JP2016001288A - Variable focus lens - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a variable focus lens having higher performance, while improving a variable focal length, and maintaining high-speed response, by the same degree of a voltage as an applied voltage to a conventional variable focus lens.SOLUTION: A variable focus lens includes: a single crystal substrate 11 having an electrooptic effect; a first electrode pair 12a, 12b formed on a surface entered by light, a first surface which is orthogonal to the surface entered by light, and a second surface facing to the first surface; and a second electrode pair 13a, 13b formed on a surface from which light is emitted, and a first surface and a second surface which are orthogonal to the surface from which light is emitted.

Description

  The present invention relates to a variable focus lens, and more particularly, to a variable focus lens in which a focal length can be changed in a variable focus lens using a single crystal of a substance having an electro-optic effect.

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

  Therefore, as described in Patent Document 1, a variable focus lens using a material capable of changing a refractive index for a transparent medium constituting an optical lens is known. Also, as described in Non-Patent Document 1, a variable focus lens that mechanically deforms the shape of an optical lens has been proposed. However, any of these variable focus lenses has a problem in that the response time required to change the focal length is limited and cannot be applied to a high-speed response of 1 ms or less.

  Therefore, as described in Patent Document 2, a variable focus lens using an optical material having an electro-optic effect has been proposed, and a high-speed response of 1 ms or less is realized.

Japanese Patent Laid-Open No. 11-064817 JP 2014-026229 A

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, in the variable focal length lens described in Patent Document 2, it is necessary to apply a larger voltage in order to further shorten the focal length. However, the higher the applied voltage, the more difficult the high speed operation. Further, there is a problem that wavefront aberration, which is important for lens performance, deteriorates as the applied voltage increases. For this reason, if the applied voltage used in the variable focus lens described in Patent Document 2 can improve the performance as a lens, specifically the focal length, a higher performance lens while maintaining a high-speed response. Can be realized.

  An object of the present invention is to provide a variable focus lens with higher performance while improving the variable focal length and maintaining a high-speed response with a voltage comparable to the applied voltage of a conventional variable focus lens.

  In order to achieve such an object, one embodiment of the variable focus lens according to the present invention includes a single crystal substrate having an electro-optic effect, a surface on which light is incident on the single crystal substrate, and a surface on which the light is incident. A first electrode pair formed on a first surface orthogonal to the first surface and a second surface opposite to the first surface, wherein the first anode has the first surface and the light The first cathode and the light incident surface are formed across the side where the incident surface is in contact, and the first cathode has a side where the second surface and the light incident surface are in contact with each other. A first electrode pair formed on the second surface and the surface on which the light is incident, a surface on which light is emitted from the single crystal substrate, and the first surface orthogonal to the surface on which the light is emitted. A second electrode pair formed on the surface and the second surface, wherein the second anode has the second surface and a surface from which the light is emitted. The second cathode is formed on the second surface and the surface from which the light is emitted. The second cathode straddles the first surface across the side where the first surface and the surface from which the light is emitted. And a second electrode pair formed on the surface from which the light is emitted, and light transmitted through the single crystal substrate by changing an applied voltage between the first and second electrode pairs. It is characterized by changing the focus of the lens.

  According to the present invention, in a variable focus lens using a single crystal material having an electro-optic effect, electrodes are continuously formed on a part of the light incident surface and the light output surface. In comparison, since the change in the refractive index inside the crystal can be increased with the same applied voltage, the focal length can be further reduced.

It is a figure which shows the structure of the conventional variable focus lens. It is a figure which shows the electric force line and refractive index modulation amount inside the conventional variable focus lens. It is a figure which shows the structure of the variable focus lens concerning one Embodiment of this invention. It is a figure which shows the electric force line and refractive index modulation amount inside the variable focus lens of this embodiment. It is the figure which compared the correlation with a focal distance and the distance between electrodes. It is a figure which shows the correlation with the focal distance of the variable focus lens of this embodiment, and the distance between electrodes. It is a figure which shows the correlation with the wavefront aberration of the variable focus lens of this embodiment, and the distance between electrodes.

  First, an electro-optic effect and optical path length modulation will be described, and then an embodiment of the present invention will be described in detail.

  FIG. 1 shows a configuration of a conventional variable focus lens. In the single crystal substrate 1 having an electro-optic effect cut out to a rectangular parallelepiped, two pairs of two surfaces parallel to the x-axis and the y-axis are opposed to each other among the surface on which the light 4 is incident and the surface orthogonal to the surface from which the light 4 is emitted. Electrode pairs are formed at intervals. A first electrode pair 2a, 2b is formed on the light incident side, and a second electrode pair 3a, 3b is formed on the light emission side. The electrodes 2a, 3a are anodes and the electrodes 2b, 3b are cathodes.

  FIG. 2 shows the lines of electric force and the amount of refractive index modulation inside the conventional variable focus lens. The shape of the electric lines of force inside the crystal when the single crystal substrate 1 is viewed from the y-axis direction is shown, and the refractive index modulation amount Δn in the z-axis direction is schematically shown on the right side thereof.

(Electro-optic effect)
As for the electro-optic effect, as described in Patent Document 2, several electro-optic effects having different orders are included. For the variable focus lens, a single crystal material that generally exhibits a primary electro-optic effect (hereinafter referred to as Pockels effect) or a secondary electro-optic effect (hereinafter referred to as Kerr effect) is used. When utilizing the Pockels effect, the refractive index change is proportional to the applied electric field. Therefore, when a material having the Pockels effect is used for the single crystal substrate 1 shown in FIG. 1, the direction of the electric lines of force is reversed, so that the change in refractive index is canceled and the function as a lens is not achieved.

  On the other hand, when the Kerr effect is used, the refractive index change is proportional to the square of the applied electric field. Therefore, even if the direction of the lines of electric force is reversed, the refractive index changes are equal and function as a lens. As shown in FIG. 2, the curvature of the electric lines of force varies depending on the location in the electro-optic crystal, so that an electric field distribution is generated inside the crystal, thereby generating a refractive index change distribution and functioning as a lens. This refractive index distribution draws a parabola in the zx plane, and functions as a convex lens when the parabola is convex upward, and functions as a concave lens when convex downward. FIG. 2 schematically shows the refractive index change Δn in the zx plane when the direction of the electric field amplitude of the light 4 incident on the single crystal substrate 1 is the z-axis direction. When the voltage applied to each electrode pair is 0, the refractive index change is 0, and when the voltage is applied, the refractive index change draws a parabola and functions as a lens.

As a substance having a Kerr effect, there is a single crystal material having a perovskite structure. If a use temperature is appropriately selected, a phase transition to a cubic phase that exhibits a Kerr effect can be achieved. For example, barium titanate (BaTiO ₃ ) undergoes a phase transition from a tetragonal phase to a cubic phase that exhibits the Kerr effect at around 120 ° C. In addition, a single crystal material mainly composed of potassium tantalate niobate (KTN: KTa _1-x Nb _x O ₃ , 0 <x <1) is made of barium titanate (from the viewpoint of an element utilizing the electro-optic effect). It has more preferable characteristics than BaTiO ₃ ).

While barium titanate (BaTiO ₃ ) has a predetermined phase transition temperature, KTN can select the phase transition temperature based on the composition ratio of tantalum and niobium, that is, the value of x in the chemical formula. If KTN is a temperature higher than the phase transition temperature, it becomes a cubic phase exhibiting a large Kerr effect, and the Kerr effect becomes larger as it is closer to the phase transition temperature. Therefore, by changing the composition ratio of tantalum and niobium, a large Kerr effect can be easily expressed by selecting the phase transition temperature near room temperature.

In addition, 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. Furthermore, it is 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. For example, cubic 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)
In FIG. 2, the characteristic of the lens is evaluated by optical path length modulation Δs obtained by integrating the refractive index modulation amount Δn over the light traveling path (length L) as in the following equation.

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

(Embodiment of the present invention)
FIG. 3 shows the lines of electric force and the amount of refractive index modulation inside the variable focus lens of this embodiment. In the single crystal substrate 11 having an electro-optic effect cut out in a rectangular parallelepiped, a first surface parallel to the x-axis and the y-axis among the surface on which the light 14 is incident and the surface orthogonal to the surface to be emitted; Two electrode pairs are formed at positions facing each other at a distance from the second surface facing the surface. The first electrode pair 12a, 12b is formed on the light incident side, and the second electrode pair 13a, 13b is formed on the light emission side. The electrodes 12a, 13a are anodes and the electrodes 12b, 13b are cathodes.

  The first electrode pairs 12a and 12b are also formed continuously on the surface on which the light 14 is incident, and the second electrode pairs 13a and 13b are formed continuously on the surface from which the light is emitted. . That is, the first electrode pair 12a, 12b is formed on a surface on which light is incident, a first surface orthogonal to the surface on which light is incident, and a second surface opposite to the first surface. The anode 12a is formed on the first surface and the light incident surface across the side where the first surface and the light incident surface are in contact with each other, and the first cathode 12b is formed on the second surface and the light incident surface. The second surface and the light incident surface are formed across the side where the surface where the light enters is in contact.

  The second electrode pairs 13a and 13b are formed on a surface from which light is emitted and a first surface and a second surface orthogonal to the surface from which light is emitted, and the second anode 13a is formed on the second surface. The second cathode 13b has a side where the first surface and the surface from which the light is emitted are in contact with each other. In addition, the first surface and the surface from which light is emitted are formed.

  FIG. 4 shows the lines of electric force and the amount of refractive index modulation inside the conventional variable focus lens. The shape of the electric lines of force inside the crystal when the single crystal substrate 11 is viewed from the y-axis direction is shown, and the refractive index modulation amount Δn in the z-axis direction is schematically shown on the right side thereof. The first and second electrode pairs have an L-shaped cross-sectional shape as viewed from the y-axis direction, and have a structure that surrounds the four corners of the single crystal substrate 11.

  The principle of the lens function is the same as that of a conventional variable focus lens. Comparing FIG. 2 and FIG. 4, it can be seen that the curvature of the electric lines of force is larger in the variable focus lens of the present embodiment. When the curvature of the electric lines of force increases, the electric field distribution inside the crystal increases, and as a result, the refractive index change near the electrode becomes larger than the refractive index change at the center of the light incident surface. That is, in the zx plane, the difference between the refractive index change at z = 0 and the refractive index change in the vicinity of the electrode becomes large. As a result, the curvature of the parabola becomes larger than the conventional one, and a greater lens effect is exhibited.

  FIG. 5 is a diagram comparing the correlation between the focal length and the interelectrode distance. In the conventional variable focus lens shown in FIG. 2, the thickness h (z-axis direction) of the single crystal substrate 1 is 4.0 mm, the length L (y-axis direction) is 6.6 mm, and the width (y-axis direction) is also It is 6.6 mm. The size of each of the first and second electrode pairs 2 and 3 is 0.8 mm in the x-axis direction and 6.6 mm in the y-axis direction from the entrance surface or the exit surface. Accordingly, the inter-electrode distance d is 2.4 mm. In such a structure, the focal length when the length of the single crystal substrate 1 in the x-axis direction is changed and the distance between the electrodes is changed accordingly is shown.

  On the other hand, in the variable focus lens of this embodiment shown in FIG. 4, the thickness h (z-axis direction) of the single crystal substrate 11 is 4.0 mm, the length L (y-axis direction) is 6.6 mm, and the width (y (Axial direction) is also 6.6 mm. The size of each of the first and second electrode pairs 12 and 13 is 0.8 mm in the x-axis direction from the entrance surface or the exit surface on two opposing surfaces parallel to the x-axis and the y-axis, and the y-axis. 6.6 mm in the direction. Accordingly, the inter-electrode distance d is 2.4 mm. On the light incident surface and light exit surface, the distance from the electrode surface is 0.5 mm in the z-axis direction and 6.6 mm in the y-axis direction. In such a structure, the focal length when the length of the single crystal substrate 1 in the x-axis direction is changed and the distance between the electrodes is changed accordingly is shown.

  At this time, the focal length was plotted by numerical calculation assuming that the voltage applied to the first and second electrode pairs was 1 kV and the relative dielectric constant was 20,000. The polarization of the incident light is a straight line, and the direction of the oscillating electric field is the z-axis direction. As can be seen from FIG. 5, at any inter-electrode distance, the variable focal length lens of the present embodiment has a focal length of about ½ or less compared to the conventional variable focal length lens.

  FIG. 6 shows the correlation between the focal length of the variable focus lens of the present embodiment and the interelectrode distance. In the variable focus lens of the present embodiment shown in FIG. 4, the thickness h (z-axis direction) of the single crystal substrate 11 is 4.0 mm, the length L (y-axis direction) is 6.6 mm, and the width (x-axis direction). ) Is also 6.6 mm. The size of each of the first and second electrode pairs 12 and 13 is 0.8 mm in the x-axis direction from the entrance surface or the exit surface on two opposing surfaces parallel to the x-axis and the y-axis, and the y-axis. 6.6 mm in the direction. Accordingly, the inter-electrode distance d is 2.4 mm. On the light incident surface and light exit surface, the distance from the electrode surface is 0.5 mm in the z-axis direction and 6.6 mm in the y-axis direction. In such a structure, by changing the size of each of the first and second electrode pairs 12 and 13 from 0.8 mm to 2.8 mm in the x-axis direction, the interelectrode distance d is set to 1 mm to The focal length when changed to 5 mm is shown.

  At this time, the voltage applied to the first and second electrode pairs 12 and 13 was 1 kV, the relative dielectric constant was 20,000, and the focal length was plotted by numerical calculation. The polarization of the incident light is a straight line, and the direction of the oscillating electric field is the z-axis direction. As can be seen from FIG. 6, the focal length decreases as the interelectrode distance decreases.

FIG. 7 shows the correlation between the wavefront aberration of the variable focus lens of this embodiment and the interelectrode distance. The correlation between the interelectrode distance and the wavefront aberration was plotted by numerical calculation under the same conditions as the calculation of the focal length shown in FIG. The polarization of the incident light is a straight line, and the direction of the oscillating electric field is the z-axis direction. It can be seen that the wavefront aberration indicating blur in the image formation of the lens takes a minimum value when the distance between the electrodes is 2.5 mm to 3.5 mm. The electrode width X in the x-axis direction (electrode width in the optical axis direction, unit mm) and the electrode width Z in the z-axis direction (width in the direction perpendicular to the optical axis and parallel to the oscillating electric field of light, unit mm) ) And generalize. The inter-electrode distance d (mm) shown in FIG. 4 has a minimum wavefront aberration between 2.5 <d <3.5 from the calculation result shown in FIG. Since the width of the single crystal substrate 11 in the x-axis direction is 6.6 mm, the electrode width in the x-axis direction is (6.6-3.5) / 2 <X <(6.6-2.5). When setting between / 2, the wavefront aberration becomes the minimum value. When all the sides are divided by the electrode width Z in the z direction when FIG. 7 is calculated = 0.5 mm, (6.6-3.5) /2/0.5 <(X / Z) <(6 .6-2.5) /2/0.5, the wavefront aberration is minimal. Rounding off the decimals, the ratio of electrode width X and Z is
3 <(X / Z) <4
In this case, the wavefront aberration takes a minimum value.

(Example)
As shown in FIG. 4, a substrate processed into a rectangular parallelepiped is cut out from a single crystal having an electro-optic effect. The single crystal substrate 11 cuts out a block from the KTN single crystal and sets the dimensions of the light incident surface and light exit surface to 6.6 mm in the y-axis direction and 4.0 mm in the z-axis direction. The two planes parallel to the xy plane are 6.6 mm in the x-axis direction and 6.6 mm in the y-axis direction. The dimensions of the two surfaces parallel to the xz plane are 4.0 mm in the z-axis direction, 6.6 mm in the x-axis direction, and the two surfaces parallel to the xy plane are 6.6 mm in the y-axis direction, the x-axis The direction is 6.6 mm.

  The size of each of the first and second electrode pairs 12 and 13 is 1.8 mm in the x-axis direction from the entrance surface or the exit surface on the two opposing surfaces parallel to the x-axis and the y-axis, and the y-axis 6.6 mm in the direction. Accordingly, the inter-electrode distance d is 3.0 mm. On the light incident surface and light exit surface, the distance from the electrode surface is 0.5 mm in the z-axis direction and 6.6 mm in the y-axis direction.

  All the surfaces of the single crystal substrate 11 are optically polished, and all the surfaces of the substrate are parallel to the (100) plane. Since the KTN single crystal has 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.

  With this variable focus lens temperature controlled at 40 ° C., laser light having a wavelength of 633 nm collimated to 3 mm in the z-axis direction is incident on the incident surface of the single crystal substrate 11. 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 1 kV is applied to each of the first and second electrode pairs 12 and 13, the light emitted from the single crystal substrate 11 is collected and functions as a cylindrical convex lens. The focal length is 51 cm.

  When no voltage is applied, there is no light collection effect and the focal length is infinite. Accordingly, the focal length is 51 cm at an applied voltage capable of a high-speed response of 1 ms or less. As shown in FIG. 5, the focal length of the conventional variable focus lens can be reduced from 100 cm to about half. The wavefront aberration is 150 nm, which can be improved to about half from the 300 nm wavefront aberration of the conventional variable focus lens.

1,11 Single crystal substrate 2,12 First electrode pair 3,13 Second electrode pair 4,12 Light

Claims (4)

A single crystal substrate having an electro-optic effect;
A first electrode pair formed on a surface on which light is incident on the single crystal substrate, a first surface orthogonal to the surface on which the light is incident, and a second surface opposite to the first surface. The first anode is formed on the first surface and the surface on which the light is incident across a side where the first surface and the surface on which the light is incident is in contact, and the first cathode is A first electrode pair formed on the second surface and the light incident surface across a side where the second surface and the light incident surface are in contact with each other;
A second electrode pair formed on a surface from which light is emitted from the single crystal substrate, the first surface and the second surface orthogonal to the surface from which the light is emitted, and the second anode is The second surface and the surface from which the light exits are formed across the side where the second surface and the surface from which the light exits are in contact, and the second cathode is formed from the first surface and the surface from which the light exits. A second electrode pair formed on the first surface and the surface from which the light exits across a side in contact with the surface from which the light exits;
A variable focus lens, wherein a focus of light transmitted through the single crystal substrate is varied by changing an applied voltage between the first and second electrode pairs. The width X of the electrode in the optical axis direction on the first and second surfaces of the anode and cathode of the first and second electrode pairs, respectively, on the surface on which the light is incident and the surface on which the light is emitted When the width Z is perpendicular to the optical axis direction and parallel to the oscillating electric field of the light,
3 <(X / Z) <4
The variable focus lens according to claim 1, wherein the relationship is satisfied. The single crystal substrate is a single crystal material having a perovskite structure, and is made of potassium tantalate niobate (KTN: KTa _1-x Nb _x O ₃ , 0 <x <1). Or the variable focus lens of 2.   In the single crystal substrate, the main component of the crystal is composed of groups Ia and Va of the periodic table, group Ia is potassium, group Va can include at least one of niobium and tantalum, 4. The variable focus lens according to claim 3, wherein the variable focus lens is made of a single crystal material containing one or plural kinds of Group Ia of the periodic table excluding potassium as an additive impurity, for example, lithium or Group IIa.

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