis 200B of the light emission side substrate 104, and the angle of the alignment axis 200B with respect to the polarization direction (X direction) of the linearly polarized light L1 incident on the polarization modulation element 20 is defined as the angle θ2 of the alignment axis 200B of the light emission side substrate 104. In addition, in one liquid crystal cell 100, the average of the angle θ1 of the alignment axis 200A of the light incident side substrate 102 and the angle θ2 of the alignment axis 200B of the light emission side substrate 104 is defined as the average alignment angle θa of the liquid crystal cell 100. In this specification, for convenience, it is described that the +X direction is 0°, and the clockwise direction from the +X direction is defined as the + (plus) direction.

[0069] In this embodiment, the nematic liquid crystal 106 is aligned antiparallel in the initial alignment state. Therefore, in one of the liquid crystal cells 100, the angle θ1 of the alignment axis 200A of the light incident side substrate 102, the angle θ2 of the alignment axis 200B of the light emission side substrate 104, and the average alignment angle θa are equal.

[0070] In the liquid crystal cells 110 - 130, as Figures 7 - 10 shown, the average alignment angles θa (the angle θ1 of the alignment axis 200A and the angle θ2 of the alignment axis 200B) are different from each other. Note that Figure 7 the light incident side substrate 102, the light emission side substrate 104, etc. are omitted.

[0071] Specifically, in the first liquid crystal cell 110 of the incident linearly polarized light L1, as Figure 8 shown, the angle θ1 of the alignment axis 200A of the light incident side substrate 102, the angle θ2 of the alignment axis 200B of the light emitting side substrate 104, and the average alignment angle θa of the liquid crystal cell 110 are 15°. In the second liquid crystal cell 120, as Figure 9 shown, the angle θ1 of the alignment axis 200A of the light incident side substrate 102, the angle θ2 of the alignment axis 200B of the light emitting side substrate 104, and the average alignment angle θa of the liquid crystal cell 120 are 45°. In addition, in the third liquid crystal cell 130, as Figure 10 shown, the angle θ1 of the alignment axis 200A of the light incident side substrate 102, the angle θ2 of the alignment axis 200B of the light emitting side substrate 104, and the average alignment angle θa of the liquid crystal cell 130 are 75°. In this embodiment, each liquid crystal cell 100 functions as a 1 / 2 wave plate in the initial alignment state. Therefore, when linearly polarized light L1 having a polarization direction in the X direction is incident on the polarization modulation element 20 in the initial alignment state, emitted light L2 having a polarization direction in the Y direction is emitted from the polarization modulation element 20.

[0072] The controller 30 of the polarization modulation device 10 applies a voltage to the liquid crystal cells 110 - 130 to switch the polarization direction of the emitted light L2 emitted from the polarization modulation element 20. When switching the polarization direction of the emitted light L2 from the Y direction to the X direction, the controller 30 applies a voltage to the liquid crystal cell 100 with the smallest absolute value of the difference between 45° and the average alignment angle θa, and this voltage is higher than the voltage applied to the other liquid crystal cells 100. In the liquid crystal cell 100 where the average alignment angle θa is close to 45°, the residual birefringence of the nematic liquid crystal 106 near the interface has a greater influence on the emitted light L2. Therefore, by applying a voltage to the liquid crystal cell 100 with the smallest absolute value of the difference between 45° and the average alignment angle θa, and this voltage is higher than the voltage applied to the other liquid crystal cells 100, the influence of the residual birefringence of the nematic liquid crystal 106 near the interface on the emitted light L2 can be reduced, and the voltage applied to the entire polarization modulation element 20 can be reduced.

[0073] In this embodiment, the controller 30 applies a voltage to the liquid crystal cell 120 where the difference between 45° and the average alignment angle θa is zero, and this voltage is higher than the voltage applied to the liquid crystal cell 110 and the liquid crystal cell 130. Details of the voltage applied by the controller 30 to each of the liquid crystal cells 110 - 130 will be described later.

[0074] Next, the operation and effects of the polarization modulation device 10 are described. First, the polarization contrast pCR is defined. The term "polarization contrast pCR" refers to the ratio of the transmitted light intensity I2 when the emitted light L2 is transmitted through an analyzer having a transmission axis parallel to the desired polarization direction of the polarization component to the transmitted light intensity I1 when the emitted light ray L2 is transmitted through an analyzer having a transmission axis perpendicular to the desired polarization direction of the polarization component. The polarization contrast pCR is expressed as pCR = I2 / I1. A higher polarization contrast pCR indicates a higher intensity of the polarization component. In the present embodiment, in the initial alignment state, the polarization modulation element 20 functions as a half-wave plate for the linearly polarized light L1, and thus the desired polarization direction in the initial alignment state is the Y direction. Further, in the ON state, the polarization modulation element 20 maintains the polarization direction of the linearly polarized light L1 and emits the light as the emitted light L2, and thus the desired polarization direction in the ON state is the X direction.

[0075] Next, the operation and effects of the polarization modulation device 10 in the ON state are described. Figure 11 Illustrated are the polarization contrast pCR and the voltages V1 - V3 respectively applied to the liquid crystal cells 110 - 130 when a voltage V2 higher than the voltages V1 and V3 applied to the liquid crystal cells 110 and 130 is applied to the liquid crystal cell 120 where the absolute value of the difference between 45° and the average alignment angle θa is minimized; and an example of the polarization contrast pCR and the applied voltage in Comparative Example 1. Figure 12 Illustrated is the polarization contrast pCR, Figure 11 the relationship between the average value of the voltages V1 - V3 shown in [] (hereinafter referred to as "average voltage value Va") and the average voltage value Va of Comparative Example 1. Figure 13 Illustrated are the polarization contrast pCR and the wavelength λ of the linearly polarized light L1 in the present embodiment (V1 = 16.9V, V2 = 22.5V, V3 = 18.1V) and Comparative Example 1 (V1 = V2 = V3 = 19.2V) when the average voltage value Va of 19.2V is applied.

[0076] Comparative Example 1 is an example in which the same voltage is applied to each of the liquid crystal cells 110 - 130 (that is, in Comparative Example 1, the average voltage value Va and the voltages V1 - V3 respectively applied to the liquid crystal cells 110 - 130 are equal). Note that Figure 11 and Figure 12 the wavelength λ of the linearly polarized light L1 in [] is 380 nm, and the voltages V1 - V3 applied to the liquid crystal cells 110 - 130 and the polarization contrast pCR are obtained by simulation.

[0077] As Figure 12As shown, when comparing the same polarization contrast pCR, the average voltage value Va of this embodiment is less than the average voltage value Va of Comparative Example 1 (for example, when the polarization contrast pCR is 20, the average voltage value Va of this embodiment is 19.2 V, and the average voltage value Va of Comparative Example 1 is 19.7 V). That is, the polarization modulation device 10 of this embodiment can emit a polarized light component with a desired polarization direction at high intensity and low voltage.

[0078] As Figure 13 shown, at all wavelengths λ, the polarization contrast pCR of this embodiment is higher than that of Comparative Example 1. That is, in the ON state, the polarization modulation device 10 of this embodiment can emit a polarized light component with a desired polarization direction at high intensity in a wide wavelength range.

[0079] Next, the operation and effects of the polarization modulation device 10 in the initial alignment state are described. Figure 14 The polarization contrast pCR of the polarization modulation device 10 (polarization modulation element 20) and the wavelength λ of the linearly polarized light L1 in the initial alignment state are shown. As Figure 14 shown, in the initial alignment state, the polarization modulation device 10 can emit a polarized light component with a desired polarization direction at high intensity in a wide wavelength range. For example, for the polarization modulation device 10, within 99.5% (382 nm - 780 mm) of the wavelength λ in the range of 380 nm - 780 nm, the polarization contrast pCR is 20 or greater.

[0080] Next, the voltages V1 - V3 that the controller 30 correspondingly applies to the liquid crystal cells 110 - 130 are described. In one example, the voltages V1 - V3 correspondingly applied to the liquid crystal cells 110 - 130 are obtained as follows.

[0081] First, each of the liquid crystal cells 110 - 130 is modeled as a birefringent plate. Specifically, as Figure 15 shown, the liquid crystal cell 110 to which a voltage is applied is modeled as a birefringent plate 310 having an optical axis 310A. In this case, the angle of the optical axis 310A with respect to the polarization direction (X direction) of the linearly polarized light L1 is defined as the angle of the optical axis 310A is 15°, which is the same as the angles θ1 and θ2 of the alignment axes 200A and 200B of the liquid crystal cell 110. In addition, as Figure 16 shown, the liquid crystal cell 120 to which a voltage is applied is modeled as a birefringent plate 320 having an optical axis 320A. In addition, as Figure 17 shown, the liquid crystal cell 130 to which a voltage is applied is modeled as a birefringent plate 330 having an optical axis 330A. The angle of the optical axis 320A with respect to the polarization direction of the linearly polarized light L1 corresponds to and the angle of the optical axis 330A with respect to the polarization direction of the linearly polarized light L1 corresponds to

[0082] Here, a voltage V1 is applied to the liquid crystal cell 110, a voltage V2 is applied to the liquid crystal cell 120, a voltage V3 is applied to the liquid crystal cell 130, the thickness of the birefringent plate 310 is defined as d1, the thickness of the birefringent plate 320 is defined as d2, the thickness of the birefringent plate 330 is defined as d3, and the refractive index anisotropy of the birefringent plates 310 - 330 is defined as Δn.

[0083] The Jones matrix of the birefringent plate is represented by the following equation (1). When viewed from above on the +Z side, the Jones matrix of the birefringent plate is represented by the following equation (2). In the birefringent plate, the optical axis is tilted by an angle with respect to the linearly polarized light L1 having a polarization direction in the X direction Therefore, the Jones matrix of each of the birefringent plates 310 - 330 is represented by the following equations (3) - (5).

[0084]

[0085] W = R(−φ)W(┏1)R(φ) (3)

[0086] W = R(−3φ)W(┏2)R(3φ) (4)

[0087] W = R(−5φ)W(┏3)R(5φ) (5)

[0088] The overall Jones matrix of the birefringent plates 310 - 330 (i.e., the Jones matrix corresponding to the entire polarization modulation element 20) is represented by the following equation (6). Here, Γ1, Γ2, and Γ3 are represented by the following equations (7) - (9). λ is the wavelength of the linearly polarized light L1.

[0089] W = R(−5φ)W(┏3)R(2φ)W(┏2)R(2φ)W(┏1)R(φ) (6)

[0090]

[0091] Next, the polarization contrast pCR is expressed using the Jones matrix. When the overall Jones matrix of the birefringent plates 310 - 330 is represented by the following equation (10), in the case where the emitted light L2 is transmitted through an analyzer having a transmission axis perpendicular to the polarization component and having a desired polarization direction (X direction), the transmitted light is represented by the following equation (11), and the transmitted light intensity I1 is represented by the following equation (12). Here, the symbol "*" represents the complex conjugate.

[0092]

[0093] In the case where the emitted light L2 is transmitted through an analyzer having a transmission axis parallel to the desired polarization direction (X direction) of the polarization component, the transmitted light is represented by the following equation (13), and the transmitted light intensity I2 is represented by the following equation (14).

[0094]

[0095]

[0096] Based on equations (6)-(9), (12), and (14), the polarization contrast pCR is represented by the following equations (15) and (16).

[0097]

[0098] Meanwhile, the relationship between the voltage V applied to the liquid crystal cell 100 in the ON state and the residual birefringence Δnd in the liquid crystal cell 100 is obtained as follows.

[0099] First, as Figure 18 shown, an alignment model is set in which the angle δ of the liquid crystal director with respect to the main surface of the substrate increases as the thickness h from the substrate to the main surface increases. When n o is the ordinary light refractive index of the liquid crystal, n e is the extraordinary light refractive index, n eff (x, y, z) is the refractive index of the liquid crystal in the alignment axis direction at the position (x, y, z), M is the number of divisions in which the interior in the thickness direction of the thickness h is divided into a plurality of regions, Δni is the refractive index anisotropy (phase difference) of the i-th divided region, Δn(x, y, z) is the refractive index anisotropy (phase difference) at the position (x, y, z), and d is the cell thickness of the liquid crystal cell 100, the birefringence Δnd is represented by the following equations (17)-(19).

[0100]

[0101] Δn(x, y, z) = n eff (x, y, z) - n o (18)

[0102]

[0103] In one example, the relationship between the applied voltage V and the angle δ of the liquid crystal director with respect to the main surface of the substrate is obtained by a liquid crystal simulator (for example, LCD Master manufactured by Shintech). For example, Figure 19The relationship between the voltage V applied to the liquid crystal cell 100 in the ON state and the residual birefringence Δnd in the liquid crystal cell 100 is obtained based on the obtained relationship between the applied voltage V and the angle δ of the liquid crystal director with respect to the main surface of the substrate and equations (17)-(19).

[0104] As Figure 11 shown, by obtaining a combination that satisfies the set polarization contrast pCR from the relationship between the applied voltage V in the ON state and the residual birefringence Δnd in the liquid crystal cell 100 and equations (15) and (16), and that minimizes the sum of the voltage V1 applied to the liquid crystal cell 110, the voltage V2 applied to the liquid crystal cell 120, and the voltage V3 applied to the liquid crystal cell 130, the voltages V1 - V3 applied to the liquid crystal cells 110 - 130 can be obtained accordingly.

[0105] As described above, the polarization modulation device 10 applies the voltage V2, which is higher than the voltages V1 and V3 applied to the other liquid crystal cells 110 and 130, to the liquid crystal cell 120 where the absolute value of the difference between 45° and the average alignment angle θa is the smallest. Therefore, the influence of the residual birefringence near the interface of the nematic liquid crystal 106 can be reduced, and the voltage applied to the entire polarization modulation element 20 can be reduced. That is, the polarization modulation device 10 can emit a polarized light component with a desired polarization direction at high intensity and low voltage. In addition, in the ON state and the initial alignment state, the polarization modulation device 10 can emit a polarized light component with a desired polarization direction at high intensity over a wide wavelength range.

[0106] Example 2

[0107] In Example 1, the angle θ1 of the alignment axis 200A, the angle θ2 of the alignment axis 200B, and the average alignment angle θa of the first liquid crystal cell 110 were 15°; the angle θ1 of the alignment axis 200A, the angle θ2 of the alignment axis 200B, and the average alignment angle θa of the second liquid crystal cell 120 were 45°; and the angle θ1 of the alignment axis 200A, the angle θ2 of the alignment axis 200B, and the average alignment angle θa of the third liquid crystal cell 130 were 75°. However, the angle θ1 of the alignment axis 200A, the angle θ2 of the alignment axis 200B, and the average alignment angle θa are not limited to this.

[0108] Figure 20 Examples of the voltages V1 - V3 and the average voltage value Va applied to the liquid crystal cells 110 - 130 accordingly in order to set the polarization contrast pCR to 20 in the polarization modulation element 20 where the angle θ1 of the alignment axis 200A, the angle θ2 of the alignment axis 200B, and the average alignment angle θa of the liquid crystal cells 110 - 130 are changed (wavelength λ: 380 nm) are shown. As in Example 1, Figure 20The example in the field is an example where a voltage V2 higher than the voltages V1 and V3 applied to the liquid crystal cells 110 and 130 is applied to the liquid crystal cell 120 with the smallest absolute value of the difference between 45° and the average alignment angle θa; and the comparative example field is an example where the same voltages V1 - V3 are applied to the liquid crystal cells 110 - 130. Note that the voltages V1 - V3 in this embodiment are obtained by the same method as the voltages V1 - V3 in Embodiment 1.

[0109] As Figure 20 shown, the average voltage value Va required to set the polarization contrast pCR to 20 in this embodiment is less than the average voltage value Va required to set the polarization contrast pCR to 20 in the comparative example. That is, in the polarization modulation device 10 of this embodiment, as in the polarization modulation device 10 of Embodiment 1, the voltage V2 is applied to the liquid crystal cell 120 with the smallest absolute value of the difference between 45° and the average alignment angle θa, and this voltage V2 is higher than the voltages V1 and V3 applied to the liquid crystal cells 110 and 130. As a result, the influence of the residual birefringence near the interface of the nematic liquid crystal 106 on the emitted light L2 can be reduced, and the voltage applied to the entire polarization modulation element 20 can be reduced.

[0110] Figure 21 shows the polarization contrast pCR and the wavelength λ of the linearly polarized light L1 of the polarization modulation element 20 (the first liquid crystal cell 110: θa = 5°, the second liquid crystal cell 120: θa = 35°, the third liquid crystal cell 130: θa = 70°) of No. 2 shown in the initial alignment state. Figure 20 shown in Figure 22 shows the polarization contrast pCR and the wavelength λ of the linearly polarized light L1 of the polarization modulation element 20 (the first liquid crystal cell 110: θa = 15°, the second liquid crystal cell 120: θa = 40°, the third liquid crystal cell 130: θa = 70°) of No. 7 shown in the initial alignment state. As Figure 20 shown in Figure 21 and 22 shown, in the initial alignment state, the polarization modulation device 10 (polarization modulation element 20) of this embodiment can also emit a polarized light component with a desired polarization direction at a high intensity in a wide wavelength range.

[0111] Embodiment 3

[0112] In Embodiment 1, the polarization modulation element 20 includes three liquid crystal cells 110 - 130. However, the polarization modulation element 20 may include a plurality of liquid crystal cells 100 as long as it does.

[0113] In this embodiment, a polarization modulation device 10 is described, which includes a polarization modulation element 20 provided with two liquid crystal cells 100 and a controller 30. Also in this embodiment, in the initial alignment state, the polarization modulation element 20 functions as a half-wave plate for linearly polarized light L1, and in the ON state, it maintains the polarization direction of the linearly polarized light L1 and emits this light as emitted light L2. In the initial alignment state, each of the two liquid crystal cells 100 functions as a half-wave plate.

[0114] As Figure 23 shown, the polarization modulation element 20 of this embodiment includes two liquid crystal cells 110 and 120. Similar to Embodiment 1, the liquid crystal cells 110 and 120 of this embodiment are stacked in sequence. Except for the angle θ1 of the alignment axis 200A of the light incident side substrate 102, the angle θ2 of the alignment axis 200B of the light emission side substrate 104, and the average alignment angle θa, the configurations of the liquid crystal cells 110 and 120 of this embodiment are the same as those of the liquid crystal cell 100 of Embodiment 1.

[0115] In the first liquid crystal cell 110 of this embodiment, as Figure 24 shown, the angle θ1 of the alignment axis 200A of the light incident side substrate 102, the angle θ2 of the alignment axis 200B of the light emission side substrate 104, and the average alignment angle θa are 24°. In the second liquid crystal cell 120 of this embodiment, as Figure 25 shown, the angle θ1 of the alignment axis 200A of the light incident side substrate 102, the angle θ2 of the alignment axis 200B of the light emission side substrate 104, and the average alignment angle θa are 63°.

[0116] Figure 26 Shows the polarization contrast pCR of the polarization modulation device 10 (polarization modulation element 20) and the wavelength λ of the linearly polarized light L1 in the initial alignment state. As Figure 26 shown, the polarization modulation device 10 of this embodiment can also emit a polarization light component with a desired polarization direction at a high intensity within a wide wavelength range.

[0117] Similar to the controller 30 of Embodiment 1, when switching the polarization direction of the emitted light L2 from the Y direction to the X direction, the controller 30 of this embodiment applies a voltage to the liquid crystal cell 100 with the smallest absolute value of the difference between 45° and the average alignment angle θa, and this voltage is higher than the voltage applied to the other liquid crystal cell 100. The controller 30 applies a voltage V2 to the liquid crystal cell 120 with the smallest absolute value of the difference between 45° and the average alignment angle θa, and this voltage V2 is higher than the voltage V1 applied to the liquid crystal cell 110.

[0118] Specifically, the controller 30 can set the polarization contrast pCR to 20 by applying a voltage of 15.22V (V1 = 15.22V) to the liquid crystal cell 110 and a voltage of 15.69V (V2 = 15.69V) to the liquid crystal cell 120. In this case, the average voltage value Va is 15.455V (Va = 15.455V). At the same time, in the case where the same voltage is applied to the liquid crystal cell 110 and the liquid crystal cell 120, in order to set the polarization contrast pCR to 20, a voltage of 15.46V (average voltage value Va = 15.46) must be applied to each of the liquid crystal cell 110 and the liquid crystal cell 120. Note that also in this embodiment, the voltages V1 and V2 applied to the liquid crystal cells 110 and 120 are obtained by the same method as in Embodiment 1. In addition, the wavelength λ of the linearly polarized light L1 is 380nm.

[0119] As described above, the polarization modulation device 10 of this embodiment applies the voltage V2, which is higher than the voltage V1 applied to the liquid crystal cell 110, to the liquid crystal cell 120 with the smallest absolute value of the difference between 45° and the average alignment angle θa. Therefore, the polarization modulation device 10 can reduce the influence of the residual birefringence near the interface of the nematic liquid crystal 106 on the emitted light L2, and can emit a polarization light component with a desired polarization direction with high intensity and low voltage.

[0120] Embodiment 4

[0121] In Embodiment 3, the angle θ1 of the alignment axis 200A, the angle θ2 of the alignment axis 200B, and the average alignment angle θa of the first liquid crystal cell 110 are 24°; and the angle θ1 of the alignment axis 200A, the angle θ2 of the alignment axis 200B, and the average alignment angle θa of the second liquid crystal cell 120 are 63°. However, the angle θ1 of the alignment axis 200A, the angle θ2 of the alignment axis 200B, and the average alignment angle θa are not limited thereto.

[0122] Figure 27 An example is shown in which in the polarization modulation element 20 where the angle θ1 of the alignment axis 200A, the angle θ2 of the alignment axis 200B, and the average alignment angle θa of the liquid crystal cells 110 and 120 are changed (wavelength λ: 380nm), the voltages V1 - V2 and the average voltage value Va applied to the liquid crystal cells 110 and 120 accordingly to set the polarization contrast pCR to 20. As in Embodiment 3, Figure 27 in the example of the embodiment field, the voltage V2, which is higher than the voltage V1 applied to the liquid crystal cell 110, is applied to the liquid crystal cell 120 with the smallest absolute value of the difference between 45° and the average alignment angle θa; and in the example of the comparative example field, the same voltages V1 and V2 are applied to the liquid crystal cells 110 and 120. The voltages V1 and V2 applied to the liquid crystal cells 110 and 120 of this embodiment are obtained by the same method as in Embodiment 1.

[0123] As Figure 27 shown, the average voltage value Va for setting the polarization contrast pCR to 20 in this embodiment is less than the average voltage value Va for setting the polarization contrast pCR to 20 in the comparative example. That is to say, also in this embodiment, the polarization modulation device 10 also applies a voltage V2 to the liquid crystal cell 120 with the smallest absolute value of the difference between 45° and the average alignment angle θa, and this voltage V2 is higher than the voltage V1 applied to the liquid crystal cell 110. As a result, the influence of the residual birefringence near the interface of the nematic liquid crystal 106 on the emitted light L2 can be reduced, and the voltage applied to the entire polarization modulation element 20 can be reduced.

[0124] Embodiment 5

[0125] In Embodiment 1, the polarization modulation element 20 includes three liquid crystal cells 110 - 130. In addition, the polarization modulation element 20 of Embodiment 1 includes one liquid crystal cell 100 (liquid crystal cell 120) with the smallest absolute value of the difference between 45° and the average alignment angle θa. However, it is sufficient for the polarization modulation element 20 to include a plurality of liquid crystal cells 100. The following configuration is possible: the polarization modulation element 20 includes a plurality of liquid crystal cells 100 with the smallest absolute value of the difference between 45° and the average alignment angle θa; and the controller 30 applies a voltage to the plurality of liquid crystal cells 100 with the smallest absolute value of the difference between 45° and the average alignment angle θα, and this voltage is higher than the voltage applied to the other liquid crystal cells 100. In this embodiment, a polarization modulation device 10 is described, which includes a polarization modulation element 20 provided with four liquid crystal cells 100 and a controller 30.

[0126] Also in this embodiment, in the initial alignment state, the polarization modulation element 20 functions as a 1 / 2 wave plate for the linearly polarized light L1, and in the ON state, it maintains the polarization direction of the linearly polarized light L1 and emits this light as the emitted light L2. In the initial alignment state, each of the four liquid crystal cells 100 functions as a 1 / 2 wave plate.

[0127] As Figure 28 shown, the polarization modulation element 20 of this embodiment includes four liquid crystal cells 110 - 140. Similar to Embodiment 1, the liquid crystal cells 110 - 140 of this embodiment are stacked in sequence. Except for the angle θ1 of the alignment axis 200A of the light incident side substrate 102, the angle θ2 of the alignment axis 200B of the light emission side substrate 104, and the average alignment angle θa, the configuration of the liquid crystal cells 110 - 140 in this embodiment is the same as the configuration of the liquid crystal cell 100 in Embodiment 1.

[0128] In the first liquid crystal cell 110 of the present embodiment, the angle θ1 of the alignment axis 200A of the light incident side substrate 102, the angle θ2 of the alignment axis 200B of the light emission side substrate 104, and the average alignment angle θa are 11.25°. In the second liquid crystal cell 120 of the present embodiment, the angle θ1 of the alignment axis 200A of the light incident side substrate 102, the angle θ2 of the alignment axis 200B of the light emission side substrate 104, and the average alignment angle θa are 33.75°. In the third liquid crystal cell 130 of the present embodiment, the angle θ1 of the alignment axis 200A of the light incident side substrate 102, the angle θ2 of the alignment axis 200B of the light emission side substrate 104, and the average alignment angle θa are 56.25°. In the fourth liquid crystal cell 140 of the present embodiment, the angle θ1 of the alignment axis 200A of the light incident side substrate 102, the angle θ2 of the alignment axis 200B of the light emission side substrate 104, and the average alignment angle θa are 78.75°.

[0129] In the present embodiment, the absolute value of the difference between 45° and the average alignment angle θa of the liquid crystal cell 120 is equal to the absolute value of the difference between 45° and the average alignment angle θa of the liquid crystal cell 130 (absolute value: 11.25). The liquid crystal cells 120 and 130 correspond to the liquid crystal cell 100 with the smallest absolute value of the difference between 45° and the average alignment angle θa. Here, the phrase "the absolute value of the difference between 45° and the average alignment angle θa" includes the case where the absolute value of the difference between 45° and the average alignment angle θa exactly matches, and also includes the case where the absolute value of the difference between 45° and the average alignment angle θa substantially matches. The case where the absolute values substantially match is, for example, the case where the difference between the absolute values of the difference between 45° and the average alignment angle θa is within the manufacturing tolerance range.

[0130] Similar to the controller 30 of Embodiment 1, when switching the polarization direction of the emitted light L2 from the Y direction to the X direction, the controller 30 of the present embodiment applies a voltage higher than the voltage applied to other liquid crystal cells 100 to the liquid crystal cell 100 with the smallest absolute value of the difference between 45° and the average alignment angle θa. In the present embodiment, since the absolute value of the difference between 45° and the average alignment angle θa of the liquid crystal cells 120 and 130 is the smallest, the controller 30 applies voltages V2 and V3 to the liquid crystal cells 120 and 130, and the voltages V2 and V3 are higher than the voltage V1 applied to the liquid crystal cell 110 and the voltage V4 applied to the liquid crystal element 140. The voltage V2 applied to the liquid crystal cell 120 and the voltage V3 applied to the liquid crystal cell 130 may be equal, or one may be higher than the other.

[0131] Figure 29Shows an example in which voltages V2 and V3 higher than voltages V1 and V4 applied to liquid crystal cells 110 and 140 are applied to liquid crystal cells 120 and 130 where the absolute value of the difference between 45° and the average alignment angle θa is minimized, the polarization contrast pCR, and voltages V1 - V4 correspondingly applied to liquid crystal cells 110 - 140; and the polarization contrast pCR and the applied voltage (wavelength λ: 380 nm) in Comparative Example 2. Figure 30 Shows Figure 29 The relationship between the polarization contrast pCR shown and the average voltage value Va and the average voltage value Va of Comparative Example 2. Comparative Example 2 is an example in which the same voltages V1 - V4 are correspondingly applied to liquid crystal cells 110 - 140. The voltages V1 - V4 applied to liquid crystal cells 110 - 140 of this embodiment are obtained by the same method as in Embodiment 1.

[0132] As Figure 30 Shown, the average voltage value Va of this embodiment is less than the average voltage value Va of Comparative Example 2 (for example, when the polarization contrast pCR is 20, the average voltage value Va of this embodiment is 24.35 V, and the average voltage value Va of Comparative Example 2 is 25.40 V). That is, the polarization modulation device 10 of this embodiment can emit a polarized light component having a desired polarization direction with high intensity and low voltage.

[0133] As described above, the polarization modulation device 10 of this embodiment applies voltages V2 and V3 to liquid crystal cells 120 and 130 where the absolute value of the difference between 45° and the average alignment angle θa is minimized, and these voltages V2 and V3 are higher than voltages V1 and V4 applied to liquid crystal cells 110 and 140. Therefore, the polarization modulation device 10 can reduce the influence of the residual birefringence near the interface of the nematic liquid crystal 106 on the emitted light L2, and can emit a polarized light component having a desired polarization direction with high intensity and low voltage.

[0134] Embodiment 6

[0135] In Embodiments 1-5, the nematic liquid crystal 106 in the liquid crystal cell 100 is in an antiparallel alignment. However, a configuration in which the nematic liquid crystal 106 in the liquid crystal cell 100 is in a twisted alignment is also possible. Similar to the polarization modulation device 10 of Embodiments 1-5, the polarization modulation device 10 of this embodiment includes a polarization modulation element 20 and a controller 30. Similar to the polarization modulation element 20 of Embodiment 1, in the initial alignment state, the polarization modulation element 20 of this embodiment functions as a half-wave plate for linearly polarized light L1 incident on the liquid crystal cell 110. Further, in the ON state, the polarization modulation element 20 of this embodiment maintains the polarization direction of the linearly polarized light L1 and emits the light as emitted light L2. The controller 30 of this embodiment applies a voltage to the liquid crystal cells 110-140 to switch the polarization direction of the emitted light L2 emitted from the polarization modulation element 20.

[0136] Similar to the polarization modulation element 20 of Embodiment 5, the polarization modulation element 20 of this embodiment includes four liquid crystal cells 110-140. Similar to the liquid crystal cell 100 of Embodiment 1, the liquid crystal cells 110-140 of this embodiment are sequentially stacked such that the light incident side substrate 102 of one liquid crystal cell 100 and the light emitting side substrate 104 of another liquid crystal cell 100 face each other. Except for the alignment of the nematic liquid crystal 106, the angle θ1 of the alignment axis 200A of the light incident side substrate 102, the angle θ2 of the alignment axis 200B of the light emitting side substrate 104, and the average alignment angle θa, the configuration of the liquid crystal cells 110-140 of this embodiment is the same as that of the liquid crystal cell 100 of Embodiment 1. Note that each of the liquid crystal cells 110-140 of this embodiment does not function as a half-wave plate.

[0137] In the initial alignment state of the liquid crystal cells 110-114, when viewed from above the +Z side, the nematic liquid crystal 106 is twisted clockwise. As Figures 31 - 34 shown, the twist angle α of the liquid crystal cells 110-140 (i.e., the twist angle of the nematic liquid crystal 106) is 22.5°. The sum of the twist angles α of the various liquid crystal cells 110-140 is 90°. In this embodiment, as will be described later, in adjacent liquid crystal cells 100, the angle θ2 of the alignment axis 200B of the light emitting side substrate 104 of one liquid crystal cell 100 and the angle θ1 of the alignment axis 200A of the light incident side substrate 102 of another liquid crystal cell 100 match. The average alignment angles θa of the liquid crystal cells 110-140 are different from each other.

[0138] Specifically, as Figure 31 shown, in the first liquid crystal cell 110, the angle θ1 of the alignment axis 200A of the light incident side substrate 102 is 0°, and the angle θ2 of the alignment axis 200B of the light emitting side substrate 104 is 22.5°. The average alignment angle θa of the liquid crystal cell 110 is 11.25°.

[0139] AsFigure 32 As shown, in the second liquid crystal cell 120, the angle θ1 of the alignment axis 200A of the light incident side substrate 102 is 22.5°, and it matches the angle θ2 of the alignment axis 200B of the light emitting side substrate 104 of the liquid crystal cell 110. The angle θ2 of the alignment axis 200B of the light emitting side substrate 104 is 45°. The average alignment angle θa of the liquid crystal cell 120 is 33.75°.

[0140] As Figure 33 shown, in the third liquid crystal cell 130, the angle θ1 of the alignment axis 200A of the light incident side substrate 102 is 45°, and it matches the angle θ2 of the alignment axis 200B of the light emitting side substrate 104 of the liquid crystal cell 120. The angle θ2 of the alignment axis 200B of the light emitting side substrate 104 is 67.5°. The average alignment angle θa of the liquid crystal cell 130 is 56.25°.

[0141] As Figure 34 shown, in the fourth liquid crystal cell 140, the angle θ1 of the alignment axis 200A of the light incident side substrate 102 is 67.5°, and it matches the angle θ2 of the alignment axis 200B of the light emitting side substrate 104 of the liquid crystal cell 130. The angle θ2 of the alignment axis 200B of the light emitting side substrate 104 is 90°. The average alignment angle θa of the liquid crystal cell 140 is 78.75°.

[0142] In this embodiment, the absolute value of the difference between 45° and the average alignment angle θa of the liquid crystal cell 120 is equal to the absolute value of the difference between 45° and the average alignment angle θa of the liquid crystal cell 130 (absolute value: 11.25). The liquid crystal cells 120 and 130 correspond to the liquid crystal cell 100 with the smallest absolute value of the difference between 45° and the average alignment angle θa (absolute value: 11.25).

[0143] Similar to the controller 30 of Embodiment 5, the controller 30 of this embodiment applies a voltage to the liquid crystal cell 100 with the smallest absolute value of the difference between 45° and the average alignment angle θa, and this voltage is higher than the voltage applied to other liquid crystal cells 100. In this embodiment, the absolute value of the difference between 45° and the average alignment angles θa of the liquid crystal cells 120 and 130 is the smallest. Therefore, the controller 30 applies voltages V2 and V3 to the liquid crystal cells 120 and 130, and these voltages V2 and V3 are higher than the voltage V1 applied to the liquid crystal cell 110 and the voltage V4 applied to the liquid crystal cell 140. The voltage V2 applied to the liquid crystal cell 120 and the voltage V3 applied to the liquid crystal cell 130 can be equal, or one can be higher than the other.

[0144] Figure 35Shows an example in which voltages V2 and V3 higher than voltages V1 and V4 applied to liquid crystal cells 110 and 140 are applied to liquid crystal cells 120 and 130 where the absolute value of the difference between 45° and the average alignment angle θa is minimized, the polarization contrast pCR, and the voltages V1 - V4 applied to liquid crystal cells 110 - 140 accordingly; and an example of the polarization contrast pCR and the applied voltage (wavelength λ: 380 nm) in Comparative Example 3. Figure 36 Shows Figure 29 The relationship between the polarization contrast pCR shown and the average voltage value Va and the average voltage value Va of Comparative Example 3. Comparative Example 3 is an example in which the same voltages V1 - V4 are applied to liquid crystal cells 110 - 140 accordingly. The voltages V1 - V4 applied to liquid crystal cells 110 - 140 of this embodiment are obtained by the same method as in Embodiment 1.

[0145] As Figure 36 Shown, the average voltage value Va of this embodiment is less than the average voltage value Va of Comparative Example 3 (for example, when the polarization contrast pCR is 20, the average voltage value Va of this embodiment is 43.7 V, and the average voltage value Va of Comparative Example 3 is 45.6 V). That is, the polarization modulation device 10 of this embodiment can also emit a polarization light component with a desired polarization direction at high intensity and low voltage.

[0146] As described above, even when the nematic liquid crystal 106 is in a twisted alignment, the polarization modulation device 10 of this embodiment applies voltages V2 and V3 to liquid crystal cells 120 and 130 where the absolute value of the difference between 45° and the average alignment angle θa is minimized, and these voltages V2 and V3 are higher than the voltages V1 and V4 applied to liquid crystal cells 110 and 140. Therefore, the polarization modulation device 10 can reduce the influence of the residual birefringence near the interface of the nematic liquid crystal 106 on the emitted light L2, and can emit a polarization light component with a desired polarization direction at high intensity and low voltage.

[0147] Modification example

[0148] Although the embodiments have been described above, various modifications can be made to the present disclosure without departing from the spirit and scope of the present disclosure.

[0149] In Embodiments 1 - 5, the nematic liquid crystal 106 of the liquid crystal cell 100 is in an antiparallel alignment. However, a configuration in which the nematic liquid crystal 106 of the liquid crystal cell 100 is in a parallel alignment is also possible.

[0150] When the average alignment angle θa of angles θ1 and θ2 of the liquid crystal cell 110 is defined as θ11, the average alignment angle θa of angles θ1 and θ2 of the liquid crystal cell 120 is defined as θ12, and the average alignment angle θa of angles θ1 and θ2 of the liquid crystal cell 130 is defined as θ13, in Example 2 ( Figure 20 ) the angles θ1 of the alignment axis 200A, the angle θ2 of the alignment axis 200B, and the average alignment angle θa of the liquid crystal cells 110 - 130 can be expressed as follows: (a) When θ11 is 5° (θ11 = 5°), θ12 = θ11 + 25° or θ12 = θ11 + 30° and θ13 = θ12 + 35°.

[0151] (b) When θ11 is 10° or 15° (θ11 = 10° or θ11 = 15°), θ12 = θ11 + 25° or θ12 = θ11 + 30° and θ13 = θ12 + 30° or θ13 = θ12 + 35°.

[0152] In Example 6, the twist angle α of the nematic liquid crystal 106 is 22.5°. However, in the case where the nematic liquid crystal 106 in the liquid crystal cell 100 forming the polarization modulation element 20 is twisted and aligned, as long as the twist direction and the twist angle α of the nematic liquid crystal 106 in each liquid crystal cell 100 are the same, and in addition, as long as the sum of the twist angles α of each liquid crystal cell 100 is 90°. In addition, as long as the thickness d of the nematic liquid crystal 106 is adjusted to the thickness at which the polarization modulation element 20 gives a phase difference of 1 / 2 wavelength to the linearly polarized light L1 in the initial alignment state.

[0153] In Example 6, the polarization modulation element 20 is not limited to a phase difference of 1 / 2 wavelength, and can give a phase difference that is an odd multiple (2×n - 1 times, where n is a natural number) of 1 / 2 wavelength.

[0154] The plurality of liquid crystal cells 100 forming the polarization modulation element 20 include at least one liquid crystal cell 100 in which the absolute value of the difference between 45° and the average alignment angle θa is different from the absolute value of the difference between 45° and the average alignment angle θa of another liquid crystal cell 100. For example, as in Example 1, in the case where the polarization modulation element 20 is formed of three liquid crystal cells 100, the absolute value of the difference between 45° and the average alignment angle θa of two liquid crystal cells 100 is equal, and the absolute value of the difference between 45° and the average alignment angle θa of another liquid crystal cell 100 is different from the absolute value of the difference between 45° and the average alignment angle θa of two liquid crystal cells 100, such a configuration is possible. In addition, as in Examples 4 and 6, in the case where the polarization modulation element 20 is formed of four liquid crystal cells 100, the absolute values of the differences between 45° and the average alignment angles θa of the four liquid crystal cells 100 are different from each other, such a configuration is possible.

[0155] For purposes of explanation, some example embodiments have been described above. Although the foregoing discussion has presented specific embodiments, those skilled in the art will recognize that changes may be made in form and detail without departing from the broader spirit and scope of the invention. Accordingly, the specification and drawings are to be regarded as illustrative rather than restrictive. Thus, the detailed description should not be considered restrictive, and the scope of the invention is defined only by the included claims and the full equivalents to which those claims are entitled.

Claims

1. A polarization modulation device, comprising: a polarization modulation element including a plurality of liquid crystal cells, each of the plurality of liquid crystal cells including a light incident side substrate, a light emitting side substrate opposite to the light incident side substrate, and a liquid crystal layer sandwiched between the light incident side substrate and the light emitting side substrate, wherein linearly polarized light enters the light incident side substrate of a first liquid crystal cell among the plurality of liquid crystal cells; and a controller that applies a voltage to each of the plurality of liquid crystal cells to switch the polarization direction of the emitted light emitted from the polarization modulation element, wherein The plurality of liquid crystal cells are sequentially stacked with the light incident side substrate of one liquid crystal cell opposing the light emitting side substrate of another liquid crystal cell, and In the case where each of the angles of the alignment axis of the light incident side substrate and the alignment axis of the light emitting side substrate is set to an angle relative to the polarization direction of the linearly polarized light, and the average value of the angles of the alignment axis of the light incident side substrate and the alignment axis of the light emitting side substrate in each of the plurality of liquid crystal cells is defined as an average alignment angle, The average alignment angles of each of the plurality of liquid crystal cells are different from each other, The plurality of liquid crystal cells include at least one liquid crystal cell having an absolute value of a difference between 45° and the average alignment angle different from the absolute value of the another liquid crystal cell, and The controller applies a voltage higher than voltages applied to other liquid crystal cells to the liquid crystal cell having the smallest absolute value.

2. The polarization modulation device according to claim 1, wherein The angle of the alignment axis of the light incident side substrate and the angle of the alignment axis of the light emitting side substrate of each of the plurality of liquid crystal cells are equal, and In a state where no voltage is applied, each of the plurality of liquid crystal cells imparts a phase difference of 1 / 2 wavelength to light having a predetermined wavelength.

3. The polarization modulation device according to claim 1, wherein When no voltage is applied, the polarization modulation element applies a phase difference of an odd number of 1 / 2 wavelength to linearly polarized light having a predetermined wavelength. The plurality of liquid crystal cells are liquid crystal cells in which the liquid crystal layer is in a twisted alignment, The twist direction and twist angle of the liquid crystal layer of each of the plurality of liquid crystal cells are the same, The sum of the twist angles of each of the plurality of liquid crystal cells is 90°, and In adjacent ones of the liquid crystal cells, the angle of the alignment axis of the light emitting side substrate of one liquid crystal cell matches the angle of the alignment axis of the light incident side substrate of the other liquid crystal cell.

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