JP2006154492A - Liquid crystal element and optical attenuator - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a liquid crystal element having stabilized zero retardation when no voltage is applied and giving approximately λ/2 retardation at a wavelength λ of incident light in the liquid crystal layer at a low voltage. <P>SOLUTION: The liquid crystal element 10 comprises a pair of transparent substrates 11, 12 having transparent electrodes formed thereon and a liquid crystal layer 18, and varies retardation value to linearly polarized light depending on a voltage applied between the transparent electrodes 13, 14. The liquid crystal of the liquid crystal layer 18 is a nematic liquid crystal having negative dielectric anisotropy; in which, when no voltage is applied on the liquid crystal, the alignment direction of the liquid crystal molecules is almost perpendicular to the surfaces of the transparent electrodes 13, 14 and the retardation value is nearly zero; and when voltage 3 to 10 V is applied on the liquid crystal layer 18, the retardation value is 0.78 μm. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a liquid crystal element and an optical attenuator, and more particularly, to a liquid crystal shutter and an optical attenuator having a high extinction ratio with low voltage driving used for optical communication.

  2. Description of the Related Art With a dramatic increase in communication demand in recent years, a wavelength division multiplexing communication system (WDM: Wavelength Division Multiplexing communication system) that enables multiplexing of a plurality of signal lights having different wavelengths to enable large-capacity transmission using a single optical fiber. Development is underway. In such a wavelength division multiplex communication system, a large number of WDM wavelength channels having wavelengths in the range of 1490 nm to 1620 nm maintain high transmittance when a voltage is applied and emit light with a high extinction ratio when no voltage is applied. An optical shutter that cuts off or an optical attenuator that can adjust the amount of light according to the applied voltage is used.

Recently, development of an element using the electro-optic effect of a liquid crystal instead of the conventional mechanical optical shutter and the optical attenuator using the magneto-optic effect has been studied. Such an element has no moving parts, has low power consumption, and is advantageous for miniaturization, as compared with a conventional mechanical optical shutter and an optical attenuator using the magneto-optical effect. Thus, the following optical attenuators using liquid crystal elements have been proposed by the inventors of the present invention (see, for example, Patent Document 1).
In the optical attenuator using a liquid crystal element, the dielectric anisotropy is the difference between the relative dielectric constant of the liquid crystal molecular long axis direction epsilon _// and the dielectric constant of the liquid crystal molecules minor axis ε _⊥ △ ε (= ε _{/ /-epsilon ⊥)} is using positive liquid crystal, and a homogeneous orientation aligned in one direction parallel to the liquid crystal molecular alignment of the liquid crystal layer when no voltage is applied to the substrate surface. In addition, a phase plate that cancels out the residual retardation of the liquid crystal layer is integrated so that the retardation value of the liquid crystal layer becomes zero at an applied voltage of 10 V or less. Further, the layer thickness of the liquid crystal layer is adjusted so that the combined retardation value of the liquid crystal layer and the retardation plate when no voltage is applied is about λ / 2 with respect to the wavelength λ of the incident light.

  In such a liquid crystal element, when linearly polarized light with a polarization direction that forms an angle of 45 ° with respect to the alignment direction of the liquid crystal molecules on the substrate surface of the liquid crystal layer is incident, the incident linearly polarized light is blocked and orthogonal to the light exit side. By disposing a polarizer that transmits linearly polarized light having a polarization direction to the light exit side, the transmittance is maximum when no voltage is applied, and an optical attenuator whose transmittance decreases as the applied voltage increases.

A diffractive polarizer that efficiently transmits specific linearly polarized light and diffracts linearly polarized light having a polarization direction orthogonal to the specific linearly polarized light has been proposed, for example, by the inventors of the present invention (see Patent Document 2).
JP 2003-66450 A JP 2003-66232 A

However, in the case of the liquid crystal element described in Patent Document 1, the applied voltage range in which the retardation value of the liquid crystal layer is zero is narrow. As a result, when the extinction ratio is defined by the ratio of the minimum transmittance Imin and the maximum transmittance Imax with respect to the applied voltage change, it is difficult to stably obtain a high extinction ratio of 40 dB or more at a low voltage.
Further, in the diffractive polarizer described in Patent Document 2, a voltage non-voltage is obtained by arranging a polarizer that transmits incident linearly polarized light and blocks linearly polarized light having a polarization direction orthogonal to the incident linearly polarized light on the light output side of the liquid crystal element. Operation to block light when applied can be obtained, but the retardation value of the liquid crystal layer when no voltage is applied is about λ / 2, so the wavelength dependence of the phase difference occurs, and a high extinction ratio is obtained in a wide wavelength band Is difficult. If the retardation of the liquid crystal layer when no voltage is applied is canceled by the integrated phase plate, the combined retardation value when no voltage is applied becomes zero, and in principle, a high extinction ratio can be obtained in a wide wavelength band. However, it is difficult to make the combined retardation value of the liquid crystal layer and the phase plate stably zero.

  The present invention has been made in view of the above circumstances, and stabilizes the retardation value of the liquid crystal layer when no voltage is applied to zero, and sets the retardation value of the liquid crystal layer to a wavelength λ of incident light at a low voltage. An object of the present invention is to provide an optical attenuator capable of realizing a high extinction ratio by being driven at a low voltage by being integrated with a diffractive polarizer while providing a liquid crystal element having approximately λ / 2.

The present invention includes a pair of transparent substrates having transparent electrodes formed on opposing planes, and a liquid crystal layer in which liquid crystal is sandwiched between the transparent substrates, and the liquid crystal layer according to a voltage applied between the transparent electrodes. In the liquid crystal element that changes the retardation value of the linearly polarized light that passes through the liquid crystal, the liquid crystal is a nematic liquid crystal having a negative dielectric anisotropy, and the orientation direction of the liquid crystal molecules when no voltage is applied to the liquid crystal is transparent. Provided is a liquid crystal element characterized in that the liquid crystal layer has a retardation value of 0.78 μm with respect to any voltage in a range of 3 V to 10 V in the liquid crystal layer substantially perpendicular to the surface of the electrode. To do.
With this configuration, the retardation value of the liquid crystal layer when no voltage is applied (V = 0; hereinafter referred to as “off state”) becomes zero, and there is almost no dependency on the wavelength and incident angle of incident light. As a result, a high light blocking performance can be obtained by arranging a polarizer that transmits linearly polarized light having a polarization direction orthogonal to the incident linearly polarized light on the light exit side (hereinafter referred to as “orthogonal Nicol”). Further, since the retardation value of the liquid crystal layer is 0.78 μm for any voltage in the range of 3 V or more and 10 V or less (V = Vp; hereinafter referred to as “on state”), it is approximately 1 for the WDM wavelength channel. This corresponds to a retardation value of / 2 wavelength, and a high transmittance can be obtained with an orthogonal Nicol arrangement. Therefore, an optical shutter having a high extinction ratio can be obtained by switching the on state and the off state by the applied voltage. In addition, an optical attenuator with variable transmittance can be obtained with respect to a change in applied voltage between the on state and the off state.
In order to realize the ON state at a low voltage, the liquid crystal layer may be thickened. However, the response speed of the change in the retardation value of the liquid crystal layer with respect to the voltage switching between the ON state and the OFF state becomes slow. is there. On the other hand, in order to generate a voltage of 10 V or less, circuit components are expensive and problematic. For this reason, it is preferable that the on-state, that is, the retardation value of the liquid crystal layer becomes 0.78 μm at any voltage in the voltage range of 3 V to 10 V, more preferably 3 V to 5 V.

  In addition, the liquid crystal element in which a vertical alignment film in which an alignment direction of liquid crystal molecules when no voltage is applied is substantially perpendicular to a surface of the transparent electrode is formed at an interface between the liquid crystal layer and the transparent electrode. provide. With this configuration, the alignment of the liquid crystal molecules in the liquid crystal layer when no voltage is applied is stable and substantially perpendicular to the surface of the transparent electrode, so that the retardation value of the liquid crystal layer is stably zero.

In addition, the liquid crystal element is provided in which the vertical alignment film is subjected to a rubbing treatment so that liquid crystal molecules are inclined by 0.5 ° or more and 3 ° or less with respect to the substrate normal. By comprising in this way, the liquid crystal molecule of the liquid crystal layer at the time of voltage application can be stably tilted in the rubbing direction, and a desired retardation value can be obtained at a low voltage.

  An optical attenuator comprising the above liquid crystal element and a polarizer disposed on at least one of the light incident side and the light exit side of the liquid crystal element, wherein the polarizer is a first linearly polarized light. And a birefringent polarization diffractive polarizer that diffracts the second linearly polarized light having a polarization direction orthogonal to the polarization direction of the first linearly polarized light I will provide a. With this configuration, since there is no heat generation due to light absorption of the polarizer, an optical attenuator can be obtained that can obtain a stable extinction ratio according to the applied voltage.

The polarization diffraction polarizer is a multilayer diffraction polarizer in which at least two polarization diffraction gratings having birefringence are stacked, and the polarization diffraction grating is an ordinary light refraction formed on a transparent substrate. birefringent material layer rate n _o and an extraordinary refractive index n _{e (n o} ≠ n _e) is processed in the periodic roughness sectional shape, at least equal isotropic refractive index in the n _o or n _e in the recess An optical attenuator is provided that is filled with a transparent material. With this configuration, the extinction ratio of the polarizer is improved, so that an optical attenuator with a wide dynamic range is obtained.

  According to the present invention, in a liquid crystal element that includes a pair of transparent substrates and a liquid crystal layer in which liquid crystal is sandwiched between the transparent substrates, and changes a retardation value for linearly polarized light according to a voltage applied between the transparent electrodes. The liquid crystal is composed of nematic liquid crystal having negative dielectric anisotropy, the alignment direction of the liquid crystal molecules when no voltage is applied to the liquid crystal is substantially perpendicular to the surface of the transparent electrode, and the retardation value of the liquid crystal layer is approximately Since the retardation value of the liquid crystal layer is 0.78 μm with respect to any voltage in the range of 3V to 10V in the liquid crystal layer, the wavelength band used in the wavelength division multiplexing communication system (for example, λ = 1.490 μm to 1.620 μm), the retardation value of the liquid crystal layer when no voltage is applied is stably zero, and the retardation value of the liquid crystal layer is reduced to a wave of incident light at a low voltage. Possible to provide a liquid crystal element to substantially lambda / 2 with respect to lambda.

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings.
[First Embodiment]
1 and 2 show a liquid crystal element 10 according to a first embodiment of the present invention. The liquid crystal element 10 includes transparent substrates 11 and 12, transparent electrodes 13 and 14, and an alignment film 15. , 16, a seal 17, and a liquid crystal layer 18.
Among these, the alignment films 15 and 16 are vertical alignment films in which the alignment direction of liquid crystal molecules is substantially perpendicular to the surface. In this vertical alignment film, for example, by introducing a long-chain alkyl group (which may be partially or fully fluorinated) into polyimide or a silane compound, an organic material that makes the liquid crystal to be used substantially vertical is stable. The orientation can be maintained. That is, an organic material such as lecithin that is amphiphilic and has a C14 to C22 long chain alkyl group and a C18 long chain alkyl group such as octadecylchlorosilane is aligned perpendicularly to the substrate surface, whereby liquid crystal molecules Stable vertical alignment can be maintained by utilizing the property that the molecular long axis of each is aligned in parallel with the long-chain alkyl group.

  According to this embodiment, by applying a voltage to the transparent electrode 13 and the transparent electrode 14 from an external AC power supply, the orientation of the liquid crystal molecules in the liquid crystal layer 18 changes, and as a result, the retardation value changes.

Next, components of the liquid crystal element 10 and an example of a manufacturing procedure will be described below.
First, the transparent electrodes 13 and 14 are formed on one side of the transparent substrates 11 and 12. The transparent electrodes 13 and 14 are made of a material such as ITO (In ₂ O ₃ + SnO ₂ ) or SnO ₂ , and these materials are formed into a film thickness of about 5 to 300 nm by a sputtering method or a vacuum evaporation method.
Further, alignment films 15 and 16 are formed on the transparent electrodes 13 and 14. The alignment films 14 and 16 are vertical alignment films in which the alignment direction of liquid crystal molecules is substantially perpendicular to the surface. As described above, this vertical alignment film is, for example, an organic material in which a liquid crystal to be used is substantially vertical by introducing a long-chain alkyl group (which may be partially or fully fluorinated) into a polyimide or a silane compound. However, stable vertical alignment can be maintained.

Here, when an AC voltage is applied from the outside to the transparent substrate 11 and the transparent substrate 12, it is preferable to regulate the alignment so that the liquid crystal alignment of the liquid crystal layer 18 described later is inclined in a certain direction (see γ in FIG. 6). . Specifically, there are the following four types of orientation regulation methods.
(1) By rubbing the surface of the vertical alignment film in a specific direction, liquid crystal molecules at the interface of the vertical alignment film are pretilted in a specific direction when no voltage is applied.
(2) Fine irregularities are formed on the transparent electrode or the alignment film so that the liquid crystal molecules are inclined in a direction regulated by the irregular shape.
(3) The transparent electrode is finely patterned in advance so that the liquid crystal molecules are inclined in the direction of the electric field distribution generated when a voltage is applied.
(4) An inorganic material such as silicon oxide, metal oxide, metal fluoride, or composite oxide is obliquely deposited to form an alignment film. Examples of the metal oxide to be used include Ta ₂ O ₅ , WO ₃ , Bi ₂ O ₃ , and the like, and examples of the composite oxide include [(1-y) SiO _X ] + [yZrO ₂ ], [ _{(1-y) SiO X]} + [yTiO 2] (0 <x <2,0 <y <1) , and the like. Of these, SiO ₂ (0 <x <2) is preferable because of excellent stability of the alignment state.
In addition, compared with (2), (3), and (4), the alignment regulation method of (1) is advantageous for ensuring high alignment uniformity. The pretilt angle at this time is about 0.5 ° to 5 °, and the tilt angle with respect to the alignment film surface is about 89.5 ° to 85 °. As described above, for example, a polyimide or silane compound (which may be partially or wholly fluorinated) has a long-chain alkyl group as a vertical alignment film material that generates liquid crystal alignment during voltage application by rubbing the surface. The organic material which makes the liquid crystal to be used substantially vertical by introducing is preferable. In the present embodiment, the vertical alignment film is used after being rubbed in an angle direction of 45 ° with respect to the X axis.
As described above, by using the vertical alignment film, the liquid crystal molecules are aligned in a substantially vertical direction with respect to the substrate surface in the off state (applied voltage V = 0), so that the retardation value of the liquid crystal layer 18 is stably zero. It becomes. As a result, by adopting the orthogonal Nicol arrangement, it is possible to realize the blocking performance determined by the level of the extinction ratio of the polarizer without depending on the wavelength and the incident angle of the incident light.

Next, an adhesive (not shown) mixed with a gap control material on one side of the transparent substrate 11 is printed and patterned to form a seal 17, which is superimposed on the transparent substrate 12 and pressed to produce an empty cell. Moreover, injection port provided in a portion of the seal 17 ordinary (not shown) the refractive index n _o and extraordinary refractive index n _{e (where,} n _o ≠ n _e) injecting a nematic liquid crystal having, this note The inlet is sealed and the liquid crystal is sealed in the cell to form a liquid crystal layer 18 having a layer thickness G, whereby the liquid crystal element 10 of this embodiment is obtained.
As shown in FIGS. 1 and 2, in order to apply a voltage to the transparent electrode 14 through the electrode 14 </ b> A formed on the transparent substrate 11 side, conductive metal particles are mixed in the seal 17 in advance and the seal is pressure-bonded. Conductivity is developed in the seal thickness direction, and the transparent electrode 14 and the electrode 14A are conducted. An external AC power source is connected to the electrode 13A connected to the transparent electrode 13 and the electrode 14A connected to the second transparent electrode 14, and an AC voltage is applied to the liquid crystal layer 18. The transparent electrodes 13 and 14 are patterned in advance, and the alignment films 14 and 16 are also patterned so as not to contact the seal 17.

Next, a dielectric anisotropy _{△ ε (= ε // -ε ⊥} ) is negative the difference between the relative dielectric constant of the liquid crystal molecular long axis direction epsilon _// and the dielectric constant of the liquid crystal molecules minor axis direction epsilon _⊥ Specific conditions for forming the liquid crystal layer 18 using nematic liquid crystal will be qualitatively described.
Here, the dielectric anisotropy △ epsilon uses a negative liquid crystal may be the electric field and the vertical direction caused by the voltage applied to align the liquid crystal alignment direction (i.e., direction of the extraordinary refractive index n _e). In the present embodiment, the surface of the alignment films 14 and 16 is rubbed in the same direction (angular direction of 45 ° with respect to the X axis) to obtain a large change in the retardation value of the liquid crystal layer 18 at a low voltage. .
In general, the wavelength λ used in the WDM communication system is a long wavelength of 1490 nm to 1620 nm, and in order to obtain a high transmittance in the ON state (applied voltage V = Vp) of the crossed Nicols arrangement, the retardation value of the liquid crystal layer 18 is It is necessary to be λ / 2. On the other hand, examples of the nematic liquid crystal having a negative dielectric anisotropy Δε include P.I. As reported by Kirsch et al. (Liquid Crystals, 26, 449 (1999)), a ring structure containing an aromatic ring is bonded with a linking group to form a molecular long axis and negative dielectric anisotropy. In the major axis, for example, a functional group having a small polarity such as an alkyl group or an alkoxy group, and one or more polar groups in the minor axis direction (a halogen atom represented by a fluorine atom or a chlorine atom, -CN (cyano ), —NO ₂ , —NCS (isothiocyanate), —CF ₃ , —OCF _3, etc.).
In order to obtain the retardation value (λ / 2) of the liquid crystal layer 18 in the on state, the average refractive index n (Vp) of the liquid crystal layer 18 in the rubbing direction of the vertical alignment film is expressed by the following formula:
_{{N (Vp) -n o}} × G = λ / 2 ··· (1)
Where n _o : ordinary light refractive index
G: Layer thickness
It is necessary to satisfy the wavelength used (in a WDM communication system).
The average refractive index n (Vp) corresponds to the optical path length / layer thickness of the liquid crystal layer 18 with respect to linearly polarized light (that is, extraordinary light polarized light) on the polarization plane in the rubbing direction.

In general, a liquid crystal having a large absolute value of dielectric anisotropy Δε is more preferable in order for the liquid crystal molecules to produce a large alignment change at a low voltage. Further, the response time from the on state to the off state is inversely proportional to the square of the layer thickness (G) of the liquid crystal layer 18, so that the thinner the layer thickness (G) of the liquid crystal layer 18, the more advantageous is the high-speed response. Therefore, in order to obtain a thin thickness (G) in the above (1) is larger average refractive index satisfying the equation n (Vp) is preferably birefringence _{△ n (= n e -n o} ) is larger the crystal. Specifically, the retardation value of the liquid crystal layer 18 described by the formula (1) is set to 0. 0 in order to be in the on state in the applied voltage Vp of 3 V or more and 10 V or less and in the wavelength range of 1490 nm to 1620 nm. 78 μm.

  Accordingly, when linearly polarized light having a polarization plane in the Y-axis direction is incident on the liquid crystal element 10 having such a configuration, the polarization state is transmitted without change in the off state. On the other hand, in the ON state, the light is transmitted through the liquid crystal element 10 as linearly polarized light having a polarization plane in the X-axis direction. Therefore, when a polarizer is arranged with crossed Nicols on the light emitting side of the liquid crystal element 10, light is blocked in the off state, high transmittance is obtained in the on state, and an optical shutter having a high extinction ratio is obtained by switching the applied voltage. Further, when the applied voltage is changed from zero to Vp, the function of an optical attenuator that can adjust the amount of emitted light from zero to the maximum can be obtained.

[Second Embodiment]
Next, a configuration example of the liquid crystal element 20 according to the second embodiment of the present invention will be described in detail with reference to a side view shown in FIG.
The liquid crystal element 20 according to the second embodiment includes polarization diffraction polarizers 20A and 20B on the outer sides of the transparent substrates 11 and 12 provided on the light incident side and the light emission side of the liquid crystal element 10 of the first embodiment. Are integrated. In addition, since it describes in the above-mentioned patent document 2 about the structure and effect of polarization diffraction type polarizer 20A, 20B, detailed description is abbreviate | omitted.
This polarization diffraction type polarizer 20A (20B) includes a transparent substrate 11 (12), a transparent substrate 23A (23B), a transparent substrate 24A (24B), and a polarization diffraction grating layer 21A (21B) sandwiched between these substrates. ) And the polarization diffraction grating layer 22A (22B).

Here, referring to a cross-sectional view of the polarization diffractive polarizer 20A (20B) in FIG. 4, an example of components and a manufacturing procedure of the polarization diffractive polarizer 20A (20B) will be described.
First, a birefringent material layer having an ordinary light refractive index n _o and an extraordinary light refractive index n _e (n _o ≠ n _e ) is provided on one side of each of the transparent substrate 23A (23B) and the transparent substrate 24A (24B). The slow axis (the direction showing the extraordinary light refractive index) is aligned in a specific direction. Then the birefringent material layer, a diffraction grating 25 which cross-sectional shape having an uneven periodic structure of the step d ₁ and the grating pitch p _1, cycle sectional shape of the step d ₂ and the grating pitch p ₂ concavo-convex structure To a diffraction grating 26 having
Then, filling the isotropic transparent material 27 having a refractive index in at least diffraction grating respective recesses n _{s (ordinary} equal to the refractive index n _o or the extraordinary refractive index n _e). In this way, the polarizing diffraction grating layer 21A (21B) and the polarizing diffraction grating layer 22A (22B) are formed on the transparent substrate 23A (23B) and the transparent substrate 24A (24B), and the transparent substrate 23A (23B). The transparent substrate 24A (24B) and the transparent substrate 11 (12) of the liquid crystal element 10 are stacked to form a polarization diffraction polarizer 20A (20B).

  The longitudinal direction of the grating convex portions of the diffraction grating 25 and the diffraction grating 26 constituting the polarization diffraction grating layers 21A, 21B, 22A and 22B forms a predetermined angle between the transparent substrate 24A and the transparent substrate 24B. Preferably it is. That is, the diffraction of the four types of polarization diffraction grating layers 21A, 22A, 21B, and 22B is prevented so that the diffracted light generated by the diffraction gratings constituting the polarization diffraction polarizers 20A and 20B is not multiplexly diffracted and superimposed on the linearly transmitted light. It is preferable to process the lattices so that the lattice longitudinal directions are all different (for example, β to ε in FIG. 6).

5A and 5B are plan views in the case where the angles formed by the diffraction grating 25 and the X axis in the longitudinal direction of the diffraction grating 26 are θ1 and θ2.
Here, for example, the refractive index _{n s} is using approximately equal isotropic transparent material 27 and the ordinary refractive index _{n o,} the retardation value _| is _{× d 2 | n e -n s} | × d 1 and _| n e -n _s Assuming that the steps d ₁ and d ₂ are approximately half the wavelength of the incident light, the amount of the linearly transmitted light is minimized with respect to the incident light of the extraordinary light polarization, and the extinction is the ratio of the transmittance of the ordinary light polarized light that is transmitted in the straight direction. The ratio is high. Since the polarizing diffraction grating layer 21A (21B) and the polarizing diffraction grating layer 22A (22B) in which the slow axis directions of the birefringent materials used for the diffraction grating are aligned are laminated, the linearly transmitted light is compared with the case of a single layer. In contrast, a polarizer having a high extinction ratio can be obtained.
As the diffraction grating 25 and the diffraction grating 26 made of a birefringent material, it is preferable to use a polymer liquid crystal having a large amount of birefringence. For example, a diffraction grating made of a polymer liquid crystal can be used to form a polymer liquid crystal layer having a constant film thickness in which molecular alignment is aligned in the direction of alignment treatment by irradiating ultraviolet rays after applying a liquid crystal monomer to the substrate surface subjected to the alignment treatment. It can be formed and processed into a lattice shape by photolithography or reactive ion etching.

Next, in the liquid crystal element 20 of the present embodiment, the orientation directions of the diffraction grating 25 and the diffraction grating 26 (that is, the direction of the extraordinary light refractive index) made of polymer liquid crystals of the polarization diffraction polarizer 20A and the polarization diffraction polarizer 20B. ) And the X axis in the longitudinal direction of the lattice, the angle θ ₁ and the angle θ ₂ , and the on-state liquid crystal molecule tilt direction of the liquid crystal layer 18 of the liquid crystal element 10 (that is, the alignment treatment direction of the alignment film 14 and the alignment film 16) An example of the relationship is shown in FIG.

First, the polymer liquid crystals of the polarization diffraction grating layer 21A and the polarization diffraction grating layer 22A in the polarization diffraction polarizer 20A are aligned in the X-axis direction in FIG. Further, the diffraction grating 25 has θ ₁ = 90 ° (α in FIG. 6), and the diffraction grating 26 has θ ₂ = 0 ° (β in FIG. 6). That is, the linearly polarized light of the polarization plane in the Y-axis direction incident on the polarization diffraction type polarizer 20A is transmitted straight without being diffracted, and the linearly polarized light of the polarization plane in the X-axis direction is diffracted in the X-axis direction and the Y-axis direction. There is almost no light that passes straight through.
On the other hand, the polymer liquid crystals of the polarization diffraction grating layer 21B and the polarization diffraction grating layer 22B in the polarization diffraction polarizer 20B are aligned in the Y-axis direction in FIG. The diffraction grating 25 has θ ₁ = 135 ° (δ in FIG. 6), and the diffraction grating 26 has θ ₂ = 45 ° (ε in FIG. 6). That is, the linearly polarized light of the polarization plane in the X-axis direction incident on the polarization diffraction type polarizer 20B is transmitted straight without being diffracted, and the linearly polarized light of the polarization plane in the Y-axis direction is diffracted in 45 ° and 135 ° directions. There is almost no light that passes straight through.

Therefore, the linearly polarized light in the polarization plane in the X-axis direction that enters the liquid crystal element 20 from the polarization diffraction type polarizer 20A side is diffracted by the polarization diffraction type polarizer 20A, and therefore there is almost no light that passes straight through the liquid crystal element 20. . On the other hand, the linearly polarized light on the plane of polarization in the Y-axis direction passes straight without being diffracted by the polarization diffraction polarizer 20 </ b> A and enters the liquid crystal element 10.
Here, when the liquid crystal element 10 is in the OFF state, the polarization state does not change and the linearly polarized light with the polarization plane in the Y-axis direction is incident on the polarization diffraction type polarizer 20B and is diffracted by the polarization diffraction type polarizer 20B. There is almost no light that passes straight through the liquid crystal element 20. On the other hand, when the liquid crystal element 10 is in the ON state, the polarization state changes, and the light enters the polarization diffractive polarizer 20B as linearly polarized light on the polarization plane in the X-axis direction without being diffracted by the polarization diffractive polarizer 20B. The liquid crystal element 20 passes straight through. Here, when the diffracted light by the polarization diffraction type polarizer 20 </ b> A is diffracted by the polarization diffraction type polarizer 20 </ b> B, the diffraction directions of the diffracted light by the respective diffraction gratings are different, so that they are not superimposed on the straight transmitted light.
That is, the liquid crystal element 20 diffracts incident light in the off state regardless of the polarization state of the incident light, and in the on state, it linearly transmits only linearly polarized light on the polarization plane in the Y-axis direction with high efficiency.
In the liquid crystal element 20 shown in FIG. 3, the polarization diffractive polarizer 20A is arranged on the light incident side. However, if the incident light is linearly polarized light with a high linearity in the polarization direction in the Y-axis direction, the polarization diffractive type is used. The polarizer 20A may not be used.

Next, FIG. 7 is a side view showing an example of an optical system configuration in the case where the liquid crystal element 20 of the present invention is used as an optical shutter or an optical attenuator in which the amount of transmitted light changes according to the applied voltage.
In this optical system, the condensing lens 1 is arranged on the exit side of the liquid crystal element 20, and when parallel light mixed with linearly polarized light in the polarization planes in the X-axis direction and the Y-axis direction is incident on the liquid crystal element 20, the optical system is turned on. The linearly polarized light on the polarization plane in the Y-axis direction that has been transmitted straight through the liquid crystal element 20 in the state is condensed on the focal plane on the optical axis of the condenser lens 1. On the other hand, the light diffracted by the liquid crystal element 20 is condensed on the focal plane outside the optical axis of the condenser lens 1. Therefore, by disposing the aperture stop 2 having an aperture on the focal plane on the optical axis of the condenser lens 1, the linearly polarized incident light on the polarization plane in the Y-axis direction is transmitted only when the liquid crystal element 20 is in the ON state. In the off state, the light shutter blocks the incident light.
Here, instead of the aperture stop 2, a photodetector having a light receiving portion corresponding to the opening may be arranged. The same function can be obtained even if the core of the optical fiber for optical transmission is arranged instead of the opening. Further, in the voltage in the intermediate range between the off state (applied voltage V = 0) and the on state (applied voltage V = Vp), the light attenuator in which the amount of light transmitted through the straight line is variable according to the magnitude of the applied voltage is obtained.
As described above, in the case of the optical system using the liquid crystal element 20 of FIG. 7, the light of the polarization plane in the X-axis direction is not used as signal light in the incident light.

Next, in order to realize an optical shutter and optical attenuator having high light utilization efficiency regardless of the polarization state of incident light, the configuration of the polarization-independent optical shutter and optical attenuator 30 provided with the liquid crystal element 20 is shown in FIG. Show.
The optical shutter and optical attenuator 30 has a configuration in which the polarization conversion element 3A and the polarization conversion element 3B are arranged on the light incident side and the light emission side of the liquid crystal element 20.
In the polarization conversion elements 3A and 3B, polarization separation mirrors 31A and 31B and total reflection mirrors 32A and 32B are formed on the inclined surface of a right-angled isosceles glass prism. It has a bonded configuration.
The polarization separation mirrors 31A and 31B are made of a dielectric multilayer film, and transmit linearly polarized light having a polarization plane in the Y-axis direction and reflect linearly polarized light having a polarization plane in the X-axis direction. Further, the half-wave plates 33A and 33B are bonded to a part of the light incident / exit surfaces of the polarization conversion elements 3A and 3B, and the linearly polarized light in the X-axis direction is converted to the straight line in the Y-axis direction. Convert to polarized light.
The light incident on the polarization separation mirror 31A of the polarization conversion element 3A becomes linearly polarized light having a polarization plane in the Y-axis direction after the linearly polarized light having the polarization plane in the X-axis direction is reflected and transmitted through the half-wave plate 33A. Incident on the liquid crystal element 20. On the other hand, of the light incident on the polarization separation mirror 31A, the linearly polarized light with the polarization plane in the Y-axis direction passes through the polarization separation mirror 31A and enters the liquid crystal element 20. That is, all the incident light of the liquid crystal element 20 is linearly polarized light with a polarization plane in the Y-axis direction.
Next, when the liquid crystal element 20 is in the ON state, as shown in FIG. 8, the polarization component that is transmitted through the liquid crystal element 20 and becomes linearly polarized light in the polarization plane in the X-axis direction is half that of the polarization conversion element 3B. The light transmitted through the wave plate 33B becomes linearly polarized light having a polarization plane in the Y-axis direction, passes through the polarization separation mirror 31B, and is emitted from the polarization conversion element 3B in the Z-axis direction. The light having the polarization plane in the X-axis direction that is reflected by the total reflection mirror 32B is reflected by the polarization separation mirror 31B and is emitted from the polarization conversion element 3B in the Z-axis direction while maintaining the polarization plane in the X-axis direction. At this time, the condenser lens 1 and the aperture stop 2 (not shown) are disposed on the light emitting side in the Z direction, as in FIG.
On the other hand, when the liquid crystal element 20 is in the OFF state, the light transmitted through the half-wave plate 33B of the polarization conversion element 3B out of the linearly polarized diffracted light of the polarization plane in the Y-axis direction that transmits the liquid crystal element 20 is X It becomes linearly polarized light of the polarization plane in the axial direction, is reflected by the polarization separation mirror 31B, and is emitted from the polarization conversion element 3B in the Y-axis direction. Further, the light of the polarization plane in the Y-axis direction reflected by the total reflection mirror 32B passes through the polarization separation mirror 31B and is emitted from the polarization conversion element 3B in the Y-axis direction with the same polarization plane. An AC voltage is applied to the liquid crystal element 20 using an AC power source.

  As a result, regardless of the normal polarization state of the incident light, when the liquid crystal element 20 is in the off state, the incident light is diffracted and there is almost no light that passes straight through the polarization conversion element 3B. In addition, when the liquid crystal element 20 is in an on state, incident light is transmitted through the polarization conversion element 3B in a straight line in the Z-axis direction without being diffracted. Here, even if the polarization separation mirrors 31A and 31B of the polarization conversion elements 3A and 3B are insufficient, the liquid crystal element 20 has a high extinction ratio, so that it is high as a polarization-independent optical shutter and an optical attenuator. Extinction ratio and dynamic range are realized.

Next, the liquid crystal element 10 according to the present invention will be specifically described with reference to FIG.
In the wavelength 1.55μm ordinary refractive index _n o (LC) is 1.49 and the extraordinary refractive index _n e (LC) is 1.63, the dielectric anisotropy _{△ ε (= ε // -ε ⊥} ) is - The nematic liquid crystal 4 was sandwiched between the transparent substrates 11 and 12 having the transparent electrodes 13 and 14 formed on one side, and the liquid crystal element 10 having a liquid crystal layer 18 having a layer thickness G of 12.5 μm was produced.
Vertical alignment films 15 and 16 made of polyimide are formed on the surfaces of the transparent electrodes 13 and 14 in contact with the liquid crystal layer 18 and are rubbed at an angle of 45 ° with the X axis. At this time, the liquid crystal molecules at the interface of the vertical alignment film of the liquid crystal layer 18 form a pretilt angle of approximately 89 ° with respect to the substrate plane, and are uniformly inclined in the angle direction of 45 ° with respect to the X axis.
In an off state in which no AC voltage is applied to the transparent electrodes 13 and 14, the retardation value of the liquid crystal layer 18 with respect to light having a wavelength λ of 1.55 μm is almost zero, and the polarization state of incident light is not changed. To Penetrate. Next, when a voltage is applied to the transparent electrodes 13 and 14, the liquid crystal molecules of the liquid crystal layer 18 are tilted in the XY plane direction at an angle direction of 45 ° with respect to the X axis according to the applied voltage, and the retardation value of the liquid crystal layer 18 is To increase. The retardation value becomes λ / 2 at an applied voltage of 3.6V.

Next, as shown in FIG. 3, polarization diffractive polarizers 20 </ b> A and 20 </ b> B are integrated on the light incident side and the light emitting side of the liquid crystal element 10 to form the liquid crystal element 20.
The polarization diffraction grating layer 21A (21B) and the polarization diffraction grating layer 22A (22B) of the polarization diffraction polarizer 20A (20B) are both a diffraction grating 25 and a diffraction grating 26 made of polymer liquid crystal. Liquid crystalline polymers, with the ordinary refractive index _n o (= 1.55) and an extraordinary refractive index _n e (= 1.67), both cross-section a rectangular shape, the film thickness of the grid convex portion 6.3μm To 6.6 μm linear grid. In addition, the grating pitch is 20 μm in all cases, and the diffraction angle of ± first-order diffracted light is 4.4 ° with respect to the wavelength λ (= 1.55 μm). The alignment direction of the polymer liquid crystal of the diffraction grating 25 and the diffraction grating 26, the angle formed with the X axis in the longitudinal direction of the grating, and the alignment direction of the liquid crystal layer 18 of the liquid crystal element 10 are the same as those in FIG. 6 described in the second embodiment. It is related. Further, a transparent adhesive 27 having a uniform refractive index n _s (= 1.55) is filled in the concave portions of the diffraction grating 25 and the diffraction grating 26, and is bonded and fixed to the transparent substrate 11 (12) and the transparent substrate 23A (23B). .

Next, the light shown in FIG. 7 in which the liquid crystal element 20 thus obtained and the condenser lens 1 are combined and an aperture stop 2 having an opening on the focal plane on the optical axis of the condenser lens 1 is disposed. A shutter or an optical attenuator is formed.
In this optical shutter or optical attenuator, parallel light incident from the polarization diffraction polarizer 20 </ b> A side condenses the emitted light on the focal plane of the condenser lens 1. The light diffracted by the liquid crystal element 20 is blocked by the aperture stop 2, and only the light that has been transmitted straight through the liquid crystal element 20 is transmitted by the photodetector through the opening of the aperture stop 2, and a photodetector (not shown). Detect at.
Here, as shown in FIG. 2, an AC power supply (not shown) is connected to the electrodes 13 A and 14 A of the liquid crystal element 20, and an AC voltage V is applied to the liquid crystal layer 18. Then, the detected light amount when the applied voltage V is changed from zero to 4 V is measured, and the ratio to the detected light amount when there is no liquid crystal element 20 is shown in FIG. 9 as the extinction ratio. In the figure, □, ●, and ○ indicate measurement results for incident light having wavelengths of 1.52 μm, 1.56 μm, and 1.60 μm, respectively.
According to FIG. 9, the liquid crystal element 20 exhibits a high extinction ratio of −50 dB or less in the off state (V = 0) with respect to incident light of 1.52 μm to 1.60 μm in the WDM wavelength region, and the on state ( V = Vp = 3.6 V), and a high transmittance of about −0.2 dB was obtained. That is, an optical shutter having a high extinction ratio can be obtained by switching between zero voltage and Vp.
Further, by changing the applied voltage from 1.8 V to 3.6 V, there is almost no wavelength dependency of the incident light, and the extinction ratio, that is, the amount of transmitted light continuously changes from about −50 dB to about −0.2 dB. It becomes a low voltage drive optical attenuator. Since the liquid crystal layer 16 has high impedance and almost no current flows, the power consumption of the liquid crystal element 20 due to voltage application is very small.

  The present invention is not limited to the embodiment described above, and can be implemented in various forms without departing from the gist of the present invention.

  The liquid crystal device of the present invention can be used as an optical shutter or an optical attenuator that requires low voltage drive, low power consumption, and high extinction ratio in the wavelength band used in the wavelength division multiplexing communication system. In particular, since the polarization diffraction type polarizer is integrated on the light incident surface and the light exit surface, an optical component that requires a stable and high extinction ratio by itself without adjusting the position with other optical components. It can be used for such purposes.

1 is a side sectional view showing a configuration of a liquid crystal element according to a first embodiment of the present invention. FIG. 2 is a cross-sectional view illustrating a configuration of the liquid crystal element illustrated in FIG. 1. The sectional side view which shows the structure of the liquid crystal element of 2nd Embodiment which concerns on this invention. FIG. 3 is a side cross-sectional view showing a configuration of a polarization diffraction type polarizer that is a component of the liquid crystal element of the present invention. (A), (B) is a top view which respectively shows the angle of the grating | lattice longitudinal direction of the diffraction grating which is a component of each polarization | polarized-light diffraction type polarizer of the liquid crystal element of this invention. The top view which shows the relationship between the birefringent material (polymer liquid crystal) orientation direction of the diffraction grating which is a component of the polarization diffraction type polarizer of the liquid crystal element of this invention, the angle of a grating | lattice longitudinal direction, and the orientation direction of a liquid crystal layer. FIG. 3 is a side sectional view showing an optical system configuration of an optical shutter or an optical attenuator using the liquid crystal element of the present invention. The sectional side view which shows the other optical system structure of the optical shutter or optical attenuator using the liquid crystal element of this invention. The graph which shows an example of the measurement result of the detected light quantity of the transmitted light when a voltage is applied to the liquid crystal element of this invention from zero to 4V.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Condensing lens 10, 20 Liquid crystal element 11, 12, 23A, 23B, 24A, 24B Transparent substrate 13, 14 Transparent electrode 13a, 14A Electrode 15, 16 Alignment film 17 Seal 18 Liquid crystal layer 2 Aperture stop 20A, 20B Polarization diffraction type Polarizer 21A, 21B, 22A, 22B Polarization diffraction grating layer 25, 26 Diffraction grating 27 Transparent adhesive (isotropic transparent material)
3A, 3B Polarization conversion element 30 Optical shutter (optical attenuator)
31A, 31B Polarization separation mirror 32A, 32B Total reflection mirror 33A, 33B 1/2 wavelength plate 4 AC power supply

Claims (5)

A pair of transparent substrates having transparent electrodes formed on opposing planes, and a liquid crystal layer in which liquid crystal is sandwiched between the transparent substrates, and the liquid crystal layer is formed according to the magnitude of a voltage applied between the transparent electrodes. In the liquid crystal element that changes the retardation value for the light of linearly polarized light that is transmitted,
The liquid crystal is a nematic liquid crystal having negative dielectric anisotropy,
The orientation direction of the liquid crystal molecules when no voltage is applied to the liquid crystal is perpendicular to or close to the surface of the transparent electrode,
The liquid crystal element, wherein a retardation value of the liquid crystal layer is 0.78 μm with respect to any voltage in the range of 3 V to 10 V in the liquid crystal layer.   2. The liquid crystal according to claim 1, wherein a vertical alignment film is formed at an interface between the liquid crystal layer and the transparent electrode so that an alignment direction of liquid crystal molecules when no voltage is applied is substantially perpendicular to a surface of the transparent electrode. element.   The liquid crystal element according to claim 2, wherein liquid crystal molecules are inclined at an angle of 0.5 ° or more and 5 ° or less with respect to the substrate normal by rubbing the vertical alignment film. An optical attenuator comprising the liquid crystal element according to any one of claims 1 to 3 and a polarizer disposed on at least one side of a light incident side or a light emission side of the liquid crystal element,
The polarizer is a birefringent polarization diffractive polarizer that linearly transmits the first linearly polarized light and diffracts the second linearly polarized light having a polarization direction orthogonal to the polarization direction of the first linearly polarized light. An optical attenuator characterized by The polarization diffraction polarizer comprises a multilayer diffraction polarizer in which at least two polarizing diffraction gratings having birefringence are laminated,
The polarizing diffraction grating, birefringent material layer having ordinary formed on a transparent substrate refractive index n _o and extraordinary refractive index _{n e (n o ≠ n e} ) is processed in the periodic roughness sectional shape together and, at least optical attenuator according to claim 4, isotropic transparent material having a refractive index equal to n _o or n _e is filled in the concave portion.

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