JP6926916B2 - Light deflector - Google Patents

Description

The present invention relates to an optical deflector, and more particularly to an optical deflector capable of deflecting light incident on a crystal having an electro-optical effect by applying a voltage to the crystal.

Light deflectors are used in various fields such as laser printers, microscopes, and projectors. As the optical deflector, one having a mechanism for rotating a polygon mirror or a galvano mirror and one using a MEMS (Micro Electro Mechanical System) are widely used. An optical deflector using a crystal having an electro-optical effect is expected to operate at a higher speed because it can deflect without a driving portion. KTN (potassium niobate tantalate: KTa _1-x Nb _x O ₃ ) is known as a substance having a large electro-optical effect, and a large change in refractive index can be obtained at a lower voltage than before. Further, when Ti or the like is used for the electrode, an electric charge can be injected into the crystal, and high-speed and wide-angle light deflection is realized by utilizing the internal electric field generated by the injected electric charge.

FIG. 1 shows the basic structure of the KTN optical deflector. The KTN optical deflector has a KTN 103 sandwiched between an electrode (cathode) 101 and an electrode (anode) 102. The light incident on the KTN light deflector and the light emitted from the KTN light deflector are referred to as incident light 104 and emitted light 105, respectively.

The electric field distribution is formed by the electric charge injected into the KTN and the electric field applied from the outside, so that the refractive index distribution is generated in the crystal.

If the electric field at the position x where the cathode is x = 0 is E (x), the electron density distribution is N (x), the permittivity is ε, and the elementary charge is e, Gauss's law is the following equation at x> 0. expressed.

Assuming that the electrons are uniformly distributed in the KTN at a density N, the crystal thickness (crystal length in the x direction) is d, and a voltage V is applied.

Therefore, E (x) is calculated as follows.

In this equation, ρ = eN, where ρ represents the charge density.

From this electric field distribution and the Kerr effect equation, the refractive index change Δn in the crystal is expressed by the following equation.

n is the refractive index and g ₁₁ is the electro-optical coefficient.

The deflection angle due to this refractive index distribution is accumulated as it propagates in the crystal. Further, the deflection angle θ (x) at the position x due to the deflection due to refraction at the crystal exit end is expressed by the following equation.

n is the refractive index and g ₁₁ is the electro-optical coefficient. L is the crystal length in the ray direction.

When light passes through a crystal in which this refractive index distribution is formed, the light is not only deflected but also focused by the refractive index distribution.

Consider the refractive index distribution when V = 0. At this time, the refractive index n _d at the distance d from the optical axis is

It is expressed as.

From the GRIN lens formula, the refractive index n _{r at a distance r}

(A is a positive constant, n ₀ is the refractive index on the optical axis), and the effective focal length f is expressed by the following equation.

From _{the equation of n d} and the effective focal length, the focal length of KTN is expressed by the following equation.

As expressed by this equation, the focal length depends on the charge density ρ, and when ρ is constant regardless of x, the focusing position is constant regardless of the position of KTN in the x direction.

JP-A-2015-31929

T. Imai, J. Miyazu, and J. Kobayashi, "Measurement of charge density distributions in KTa_1-xNb_xO_3 optical beam deflectors," Opt. Mater. Express 4, 976 (2014).

Non-Patent Document 1 discloses that the charge density of KTN is not uniform due to the charge injection method. When the charge density is not uniform, there is a problem that the focal length differs depending on the transmission position in the KTN, that is, the applied voltage at the time of deflection, and the focal length is not focused on one surface, which leads to deterioration of image quality when used in a microscope or the like.

On the other hand, in Patent Document 1, a varifocal lens whose focal length changes in synchronization with the voltage for light deflection is arranged on the exit side of the KTN crystal, and the light is focused independently of the voltage applied to the KTN crystal. A method of making the characteristics constant is disclosed. However, this method requires the addition of a varifocal lens and a device associated therewith, which causes a problem that the configuration of the optical deflector becomes complicated.

The aspect of the present invention has been made to solve such a problem, and an object of the present invention is to provide an optical deflector that compensates for the non-uniformity of the lens effect in an optical deflector having a lens effect. To do.

In order to achieve this object, the optical element is characterized in that the crystal length is processed so as to compensate the distribution of the internal charge density in the optical deflector of the electro-optical crystal.

In order to achieve the above object, one aspect of the optical deflector of the present invention is:
Crystals with electro-optic effect and
With at least one pair of electrodes sandwiching the crystal to apply an electric field within the crystal,
An incident surface for incident light into the crystal and
It has an exit surface for emitting the light from the inside of the crystal.
In an optical deflector that deflects the light in the electric field direction and the charge density distribution in the crystal is unevenly distributed in the electric field direction.
The distance between the incident surface and the exit surface changes according to the distribution of charge density in the electric field direction, and the change in the distance causes the focal length of light incident on the incident surface and emitted from the exit surface. It is characterized by being constant.

The higher the charge density, the shorter the distance between the incident surface and the exit surface, and the lower the charge density, the longer the distance between the incident surface and the exit surface.

At least one of the exit surface and the entrance surface is inclined with respect to the surface of the electrode.

At least one of the emitting surface and the incident surface has a radius of curvature.

It has a reflective surface that is parallel to the electric field and faces it.
A plurality of light incident from the incident surface is reflected by the opposing reflecting surfaces,
It is characterized in that it is emitted from the exit surface.

According to the mode of the present invention, the change in the lens effect in the deflection direction is changed by processing the crystal length of the light deflector according to the charge density so that the focal length of the crystal of the light deflector becomes constant in the lens effect. Can be compensated.

It is a figure which shows the basic structure of a KTN light deflector. (a) It is a figure which shows the relationship between the position in the x direction of an optical deflector, and the current density ρ (x). (b) It is a figure which shows the relationship between the x-direction position of an optical deflector, and a crystal length L. (a) It is a figure which shows an example of the basic structure of the KTN light deflector which concerns on Example 1. FIG. (b) It is a figure which shows another example of the basic structure of the KTN light deflector which concerns on Example 1. FIG. (a) It is a figure which shows the relationship between the position in the x direction of an optical deflector, and the current density ρ (x). (b) It is a figure which shows the relationship between the x-direction position of an optical deflector, and a crystal length L. (a) It is a figure which shows the relationship between the position in the x direction of an optical deflector, and the current density ρ (x). (b) It is a figure which shows the relationship between the x-direction position of an optical deflector, and a crystal length L. (a) It is a figure which shows an example of the basic structure of the KTN light deflector which concerns on Example 2. FIG. (b) It is a figure which shows another example of the basic structure of the KTN light deflector which concerns on Example 2. FIG. (a) It is a figure which shows an example of the basic structure of the KTN light deflector which concerns on Example 3. FIG. (b) It is a figure which shows the inclination figure of the basic structure of FIG. 7 (a).

Hereinafter, the form of the optical deflector of the present invention will be described in detail with reference to the drawings. However, the present invention is not limited to the contents described in the examples shown below, and it is obvious to those skilled in the art that various forms and details can be changed without departing from the spirit of the invention disclosed in the present specification and the like. .. In addition, the configurations according to different examples can be combined and implemented as appropriate.

In this embodiment, an example will be described in which the charge density monotonically increases from one electrode (anode) 301a, 301b to the other electrode (cathode) 302a, 302b, that is, has a monotonically increasing distribution in the x direction.

When the equation of the focal length of KTN is modified, the crystal length when the charge density ρ has a distribution in the x direction and the KTN has a focal length f _{KTN is}

It can be expressed as.

Consider a crystal with a crystal thickness of 1.2 mm in the x-direction.

Figures 2 (a) and 2 (b) show the crystal length L at _{which the focal length f KTN} is constant when ρ (x) calculated from the above equation changes linearly. Note that FIG. 2A shows the relationship between the x-direction position of the optical deflector and the current density ρ (x), and FIG. 2B shows the relationship between the x-direction position of the optical deflector and the crystal length L. Is shown.

By shortening the crystal length as the charge density increases as shown in FIGS. 2 (a) and 2 (b), f _KTN can be kept constant in the x direction.

In FIGS. 3A and 3B, crystals processed so as to keep the focal length constant when the above-mentioned charge densities are held between the electrodes (anode) 301a and 301b and the electrodes (cathodes) 302a and 302b are shown. show. The dotted line represents the crystal end faces 303a and 303b and the emitted light when not processed, and the solid line represents the crystal end face and the emitted light when processed. In the case of the dotted line, the focal length differs depending on the transmission position in the KTN, but the focal length can be made uniform by processing.

When the crystal length of the optical deflector is processed in this way, it may be compensated by diagonally processing only one of the emitting side and the incident side as shown in FIG. 3 (a), or as shown in FIG. 3 (b). It may be compensated by using diagonal processing on both the entrance and exit sides.

Further, when ρ (x) changes in a quadratic function, it becomes as shown in FIGS. 4 (a) and 4 (b). FIG. 4A shows the relationship between the x-direction position of the optical deflector and the current density ρ (x), and FIG. 4B shows the relationship between the x-direction position of the optical deflector and the crystal length L. ..

In this case, the KTN entrance / exit surface is processed into a curved surface. In the case of a curved surface, there is also a lens effect caused by the shape, so the crystal length and curvature may be processed in consideration of this effect.

In this embodiment, an example in which the charge density has a non-monotonically increasing distribution from one electrode to the other will be described.

When calculated in the same manner as in Example 1, the _{crystal lengths at which f KTN} is constant when ρ (x) changes in a quadratic function are as shown in FIGS. 5 (a) and 5 (b). Note that FIG. 5A shows the relationship between the x-direction position of the optical deflector and the current density ρ (x), and FIG. 5B shows the relationship between the x-direction position of the optical deflector and the crystal length L. Is shown.

In this way, when processing the crystal length of the crystal between the electrodes (anode) 601a and 601b and the electrodes (cathodes) 602a and 602b, as in Example 1, the exit side or the incident side is as shown in FIG. 6A. Compensation may be performed by applying curved surface processing to only one of the sides, or compensation may be performed by performing curved surface processing on both the entrance and exit sides as shown in FIG. 6 (b).

The case where the distribution is symmetric with respect to the center in the x direction has been calculated, but in the case where the distribution is asymmetric, the crystal length may be processed according to the distribution of ρ (x).

In this embodiment, as shown in FIGS. 7 (a) and 7 (b), the reflection coat 701 is arranged on a part of the planes facing each other in the direction parallel to the electric field of the KTN 705 sandwiched between the pair of electrodes 704, and the incident window is provided. A structure will be described in which the incident light 702 incident from the above is reflected in the KTN by multiple times and multiple reflections, and the emitted light 703 is emitted from the exit window.

In this case as well, the same effect of making the focal length uniform can be obtained by applying the same crystal length processing as in Examples 1 and 2 to the incident window and / and the exit window 706. (Fig. 7 (b))

In this case, the distance between the entrance / exit surfaces is adjusted in consideration of the distribution of the charge density over the entire length of the folded optical path, and the distance between the entrance / exit surfaces is adjusted according to the number of times of folding.

By processing only the window to match the crystal length to the charge density and keeping the other surfaces parallel, internal reflection becomes easier than when the entire end surface is processed.

In Examples 1 to 3, the optical deflector using KTN has been described, but KLTN (K _1-y Li _y Ta _1-x Nb _x O ₃ ) and other crystals having an electro-optical effect can be used. It may be the optical deflector used.

The present invention can provide a light deflector that compensates for the non-uniformity of the lens effect in a light deflector having a lens effect.

101 Electrode (cathode)
102 Electrode (anode)
103 KTN
104 Incident light 105 Emission light 303a, 303b Crystal end face 301a, 301b electrode (anode) when not processed
302a, 302b Electrodes (cathodes)
601a, 601b Electrodes (anodes)
602a, 602b Electrodes (cathodes)
701 Reflective coat 702 Incident light 703 Emission light 704 Pair of electrodes 705 KTN
706 Incident window and / and exit window

Claims (5)

Crystals with electro-optic effect and
With at least one pair of electrodes sandwiching the crystal to apply an electric field within the crystal,
An incident surface for incident light into the crystal and
It has an exit surface for emitting the light from the inside of the crystal.
In an optical deflector that deflects the light in the electric field direction and the charge density distribution in the crystal is unevenly distributed in the electric field direction.
The distance between the incident surface and the exit surface changes according to the distribution of charge density in the electric field direction, and the change in the distance causes the focal distance of light incident on the incident surface and emitted from the exit surface. An optical deflector characterized by being constant. The first aspect of claim 1, wherein the higher the charge density, the shorter the distance between the incident surface and the exit surface, and the lower the charge density, the longer the distance between the incident surface and the exit surface. Light deflector. The light deflector according to claim 1, wherein at least one of the exit surface and the entrance surface is inclined with respect to the surface of the electrode. The light deflector according to claim 1, wherein at least one of the emitting surface and the incident surface has a radius of curvature. It has a reflective surface that is parallel to the electric field and faces it.
A plurality of light incident from the incident surface is reflected by the opposing reflecting surfaces,
The optical deflector according to claim 1, wherein the light is emitted from the exit surface.

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