JPS63229403A - Variable focus mirror - Google Patents

Abstract

PURPOSE:To obtain a variable focus mirror which can largely change a focal length at a high speed by providing a light reflector, the surface of which is covered with a material for reflecting light and which is constituted of a piezo-electric material and a voltage impressing means which impresses a voltage to the piezo-electric material. CONSTITUTION:A reflecting film 2 constituted of a metal or the like is formed on the surface of the disk-shaped thin piezo-electric plate 1 constituted of a ceramic piezo-electric material, etc., and the piezo-electric plate 1 is fixed by a holder 3 constituted of, for example, a metal, etc. An electrode 4 is formed on the piezo-electric plate 1. The piezo-electric plate 1 changes to a recessed shape when the voltage is impressed to the electrode 4 formed on the piezo-electric plate 1 by using a power supply 5. The variable focus mirror is realized by the reflecting film 2 on the surface when light is entered to such surface. The piezo-electric plate 1 having, for example, 25mm diameter, changes by 80mum in the central part when 100V voltage is impressed to the plate 1. The variable focus mirror of which the focal length changes continuously from the infinity to 48cm at <=1msec response speed at this time is obtd. The mirror which largely changes the focal length at a high speed is thereby obtd.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a variable focus mirror that uses piezoelectric material to change its focal length at high speed.

[Prior art and its problems]

Variable focus optical systems have already been put into practical use as zoom lenses. However, when such a variable focus optical system that mechanically moves the lens is used, for example, as an objective lens of an optical disk head, the response speed is too slow. On the other hand, a method has been proposed in which the focal length is changed by changing the refractive index of the substance inside the lens without changing the position or shape of the lens. For example, methods for changing the orientation of liquid crystals,
Research is underway into methods of varying the voltage applied to ferroelectric crystals such as lithium niobate. However, at present, the value of the refractive index that can be changed by these methods is small, and the amount of change in focal length is also small. An optical system that can change the focal length at such high speed and greatly does not yet exist.

An object of the present invention is to provide a variable focus mirror that uses piezoelectric material and can change its focal length at high speed and greatly.

[Structure of the invention]

The variable focus mirror of the present invention is characterized by comprising a light reflector whose surface is covered with a material that reflects light and is made of a piezoelectric material, and a voltage applying means for applying a voltage to the piezoelectric material. There is.

[Effect]

Piezoelectric materials generate electric polarization in response to stress caused by external forces, and by utilizing this property in reverse, the material can be deformed by changing the voltage applied to the material. The relationship between electric polarization due to piezoelectricity and stress is expressed by the piezoelectric constant dik, and the component of the stress tensor is expressed as Xm (k=
1.2. ...6), the relationship of the following equation holds true, assuming that the component of the electrically polarized bac 1 - is Pi (i=1.2.3).

P, = dikXk (1) A variable focus mirror can be made using such a piezoelectric material. For example, when a highly reflective film 12 is attached to a circular piezoelectric plate 11 as shown in FIG. 2 and a voltage is applied, the piezoelectric plate 11 is deformed into a concave shape by the light 14. The deformation w (r) at the coordinate r when the circumference of the piezoelectric plate 11 is fixed with the holder 13 is expressed by the following equation.

Here, P is pressure, D, c2. c is expressed by the following formula.

Here, E represents the Young's modulus of the reflective film 12, ν represents the Poisson's ratio of the reflective film 12, h represents the thickness of the piezoelectric plate 11, and a represents the radius of the piezoelectric plate 11.

When formulas (2) to (5) are rearranged, the following formula is obtained.

...(6) Now, considering only the center part, equation (6) becomes. Also,
The maximum stress at this time is expressed as. A pressure within a range in which the maximum stress σ does not exceed the fatigue limit of the material of the piezoelectric plate 11 is applied to the reflective film 12, and the amount of deformation at the center at that time can be determined from equation (7). The amount of volume change ΔV required at this time is obtained from the following equation.

〔Example〕

The present invention will be explained in detail below.

FIG. 1 is a perspective view showing an embodiment of the variable focus mirror of the present invention. For example, a reflective film 2 made of metal or the like is formed on the surface of a circular thin piezoelectric plate 1 made of ceramic piezoelectric material, etc., and the piezoelectric plate 1 is held by a holder 3 made of metal or the like. to be fixed. Electrodes 4 are formed on the piezoelectric plate 1 .

In the variable focus mirror configured as described above, when a voltage is applied to the electrode 4 formed on the piezoelectric plate 1 using the power source 5, the piezoelectric plate 1 changes into a concave shape. When light is incident on this surface, a variable focus mirror can be realized by the reflective film 2 on the surface. For example, when a voltage of 100 V is applied to a piezoelectric plate 1 with a diameter of 25 mm, the piezoelectric plate 1 has a voltage of 80 μm at the center.
m changed. At this time, it was possible to realize a variable focus mirror whose focal length changes continuously from infinity to 48 cm at a response speed of less than 1 m5ec. Furthermore, by using a piezoelectric plate with a large diameter and increasing the applied voltage, a mirror with a short focus can be realized.

〔Effect of the invention〕

As described in detail above, according to the variable focus mirror of the present invention, it is possible to realize a mirror that is fast and has a large amount of change in focal length.

[Brief explanation of drawings]

FIG. 1 is a perspective view showing an embodiment of the variable focus mirror of the present invention, and FIG. 2 is a diagram showing the principle of the variable focus mirror. 1.11... Piezoelectric plate 2.12... Reflective film 3.13... Holder 4... Electrode 5... Power supply 14... light

Claims (1)

[Claims] (1) A variable focus mirror comprising: a light reflector whose surface is covered with a material that reflects light and is made of a piezoelectric material; and a voltage applying means for applying a voltage to the piezoelectric material.