JP2012042900A - Polarization independent variable focus lens - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a variable focus lens capable of changing a focal length at high speed without depending on polarized light of incident light.SOLUTION: A polarization independent variable focus lens includes an electrooptical material including a single crystal having inversion symmetry, multiple electrode pairs each of which is formed on a first surface of the electrooptical material and a second surface opposed to the first surface and formed at positions facing each other, a first basic unit element, in which a cylindrical variable focus lens that varies a focal point of light to be passed through the electrooptical material by changing applied voltages between the electrode pairs is defined as a unit element, a polarization rotating element which rotates linearly polarized light of light which has been passed through the first basic unit element by 90 degrees, and a second basic unit element which is arranged such that light which has been passed through the polarization rotating element is converged in the same direction with the first basic unit element.

Description

  The present invention relates to a variable focus lens, and more particularly to a variable focus lens that can change a focal length using an optical material having an electro-optic effect.

  Conventionally, optical components such as optical lenses and prisms are optical devices such as cameras, microscopes, and telescopes, electrophotographic recording devices such as printers and copiers, optical recording devices such as DVDs, optical devices for communication, industrial use, etc. It is used for. A normal optical lens has a fixed focal length. However, a lens that can adjust the focal length according to the situation, a so-called variable focus lens may be used in the above-described devices and apparatuses. The conventional variable focus lens mechanically adjusts the focal length by combining a plurality of lenses. However, such a mechanical variable focus lens has a limit in extending the application range from the viewpoint of response speed, manufacturing cost, miniaturization, power consumption, and the like.

  Therefore, a variable focus lens in which a material capable of changing the refractive index is applied to the transparent medium constituting the optical lens, a variable focus lens that mechanically deforms the shape of the optical lens, instead of moving the position of the optical lens, etc. It was issued. As the former variable focus lens, a variable focus lens using liquid crystal as an optical lens has been proposed. This variable focus lens encloses the liquid crystal in a container made of a transparent material by sandwiching the liquid crystal between two glass plates. When the inside of the container is processed into a spherical surface and the liquid crystal is molded into a lens shape, a variable focus lens can be configured. A transparent electrode is provided inside the container, and the refractive index is controlled by applying an electric field to the liquid crystal, and the focal length is variably controlled (see, for example, Patent Document 1).

  As the latter variable focus lens, liquid is often used as the material of the deformable lens. For example, the variable focus lens described in Non-Patent Document 1 has a structure in which a liquid such as silicon oil is sealed in a space sandwiched between glass plates. The glass plate is thinly processed. By applying pressure to the glass plate with a lead zirconate titanate (PZT) piezo actuator from the outside, the lens composed of the oil and the entire glass plate is deformed, and the focal position is adjusted. Control.

Japanese Patent Laid-Open No. 11-64817

Takashi Kaneko et al., "Long focal depth visual mechanism using variable focus lens", Denso Technical Review, Vol.3, No.1, p.52-58, 1998

  However, the conventional variable focus lens includes any one of a variable focus lens that mechanically adjusts the focal length, a variable focus lens that controls the refractive index by applying an electric field to liquid crystal, and a variable focus lens that deforms the lens by a PZT piezoelectric actuator. However, the response speed required to change the focal length is limited, and there is a problem that it cannot be applied to a high-speed response of 1 ms or less.

  An object of the present invention is to provide a variable focus lens that can change the focal length at high speed without depending on the polarization of incident light.

  In order to achieve such an object, an embodiment of the present invention is directed to an electro-optic material made of a single crystal having inversion symmetry, a first surface of the electro-optic material, and facing the first surface. A plurality of electrode pairs formed at positions facing each of the second surfaces, and changing the applied voltage between the electrode pairs to change the focal point of the light transmitted through the electro-optic material. Using the focal lens as a basic unit element, a first basic unit element, a polarization rotating element that rotates the linearly polarized light of the light transmitted through the first basic unit element by 90 degrees, and light transmitted through the polarization rotating element, And a second basic unit element arranged so as to collect light in the same direction as the first basic unit element.

The electro-optic material is preferably a perovskite single crystal material, typically potassium tantalate niobate (KTN: KTa _1-x Nb _x O ₃ , 0 <x <1). In the electro-optic material, the main component of the crystal is composed of groups Ia and Va in the periodic table, group Ia is potassium, and group Va can contain at least one of niobium and tantalum. Furthermore, it is also possible to include one or more members of Group Ia of the periodic table excluding potassium as an additive impurity, for example, lithium, or Group IIa.

  As described above, according to the present invention, an electro-optic material made of a single crystal having inversion symmetry and two sets of electrodes formed on the surface of the electro-optic material, the applied voltage between the electrode pair By configuring the variable focus lens, the focal point of the emitted light can be varied, and by arranging one or two pairs of this variable focus lens and a half-wave plate, it does not depend on the direction of polarization. A variable focus lens can be realized.

It is a figure which shows the structure of the variable focus lens concerning the 1st Embodiment of this invention. It is a figure for demonstrating the principle of the variable focus lens concerning 1st Embodiment. It is a figure which shows the example of the optical path length of the variable focus lens concerning 1st Embodiment. It is a figure which shows the example of polarization independence of a variable focus lens. It is a figure which shows the difference in the optical path length distribution of the variable focus lens by two numerical calculation methods. It is a figure which shows the structure of the polarization independent variable focus lens concerning the 2nd Embodiment of this invention. It is a figure which shows the structure of the biaxial polarization independent variable focus lens concerning the 3rd Embodiment of this invention. It is a figure which shows the structure of the polarization independent variable focus lens concerning the 4th Embodiment of this invention. It is a figure which shows the structure of the variable focus lens concerning the 6th Embodiment of this invention. It is a figure which shows the structure of the variable focus lens concerning the 8th Embodiment of this invention. It is a figure which shows the structure of the polarization independent variable focus lens concerning the 9th Embodiment of this invention. 6 is a diagram illustrating a configuration of a biaxial polarization-independent variable focus lens according to Example 3. FIG.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. The variable focus lens of this embodiment is composed of an electro-optic material and an electrode attached thereto. By utilizing the electro-optic effect, a much faster response speed can be obtained as compared with a conventional variable focus lens.

  The present invention provides a variable focus lens independent of polarization by combining two or more polarization dependent variable focus lenses. First, a polarization-dependent variable focus lens that is a structural unit will be described.

(Configuration of cylindrical variable focus lens)
FIG. 1 shows the configuration of a variable focus lens according to the first embodiment of the present invention. Four strip-shaped electrodes are formed on the upper surface (first surface) and the lower surface (second surface) of the substrate 1 obtained by processing the electro-optic material into a plate shape. An anode 2 (first anode) is disposed as an upper electrode on the light incident side, and a cathode 3 (first cathode) is disposed as a lower electrode across the substrate 1. Further, another pair of electrodes is disposed on the light emission side, with a distance from these electrode pairs, the upper electrode being the cathode 4 (second cathode), and the lower electrode being the anode 5 (second anode). It is. The four strip-shaped electrodes have a shape in which all the sides in the longitudinal direction are parallel.

  Light enters from a surface (third surface) perpendicular to the surface on which the electrodes are arranged, travels in the x-axis direction inside the substrate 1, and between the anode 2 and the cathode 3, the longitudinal direction of these strip electrodes Transmits in the vertical direction. Next, after passing between the cathode 4 and the anode 5, it is set so as to be emitted into the air from the surface (fourth surface) opposite to the incident surface.

  In such a configuration, a voltage is applied between the anode and the cathode. The direction of applying a voltage (z-axis direction) is opposite between the light incident side electrode pair and the light emission side electrode pair. The potentials of the anode 2 and the anode 5 may be different, and the potentials of the cathode 3 and the cathode 4 are the same. The lower potential of the anodes 2 and 5 is set to be higher than the higher potential of the cathodes 3 and 4.

  At this time, an electric field distribution is generated between these electrodes, and the refractive index is modulated by the electro-optic effect of the substrate 1. When light is transmitted through the refractive index modulated portion, the light is bent by this refractive index distribution, and as a result, the light is condensed or diverged. When condensed, according to the structure of FIG. 1, it functions as a cylindrical convex lens, and when diverged, it functions as a cylindrical concave lens. In this way, the light is condensed or diverged in the uniaxial direction, and thus is called uniaxial modulation. Further, since the degree of bending of light changes depending on the applied voltage, the focal length can be controlled by the voltage.

  The electro-optic effect responds in a time of 1 μs or less even if it is late estimated from the application of the voltage. Therefore, it is possible to realize a variable focus lens that responds significantly faster than the conventional variable focus lens. As described above, the element shown in FIG. 1 is a cylindrical variable focus lens and is a basic unit constituting various lenses. In order to realize a normal spherical lens, two cylindrical variable focus lenses and a half-wave plate inserted between them are combined as basic unit elements, and the two basic unit elements are 90.degree. Arrange them at an angle of degrees. In this way, the light is condensed or diverged in the biaxial direction, which is called biaxial modulation. In the present embodiment, the material of the substrate 1 is characterized by using a material made of a crystal having inversion symmetry, among materials having an electro-optic effect, and the reason will be described later.

  Hereinafter, the state of refractive index modulation and the function as a lens will be described in detail with reference to FIG. FIG. 2 shows a side view of the variable focus lens shown in FIG. 1 as viewed from the y-axis direction. The substrate 1 has a uniform refractive index when no voltage is applied to the four electrodes, so that light passes through without being modulated. Therefore, there is no lens function. However, when a plane wave is incident, the wavefront of the light emitted from the substrate 1 remains a plane, and it can be regarded as a lens with an infinite focal length considering that the radius of curvature is infinite.

  When voltages are applied to the four electrodes, electric lines of force 6 as shown in FIG. 2 are generated between these electrodes. The electric lines of force 6 are generated not only between the anode 2 and the cathode 3 and between the cathode 4 and the anode 5 but also widely spread outside these electrodes. The generation of electric lines of force means that an electric field is generated. At this time, since the substrate 1 has an electro-optic effect, the refractive index is modulated at a portion where an electric field is generated inside the substrate 1. Inside the substrate 1, in the vicinity of the four electrodes, that is, in the vicinity of the surface of the substrate 1, the electric field is large and the refractive index change is large. On the other hand, in the central part of the substrate 1 (near the center in all axial directions), the electric field is relatively small and the refractive index change is small.

  The right side of FIG. 2 schematically shows a refractive index modulation curve 7 representing a distribution of refractive index change. The vertical axis of the refractive index modulation curve is the z-axis coordinate, and the horizontal axis is the refractive index change Δn from when no voltage is applied. FIG. 2 shows that the refractive index changes in the negative direction as a whole, but the modulation is large in the vicinity of the surface of the substrate 1, and therefore the refractive index change Δn is small. On the other hand, the modulation is small in the vicinity of the central portion, and therefore the change in refractive index Δn is not as small as the vicinity of the surface. When light passes through such a refractive index distribution, it functions as a convex lens because the speed of light near the surface is higher than the speed of light at the center of the substrate 1. That is, the focal point moves from an infinite focal length when no voltage is applied to a finite focal length.

(Electro-optic material)
The electro-optic effect includes several different-order electro-optic effects, but generally, a first-order electro-optic effect (hereinafter referred to as Pockels effect) is used. In the Pockels effect, the refractive index change is proportional to the electric field. In the configuration shown in FIGS. 1 and 2, the direction of the electric field is reversed and the refractive index distribution is also reversed between the anode 2 and the cathode 3 and between the cathode 4 and the anode 5. Therefore, when the Pockels effect is used, when light passes between these two electrode pairs, the deflection of the light due to the refractive index distribution is canceled out between positive and negative, and the lens function is not achieved.

  On the other hand, when the secondary electro-optic effect (hereinafter referred to as the Kerr effect) is used, the refractive index change is proportional to the square of the electric field. Therefore, even if the direction of the electric field is reversed between the anode 2 and the cathode 3 and between the cathode 4 and the anode 5, the refractive index distribution is the same, so that the light deflection is canceled out. We will strengthen each other.

  Many electro-optic materials do not have inversion symmetry and develop a Pockels effect. On the other hand, some electro-optic materials have inversion symmetry, do not exhibit the Pockels effect, and the Kerr effect is dominant. Therefore, it is important to use a material having inversion symmetry as the electro-optic material constituting the substrate 1 of the present embodiment.

  In general, when an electric field is applied from the outside to a dielectric, polarization proportional to the electric field is generated, but when the electric field is removed, the polarization returns to zero. However, there are substances that retain finite polarization even when the electric field is removed. Polarization that exists even without an external electric field is called spontaneous polarization. There exists a substance capable of reversing the direction of this spontaneous polarization by an external electric field, which is called a ferroelectric.

  A single crystal having inversion symmetry refers to a crystal that has the same arrangement as the original arrangement of atoms when the arrangement of atoms is inverted in the x, y, z coordinate system around a certain origin. When a crystal having spontaneous polarization is inverted on the coordinate axis, the direction of spontaneous polarization is inverted, so that such a crystal cannot be said to have inversion symmetry. Therefore, since the ferroelectric has spontaneous polarization, it does not have inversion symmetry.

  On the other hand, there are substances that have spontaneous polarization but cannot be inverted by an external electric field. Such a material does not have inversion symmetry, but is not a ferroelectric material. Therefore, not all materials that do not have inversion symmetry are ferroelectric materials. Further, it cannot be a ferroelectric and has inversion symmetry.

As an electro-optic material having inversion symmetry, there is a single crystal material having a perovskite crystal structure. The perovskite single crystal material becomes a cubic phase having inversion symmetry in the use state if the use temperature is appropriately selected. In the cubic phase, the Pockels effect is not expressed and the Kerr effect is dominant. For example, even the most well-known barium titanate (BaTiO ₃ , hereinafter referred to as BT) may exceed the temperature at which the phase transition from the tetragonal phase to the cubic phase (hereinafter referred to as the phase transition temperature) occurs at around 120 ° C. For example, it becomes a cubic phase and exhibits the Kerr effect.

A single crystal material mainly composed of potassium tantalate niobate (KTN: KTa _1-x Nb _x O ₃ , 0 <x <1) has more preferable characteristics. BT has a predetermined phase transition temperature, whereas KTN can select a phase transition temperature depending on the composition ratio of tantalum and niobium. Thereby, the phase transition temperature can be set near room temperature. KTN has a cubic phase at a temperature higher than the phase transition temperature, has inversion symmetry, and has a large Kerr effect. Even in the same cubic phase, the Kerr effect becomes overwhelmingly closer to the phase transition temperature. For this reason, setting the phase transition temperature around room temperature is very important for easily realizing a large Kerr effect.

Further, as a single crystal material related to KTN, the main component of the crystal is composed of periodic group Ia group and Va group, group Ia is potassium, and group Va includes at least one of niobium and tantalum. Materials can be used. Moreover, 1 or multiple types of periodic table group Ia except potassium as an additional impurity, for example, lithium, or IIa group can also be included. For example, a cubic phase KLTN (K _1-y Li _y Ta _1-x Nb _x O ₃ , 0 <x <1, 0 <y <1) crystal may be used.

(Optical path length modulation)
Here, the optical path length modulation will be described in detail. In the configuration of FIG. 2, the characteristics of the lens are evaluated by an optical path length modulation Δs obtained by integrating the refractive index change Δn over the light traveling path (length L) as in the following equation.

However, in the configuration of FIG. 2, there are two types of polarized light, when the direction of the optical electric field is the y-axis direction and when the direction is the z-axis direction. In each case, the refractive index modulation Δn felt by the light is different, so the optical path length modulation Δs is also different.

  FIG. 3 shows an example of the optical path length of the variable focus lens according to the first embodiment. The vertical axis represents the optical path length modulation Δs obtained by numerical calculation when the direction of the optical electric field is the z-axis direction. The relative dielectric constant is 20,000, the length L of the substrate 1 is 7 mm, the thickness of the substrate in the z-axis direction is 4 mm, the width of the four electrodes is 0.8 mm, the distance between the electrodes on the same plane is 4 mm, and the voltage is It was calculated as 1000V. The horizontal axis of FIG. 3 shows the displacement from the origin with the center of the substrate 1 in the z coordinate shown in FIG. 2 as the origin. The distribution of Δs forms an upward convex curve, and this element functions as a cylindrical convex lens. In this example, the lens is a convex lens. However, since the optical path length modulation differs depending on the polarization as described above, the lens may be a concave lens.

(Polarization-independent variable focus lens)
Single crystal materials with inversion symmetry tend to be isotropic and the lens effect does not depend on polarization, but as described above, the electric field component generated by the applied voltage and the light Depending on whether or not the electric field component is parallel, the lens effect differs, resulting in a variable focus lens depending on polarization. In order to make the lens effect independent of the polarization, the light is divided into two polarizations whose directions of the oscillating electric field are orthogonal to each other, and each of them is modulated by a polarization-dependent variable focus lens, and then, two times again. It is necessary to synthesize polarized light.

  FIG. 4 shows an example of making the variable focus lens polarization independent. In the optical system for making polarization independent, the incident light 8 is demultiplexed into two mutually orthogonal polarization components by a deflecting beam splitter (hereinafter abbreviated as PBS) 11. The penetrating branch 20 has an optical electric field parallel to the paper surface, and the reflecting branch 21 has an optical electric field perpendicular to the paper surface. The branch 20 is reflected by the mirror 12 and enters the basic unit element 13 of the variable focus lens described with reference to FIGS. At this time, the branch 20 is modulated in the same optical path length distribution as that in FIG. 3 because the optical electric field is perpendicular to the two surfaces on which the electrodes are formed in the basic unit element 13.

  When one of the branches 21 is reflected by the mirror 14 and directly enters the basic unit element 15, the optical electric field is parallel to the electrode surface of the basic unit element 15, so that it has a polarization relationship with the branch 20. Are thus different and thus subject to modulation of different optical path length distributions. In order to make the modulation of the branch 21 equal to that of the branch 20, it is necessary to rotate the optical electric field direction by 90 degrees using the half-wave plate 16. As a result, the branch 20 and the branch 21 are subjected to the modulation of the same optical path length distribution, so if they are combined by another PBS 17 to be the outgoing light 22, both polarizations are subjected to the same modulation. It functions as a polarization-independent variable focus lens.

  However, since the light of the branch 21 enters the PBS 17 depending on the state of polarization, it travels straight as it is, so that the light of the branch 20 needs to be reflected by the PBS 17 in order to be combined with the light of the branch 20. is there. For this purpose, the half-wave plate 18 is used to rotate the polarized light by 90 degrees. Alternatively, the half-wave plate 18 is not inserted, and the light of the branch 20 is transmitted straight through the PBS 17 as it is, the half-wave plate is inserted between the basic unit element 15 and the PBS 17, and the polarization of the branch 21 is once again 90. Rotate it back and put it back. Since the light of the branch 21 is reflected by the PBS 17, it can be combined with the light of the branch 20.

  As described above, in the simple method, it is necessary to combine two PBSs, two wavelength plates, and two mirrors in addition to the two basic unit elements of the variable focus lens that shares the modulation of each polarization. However, it is difficult to reduce the size. The polarization-independent variable focus lens described with reference to FIG. 4 has a cylindrical operation. In order to realize biaxial focusing similar to that of a normal spherical lens, the optical system is further complicated.

  However, according to the present invention, this optical system can be remarkably simplified, and the number of parts can be reduced, and a small and low-cost polarization-independent variable focus lens can be realized. The reason and the optical system will be described below.

  As described above, the principle of the present invention is to generate a refractive index distribution from an electric field distribution and to function as a lens by the Kerr effect of a single crystal material having inversion symmetry represented by KTN. The refractive index modulation by the Kerr effect has conventionally been expressed as follows by the linear combination of the squares of the components of the electric field vector.

The refractive index modulation [Delta] n _y, if the direction of the optical electric field in the y-axis direction, [Delta] n _z is the direction of the optical field is the case of the z-axis direction. N ₀ is a refractive index before modulation, and s ₁₁ and s ₁₂ are electro-optic coefficients.

  However, a single crystal material having inversion symmetry exhibits an electrostrictive effect simultaneously with the Kerr effect when an electric field is applied. The electrostrictive effect is a phenomenon in which a crystalline material is distorted when an electric field is applied, and the strain (strain) is proportional to the square of the electric field. Furthermore, not only a single crystal material having inversion symmetry but also a substance generally exhibits a so-called photoelastic effect in which when a strain is generated, a refractive index change is proportional to the strain. For this reason, when an electric field is applied to a single crystal material having inversion symmetry, distortion occurs due to the electrostrictive effect, and as a result, the refractive index changes due to the photoelastic effect. Since the strain is proportional to the square of the electric field and the refractive index is proportional to the strain, only the cause and the result are equivalent to the simple Kerr effect. That is, the apparent Kerr effect usually includes a component obtained by combining the electrostrictive effect and the photoelastic effect.

  Furthermore, it has been found that the components of the electrostrictive effect and the photoelastic effect are dominant components of the apparent Kerr effect. If components other than the dominant electrostrictive effect and photoelastic effect are ignored, the change in the refractive index of KTN is expressed as follows.

Here, p ₁₁ and p ₁₂ are photoelastic coefficients. Three of e _xx , e _yy , and e _zz are distortion tensor components, which are equivalent to linear expansion coefficients in the x-axis direction, the y-axis direction, and the z-axis direction, respectively. When the electric field is applied uniformly, according to the electrostrictive effect, these distortions are expressed by a linear combination of E _x ² and E _z ² which are the squares of the electric field components as in the following equation.

Substituting this equation (3) into equation (2) eventually yields an equation equivalent to equation (1), whose properties are completely consistent with the previous description of the properties of the electro-optic material. However, when the electric field is distributed as in the substrate 1 of the variable focus lens of the present invention, the strain is strongly influenced by the elasticity of the crystal. For this reason, even if equation (2) holds, equation (3) does not hold, and therefore equation (1) does not hold.

  FIG. 5 shows the difference in the optical path length distribution of the variable focus lens by the two numerical calculation methods. It shows the difference between the calculation result of the simple Kerr effect and the numerical calculation of strain. A solid line indicates a case where the optical electric field is parallel to the y axis (y polarization), and a broken line indicates a case where the optical field is parallel to the z axis (z polarization). The plot shows the optical path length distribution calculated according to the conventional simple Kerr effect equation (1), and the square plot shows the optical path length distribution calculated according to equation (2) by numerically calculating the strain taking elasticity into consideration. In the case of z-polarized light, although the lens effect is slightly stronger in the case of distortion calculation, the convex lens function is shown in the same manner as in both cases. On the other hand, in the case of y-polarized light, the calculation result by the conventional Kerr effect is a downward convex shape and shows a concave lens function, whereas the distortion calculation shows that the effect as a lens is very small. Yes. In other words, according to a more correct calculation, it can be seen that y-polarized light is hardly modulated and passes.

(Configuration of polarization-independent variable focus lens)
FIG. 6 shows the configuration of a polarization-independent variable focus lens according to the second embodiment of the present invention. Using the above knowledge, a polarization-independent variable focus lens can be realized with a simple configuration described below. The polarization-independent variable focus lens has a basic unit element 31, a half-wave plate 32, and another basic unit element 33 of the cylindrical variable focus lens described with reference to FIGS. This is a simple structure. In the spherical lens described in “Configuration of Cylindrical Variable Focus Lens”, two basic unit elements and a half-wave plate are combined and arranged so as to form an angle of 90 degrees with respect to the optical axis. On the other hand, in the configuration of FIG. 6, the two basic unit elements are arranged so as to collect light in the same direction without rotating. In any configuration, the two basic unit elements and the half-wave plate are the same, but the arrangement of the basic unit elements is different. The spherical lens performs biaxial modulation, but the polarization-independent variable focus lens has one axis. It differs in that it modulates.

  As in the case of FIG. 4, the incident light 34 has an arbitrary polarization component, and may be considered as being separated into two components, y-polarization and z-polarization. Both components enter the basic unit element 31 together, but the y-polarized light passes through without being modulated, as described with reference to FIG. On the other hand, the z-polarized light is modulated and emitted so as to be condensed by the convex lens function of the basic unit element 31. Both components transmitted through the basic unit element 31 are incident on the half-wave plate 32 together. When the light passes through the half-wave plate 32, the direction of the optical electric field is rotated by 90 degrees. . Since the light component modulated so as to be condensed by the basic unit element 33 is changed to y-polarized light, it passes through the basic unit element 33 this time. Since the light component that has not been modulated by the basic unit element 31 is now z-polarized light, it is modulated so as to be condensed. For this reason, both of the two polarization components constituting the incident light 34 are similarly subjected to the condensing modulation and emitted as the outgoing light 35, so that the condensing modulation is similarly performed regardless of the polarization of the incident light. can do.

  As described above, the polarization-independent variable focus lens described with reference to FIG. 6 is a polarization-independent cylindrical lens, but expanding this to a biaxially operated lens having the same function as a spherical lens is Easy to do. As a matter of course, if the two configurations in FIG. 6 are placed in series on the optical axis and arranged so as to form an angle of 90 degrees with respect to the optical axis, a polarization-independent variable focus lens of two-axis operation is configured. be able to.

  FIG. 7 shows the configuration of a biaxial polarization-independent variable focus lens according to the third embodiment of the present invention. Two basic unit elements, a half-wave plate, and another set of basic unit elements, which are arranged such that the direction of voltage application, that is, the normal of the electrode surface forms an angle of 90 degrees with respect to the optical axis, combine. In this way, one wave plate can be reduced. Hereinafter, light modulation will be described step by step.

  As in FIG. 6, the incident light 46 is considered divided into a y-polarized component and a z-polarized component. The y-polarized component at this time is referred to as a light component 1, and the z-polarized component is referred to as a light component 2. Since the light component 1 is y-polarized light, it is not modulated by the basic unit element 41. However, when the light then enters the basic unit element 42, it undergoes condensing modulation in the y-axis direction. When the light subsequently enters the half-wave plate 43, it changes to z-polarized light. When the light then enters the basic unit element 44, since it is z-polarized light, it undergoes condensing modulation in the z-axis direction, and biaxial condensing modulation is applied together with the previous modulation. Since it is z-polarized light, the last basic unit element 45 passes through, and as a result, the light is emitted in a state of being subjected to biaxial condensing modulation.

  Consider one light component 2. Since this component is z-polarized light, the first basic unit element 41 performs light-gathering modulation in the z-axis direction, but the next basic unit element 42 passes through. Next, the light is converted into y-polarized light by the half-wave plate 43 and then enters the basic unit element 44. Here, no modulation is applied. Finally, when proceeding to the basic unit element 45, since it is y-polarized light, condensing modulation in the y-axis direction is added here. For this reason, the light is finally emitted in a state where it has undergone both-axis condensing modulation. In summary, both the light component 1 and the light component 2 are subjected to two-axis condensing modulation and are emitted as the emitted light 47, so that it functions as a polarization-independent biaxial variable focus lens.

  In this configuration, what is important is that the basic unit elements 41 and 44 that condense light in the same direction are arranged on both sides of the half-wave plate 43 and are different from this condensing direction, but are also in the same direction. In other words, the basic unit elements 42 and 45 that condense light are disposed on both sides of the half-wave plate 43. Therefore, the basic unit elements 41 and 42 may be interchanged in the left and right in the drawing, and the basic unit elements 44 and 45 may be interchanged.

  FIG. 8 shows a configuration of a polarization-independent variable focus lens according to the fourth embodiment of the present invention. It is another application example of the second embodiment. In this optical system, incident light 55 passes through the beam splitter 51 and then enters the basic unit element 52. Next, the light passes through the quarter-wave plate 53, is reflected by the mirror 54, and returns to the original path. The light passes through the quarter-wave plate 53 and the basic unit element 52 in order, and is finally reflected by the beam splitter 51 and output as outgoing light 56. Since the light is transmitted through the quarter-wave plate 53 twice, the light is incident again on the basic unit element 52 with the polarization rotated by 90 degrees as in the case of transmitting through the half-wave plate. From this, it can be seen that the same effect as in FIG. 6 can be obtained. The beam splitter 51 is preferably one that does not depend on polarization. The loss of optical power due to the beam splitter 51 is a disadvantage of this configuration, but is characterized in that the number of basic unit elements can be reduced to half. As a matter of course, if another basic unit element is added, two axes of light can be condensed.

(Place electrode)
In the embodiments of the variable focus lens described so far, the basic unit element has the anode 2 and the cathode 4 arranged on the upper surface of the substrate 1 and the cathode 3 and the anode 5 arranged on the lower surface. However, the basic unit element may have a configuration in which both the electrodes on the upper surface are anodes and the electrodes on the lower surface are both cathodes. The lens effect is small compared to the variable focus of the first embodiment, but the function is the same (fifth embodiment).

  FIG. 9 shows the configuration of a variable focus lens according to the sixth embodiment of the present invention. The arrangement of the electrodes is the same as that of the first embodiment of FIG. 1, but the light incident and emission directions are different. As shown in FIG. 9, light is incident between the anode 2 and the cathode 4 on the upper surface of the substrate 1, travels in the substrate 1 in the z-axis direction, and from between the cathode 3 and the anode 5 on the lower surface. Set to emit into the air. Furthermore, in the sixth embodiment, both of the upper electrodes may be anodes and the lower electrodes may be both cathodes, and conversely, the upper surface may be a cathode and the lower surface may be an anode (seventh embodiment). Form).

  FIG. 10 shows the configuration of a variable focus lens according to the eighth embodiment of the present invention. This is a further application of the basic unit element of the first embodiment, and is configured in series along the optical axis direction. A plurality of electrode pairs 62a, 62b, 63a, 63b, 64a, 64b are arranged on one substrate 61, and voltages having opposite polarities are applied to electrode pairs adjacent to each other. Similarly to the fifth embodiment, a voltage having the same polarity may be applied to all electrode pairs on the upper surface, and a voltage having the same polarity may be applied to all electrode pairs on the lower surface. Of course, the voltage may be applied in an arbitrary pattern. By configuring the element in this way, a large lens effect can be obtained even at a lower voltage. The number of electrode pairs may be even or odd.

(Polarization rotation element)
6 and 7, the half-wave plate plays an important role. This half-wave plate is more generally a polarization rotation element that rotates polarized light by 90 degrees. Any element that rotates the polarized light by 90 degrees may be used. In addition to a half-wave plate as a representative example, a Faraday rotation element or the like can be used instead.

  The half-wave plate is an optical element that generates a phase shift corresponding to half the wavelength, that is, a phase shift of π radians, between two polarized waves orthogonal to each other. Typically, it consists of a birefringent material processed into a plate shape. A single crystal material having inversion symmetry such as KTN usually does not have birefringence. However, when an electric field is applied in one direction, birefringence occurs in a direction parallel to the electric field and in a direction perpendicular thereto. . By utilizing this property, a half-wave plate can be formed by KTN.

  FIG. 11 shows the configuration of a polarization-independent variable focus lens according to the ninth embodiment of the present invention. As an application of the second embodiment shown in FIG. 6, a first basic unit element 71, a KTN half-wave plate 72, and a second basic unit element 73 are arranged in series along the optical axis direction. . The shape of the KTN half-wave plate 72 is a rectangular parallelepiped shape, and an electrode film is formed over almost the entire surface on two surfaces facing each other. By applying a voltage to the electrode pair, an electric field perpendicular to these two surfaces is uniformly formed. The direction of the electric field is arranged so as to form an angle of 45 degrees with respect to the voltage application direction of the first basic unit element 71 and the second basic unit element 73, that is, the normal line of the electrode surface. As a result, the polarization of the light transmitted through the first basic unit element 71 is rotated by 90 degrees.

  When the half-wave plate is also composed of KTN as in the case of the cylindrical variable focus lens that is the basic unit element described above, a substrate made of three electro-optic materials is integrally molded, and an electrode for the first basic unit element 71 is formed. An electrode for the KTN half-wave plate 72 and an electrode for the second basic unit element 73 are sequentially arranged and attached. In this way, an integrated polarization-independent variable focus lens can be configured. Further, as an application of the third embodiment shown in FIG. 7, it is possible to integrally mold the four basic unit elements of the variable focus lens and the half-wave plate.

(Electrode material)
When a high voltage is applied to the electro-optic material, charges are injected from the electrodes, and space charges can be generated in the crystal. This space charge causes a gradient in the magnitude of the electric field in the direction in which the voltage is applied, so that a gradient also occurs in the modulation of the refractive index. Therefore, in order to obtain a desired refractive index distribution for causing the electro-optic material to function as a lens, or to prevent light transmitted through the electro-optic material from being deflected, when a voltage is applied to the substrate 1, It is better that no space charge is formed inside the substrate 1.

  Since the amount of space charge depends on the carrier injection efficiency, the carrier injection efficiency injected from the electrode should be small. When the carriers contributing to electrical conduction in the electro-optic crystal are electrons, as the work function of the electrode material increases, the electrode and the substrate approach a Schottky junction and the carrier injection efficiency decreases. Therefore, the electrode is preferably a material that forms a Schottky junction with the electro-optic material. Specifically, when the carrier contributing to electrical conduction in the electro-optic crystal is an electron, the work function of the electrode material is preferably 5.0 eV or more. For example, as an electrode material having a work function of 5.0 eV or more, Co (5.0), Ge (5.0), Au (5.1), Pd (5.12), Ni (5.15), Ir (5.27), Pt (5.65), Se (5.9) can be used. The parentheses indicate work functions, and the unit is eV.

  On the other hand, when the carriers contributing to electrical conduction in the electro-optic crystal are holes, the work function of the electrode material is preferably less than 5.0 eV in order to suppress the injection of holes. For example, Ti (3.84) or the like can be used as an electrode material having a work function of less than 5.0 eV. Since the Ti single-layer electrode is oxidized and becomes high resistance, generally, the Ti layer and the electro-optic crystal are bonded using an electrode in which Ti / Pt / Au are sequentially laminated. Furthermore, transparent electrodes such as ITO (Indium Tin Oxide) and ZnO can also be used.

  As shown in the first embodiment (FIG. 1), the anode 2, the cathode 3, the cathode 4, and the anode 5 are formed on the upper and lower surfaces of the substrate 1 obtained by processing the electro-optic material into a plate shape. The substrate 1 is cut out from a KTN single crystal and formed into a shape of 7 mm × 7 mm × (thickness T =) 4 mm. All six surfaces of the substrate 1 are parallel to the (100) plane of the crystal and optical polishing is performed. Since this KTN single crystal had a phase transition temperature of 35 ° C., it is used at 40 ° C., which is slightly higher than this. The relative dielectric constant at this temperature is 20,000. The four electrodes have a strip shape of 0.8 mm × 7 mm, and the distance between the electrodes on the same plane is 4 mm. The two electrode pairs are formed by depositing platinum (Pt) on the 7 mm × 7 mm surface of the substrate 1. Each side of the electrode is parallel to the side of the substrate 1.

  Collimated laser light is incident on the cylindrical variable focus lens while the temperature is controlled at 40 ° C. The polarization of light is a straight line, and the direction of the oscillating electric field is the z-axis direction. When a voltage of 1000 V is applied between the upper and lower electrodes, the light emitted from the substrate 1 is condensed in the z-axis direction and functions as a cylindrical convex lens. The focal length is 59 cm.

  Using this polarization-dependent variable focus lens as a basic unit element, two of the same specifications were produced, and combined with a quartz half-wave plate to produce the optical system of the second embodiment (FIG. 6). As in the previous experiment, collimated laser light is incident with the temperature of the two basic unit elements controlled at 40 ° C. The polarization of light is a straight line, and the direction of the oscillating electric field is the z-axis direction. When a voltage of 1000 V is applied between the upper and lower electrodes, the light emitted from the substrate 1 is condensed in the z-axis direction and functions as a cylindrical convex lens. The focal length is 59 cm as before. Here, when the applied voltage is 500 V, the light condensing effect is reduced and the focal length is 290 cm. In addition, when no voltage is applied, there is naturally no light collecting effect and the focal length is infinite. Therefore, the focal length can be changed from infinity to 59 cm by changing the applied voltage from 0V to 1000V. Since changing the focal length only changes the applied voltage, the response time is 1 μs or less, which is an improvement of three orders of magnitude or more compared to the response time of the conventional variable focus lens. The above characteristics are constant regardless of the rotation angle even when the polarization direction of the incident laser beam is rotated.

  Further, four basic unit elements having the same specifications were produced and combined with a quartz half-wave plate to produce the optical system of the third embodiment (FIG. 7). As in the previous experiment, collimated laser light is incident with the temperature of the four basic unit elements controlled at 40 ° C. The polarization of light is a straight line, and the direction of the oscillating electric field is the z-axis direction. When a voltage of about 1000 V is applied between the upper and lower electrodes, the light emitted from the substrate 1 is condensed in both the y-axis and z-axis directions and functions in the same manner as a spherical convex lens. Since the optical axis positions of the basic unit elements are not equal, the voltage applied to each element was adjusted within a range of ± 50V. The focal length is 59 cm as before. Therefore, by changing the applied voltage from 0 V to 1000 V, the focal length can be changed from infinity to 72 cm, and even if the polarization direction of the incident laser light is rotated, it is similarly changed regardless of the rotation angle. be able to.

  In the basic unit element fabricated in Example 1, as shown in the fifth embodiment, both the upper surface electrodes are used as anodes and the lower surface electrodes are both used as cathodes. Collimated laser light is incident on the cylindrical variable focus lens while the temperature is controlled at 40 ° C. The polarization of light is a straight line, and the direction of the oscillating electric field is the z-axis direction. When a voltage of 1000 V is applied between the upper and lower electrodes, the light emitted from the substrate 1 is condensed in the z-axis direction and functions as a cylindrical convex lens. The focal length is 115 cm.

  Using the four basic unit elements, the optical system of the third embodiment (FIG. 7) was produced in the same manner as in Example 1. The focal length was 115 cm as before, and the lens effect was somewhat small, but the focal point control function did not change even when the polarization state was changed, as in Example 1.

  The basic unit element produced in Example 1 is used as the cylindrical variable focus lens shown in the sixth embodiment (FIG. 9). Collimated laser light is incident on the cylindrical variable focus lens while the temperature is controlled at 40 ° C. The polarization of light is a straight line and the direction of the oscillating electric field is the x-axis direction. When a voltage of 1000 V is applied between the upper and lower electrodes, the light emitted from the substrate 1 is condensed in the x-axis direction and functions as a cylindrical convex lens. The focal length is 104 cm.

  FIG. 12 shows a configuration of a biaxial polarization-independent variable focus lens according to the third embodiment. Two basic unit elements 81 and 82 in which the xy plane on which the electrodes are formed is arranged at an angle of 90 degrees with respect to the optical axis (z axis), a half-wave plate 83, and another set The basic unit elements 84 and 85 are combined. As a result, the incident light 86 having the y-polarized component and the z-polarized component is emitted as the outgoing light 87 in a state where the biaxial condensing modulation is applied, so that it functions as a polarization-independent biaxial variable focus lens. To do.

  Collimated laser light is incident with the temperature of the four basic unit elements controlled at 40 ° C. The polarization of light is a straight line and the direction of the oscillating electric field is the x-axis direction. When a voltage of about 1000 V is applied between the upper and lower electrodes, the emitted light 87 is condensed in both the x-axis and y-axis directions and functions in the same manner as a spherical convex lens. Since the optical axis positions of the basic unit elements are not equal, the voltage applied to each element was adjusted within a range of ± 50V. The focal length is 104 cm. Even if the polarization direction of the incident laser beam is rotated, the incident laser beam can be condensed similarly regardless of the rotation angle.

  In the basic unit element fabricated in Example 3, as shown in the seventh embodiment, both the upper surface electrodes are used as anodes and the lower surface electrodes are both used as cathodes. Using four of these basic unit elements, the optical system of FIG. The focal length was 114 cm, and the lens effect was somewhat small, but the focus control function did not change even when the polarization state was changed, as in Example 3.

1, 31, 61 Substrate 2, 5, 62a, 63a, 64a Anode 3, 4, 62b, 63b, 64b Cathode 6 Electric field lines 7 Refractive index modulation curve 8, 19, 34, 46, 55, 86 Incident light 11, 17 Polarizing beam splitter 12, 14, 54 Mirror 13, 15, 31, 33, 41, 42, 44, 45, 52, 71, 73, 81, 82, 84, 85 Basic unit elements 16, 18, 32, 43, 53,83 half-wave plate 20,21 branch 22,35,47,56,87 outgoing light 51 beam splitter 53 quarter-wave plate 73 KTN half-wave plate

Claims (13)

An electro-optic material made of a single crystal having inversion symmetry;
A plurality of electrode pairs formed at positions facing the first surface of the electro-optic material and the second surface facing the first surface,
A cylindrical variable focus lens that changes the focus of light transmitted through the electro-optic material by changing an applied voltage between the electrode pair is a basic unit element.
A first basic unit element;
A polarization rotation element that rotates the linearly polarized light of the light transmitted through the first basic unit element by 90 degrees;
A polarization-independent variable focus lens comprising: a second basic unit element arranged so as to collect light transmitted through the polarization rotation element in the same direction as the first basic unit element . An electro-optic material made of a single crystal having inversion symmetry;
A plurality of electrode pairs formed at positions facing the first surface of the electro-optic material and the second surface facing the first surface,
A cylindrical variable focus lens that changes the focus of light transmitted through the electro-optic material by changing an applied voltage between the electrode pair is a basic unit element.
A first basic unit element and a second basic unit element in which the voltage application direction of the electrode pair forms an angle of 90 degrees with respect to the optical axis;
A polarization rotation element that rotates the linearly polarized light of the light transmitted through the second basic unit element by 90 degrees;
A third basic unit element and a fourth basic unit element for condensing the light transmitted through the polarization rotation element, in which the voltage application direction of the electrode pair forms an angle of 90 degrees with respect to the optical axis. A polarization-independent variable focus lens, comprising: The basic unit element is:
A first anode formed on a first surface of the electro-optic material;
A first cathode formed on a second surface opposite to the first surface and formed at a position facing the first anode;
A second cathode formed on the first surface and spaced from the first anode;
A second anode formed on the second surface, facing the second cathode, and spaced apart from the first cathode;
When light is incident from a third surface orthogonal to the first surface, the light passes through the first electrode pair composed of the first anode and the first cathode, and then the second The optical axis is set so that light passes through a second electrode pair consisting of an anode and the second cathode, and light is emitted from the fourth surface facing the third surface,
It is a cylindrical variable focus lens that varies the focus of light emitted from the fourth surface of the electro-optic material by changing an applied voltage between the first and second electrode pairs. The polarization independent variable focus lens according to claim 1 or 2. The basic unit element is:
A first anode formed on a first surface of the electro-optic material;
A first cathode formed on a second surface opposite to the first surface and formed at a position facing the first anode;
A second cathode formed on the first surface and spaced from the first anode;
A second anode formed on the second surface, facing the second cathode, and spaced apart from the first cathode;
When light is incident on the first surface between the first anode and the second cathode and passes through the electro-optic material, the first cathode and the second anode The optical axis is set so that light is emitted from the second surface between
It is a cylindrical variable focus lens that varies the focus of light emitted from the second surface of the electro-optic material by changing the applied voltage between the two anodes and the two cathodes. The polarization independent variable focus lens according to claim 1 or 2. The basic unit element is:
A first anode formed on a first surface of the electro-optic material;
A first cathode formed on a second surface opposite to the first surface and formed at a position facing the first anode;
A second anode formed on the first surface and spaced from the first anode;
A second cathode formed on the second surface, facing the second anode, and spaced apart from the first cathode;
When light is incident from a third surface orthogonal to the first surface, the light passes through the first electrode pair composed of the first anode and the first cathode, and then the second The optical axis is set so that light passes through a second electrode pair consisting of an anode and the second cathode, and light is emitted from the fourth surface facing the third surface,
It is a cylindrical variable focus lens that varies the focus of light emitted from the fourth surface of the electro-optic material by changing an applied voltage between the first and second electrode pairs. The polarization independent variable focus lens according to claim 1 or 2. The basic unit element is:
A first anode formed on a first surface of the electro-optic material;
A first cathode formed on a second surface opposite to the first surface and formed at a position facing the first anode;
A second anode formed on the first surface and spaced from the first anode;
A second cathode formed on the second surface, facing the second anode, and spaced apart from the first cathode;
When light is incident on the first surface between the first anode and the second anode and passes through the electro-optic material, the first cathode and the second cathode The optical axis is set so that light is emitted from the second surface between
It is a cylindrical variable focus lens that varies the focus of light emitted from the second surface of the electro-optic material by changing the applied voltage between the two anodes and the two cathodes. The polarization independent variable focus lens according to claim 1 or 2. The basic unit element is
2N electrodes formed on the surface of the electro-optic material,
When 1 ≦ k ≦ N−1, a second electrode is formed on the first surface of the electro-optic material, and the second electrode is opposite to the first surface with the kth electrode from the light incident side as the kth anode. The electrode formed on the surface and facing the kth anode is the kth cathode,
The electrode formed on the first surface and spaced apart from the kth anode is the (k + 1) th cathode, and is formed on the second surface and faces the (k + 1) th cathode. And the k + 1th anode is an electrode spaced from the (k + 1) th cathode,
When light is incident from a third surface orthogonal to the first surface, it is formed between the electrode pair composed of the kth anode and the kth cathode, and the Nth anode and the Nth cathode. The optical axis is set so that light is emitted from the fourth surface that passes between the electrode pair and faces the third surface,
The cylindrical variable focus lens according to claim 1, wherein the lens is a cylindrical variable focus lens that changes a focus of light emitted from the fourth surface of the electro-optic material by changing an applied voltage between the kth and Nth. Alternatively, the polarization-independent variable focus lens according to 2.   8. The polarization independent variable focus lens according to claim 1, wherein the electro-optic material is a perovskite single crystal material. The electro-optical material, potassium tantalate niobate _{(KTN: KTa 1-x Nb} x O 3, 0 <x <1) polarization independent variable focus lens according to claim 8, characterized in that a.   The electro-optic material is characterized in that the main component of the crystal is composed of groups Ia and Va in the periodic table, group Ia is potassium, and group Va includes at least one of niobium and tantalum. The polarization-independent variable focus lens according to claim 8.   The polarization-independent variable focus lens according to claim 10, wherein the electro-optic material further includes one or more of Group Ia or Group IIa of the periodic table excluding potassium as an additive impurity.   4. The first and second anodes, the first and second cathodes, and the 2N electrodes are made of a material that forms a Schottky junction with the electro-optic material. The polarization-independent variable focus lens according to any one of 7.   The first and second anodes, the first and second cathodes, and the 2N electrodes have a strip shape, and all of the longitudinal sides thereof are parallel to each other. Item 13. The polarization independent variable focus lens according to Item 12.

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