JP2019015930A - Electro-optic light deflector - Google Patents

Abstract

To provide an electro-optic light deflector which gives a straight trajectory of deflection of a beam exiting therefrom.SOLUTION: An electro-optic light deflector 300 is constituted by an electro-optic crystal 302 and two antireflection films 303, two high reflection films 304 and an electrode 305 for applying an electric field. The direction of electric field application is set to be parallel to an X axis direction. An entrance surface and an exit surface of the electro-optic crystal 302 are parallel to each other, and beams to be incident to the entrance surface and the exit surface are each controlled to be incident perpendicular respectively to the entrance surface and to the exit surface in a YZ plane. That is, the beam incident to at least the exit surface is perpendicular to the line of intersection between the exit surface and the YZ plane. Surfaces of the electro-optic crystal 302 opposing to each other where the high reflection films 304 are disposed are angled with respect to the entrance surface and the exit surface and are made parallel to each other, by a cutting process or the like on the electro-optic crystal 302.SELECTED DRAWING: Figure 3

Description

  The present invention relates to an electro-optic light deflector that changes the propagation direction of light.

  An optical deflector is a device that changes the propagation direction of light. There are broadly divided into a reflection type and a transmission type. As a typical reflection type optical deflector, there is a device using a reflecting mirror whose inclination changes, such as a galvano mirror or a polygon mirror.

  On the other hand, the transmission type optical deflector can be divided into a diffraction type and a refractive index distribution type. In the diffractive type, the refractive index modulation generated by an acoustic wave becomes a diffraction grating and diffracts light to change its propagation direction. Since the light diffraction phenomenon due to the acoustic wave is called an acoustooptic effect, an optical deflector using the acoustooptic effect is called an acoustooptic deflector here.

  In the refractive index distribution type, the refractive index inside the deflector changes monotonously in a specific direction, and the wavefront of the propagating light tilts, thereby changing the propagation direction. Since this refractive index change is generally caused by an electro-optic effect, an optical deflector using this electro-optic effect is referred to herein as an electro-optic light deflector.

  The electro-optic effect refers to a phenomenon in which the refractive index of a substance changes depending on the strength of an electric field, and a crystal having an electro-optic effect is called an electro-optic crystal. When electric charge such as electrons is injected into the electro-optic crystal by some method and an electric field is applied to the electro-optic crystal into which the charge has been injected, a non-uniform electric field distribution is formed inside by electrostatic shielding. . Due to the combined effect of the non-uniform electric field distribution and the electro-optic effect, a refractive index distribution depending on the electric field distribution is formed inside the crystal. By utilizing this refractive index distribution, the light propagation direction can be changed inside the crystal.

  As a typical electro-optic light deflector, an optical deflector using potassium tantalate niobate (KTN) can be cited (see Patent Document 1). KTN is a perovskite single crystal having inversion symmetry and exhibits a Kerr effect as a secondary electro-optic effect as electro-optic characteristics. KTN exhibits a structural phase transition in which the crystal structure changes at a specific crystal temperature. At this time, the dielectric constant of the crystal significantly increases in accordance with Curie-Wise's law, so that a large electro-optic effect can be obtained.

  Optical deflectors having a wide deflection angle are required for applications such as a wavelength swept light source and a line sensor used in optical coherence tomography (OCT). The size of the deflection angle of an electro-optic light deflector is proportional to the distance that light propagates through the refractive index distribution that depends on the electric field distribution. is there. This is because a multi-pass configuration in which a reflective film is attached to the crystal surface and the beam is folded inside the crystal to pass the crystal multiple times is effective for widening the angle. it is obvious. By using this multipath configuration, a propagation distance that is times the number of paths can be obtained, and a wide deflection angle can be obtained.

  Non-Patent Document 1 discloses a wavelength swept light source used for OCT. This light source comprises a semiconductor optical amplifier, a KTN optical deflector, a diffraction grating and other optical elements. Non-Patent Document 1 shows that the light source performance depends on the deflection angle of the optical deflector, and a multi-path configuration is adopted to obtain a wide deflection angle.

Japanese Patent No. 4751389

Masahiro Ueno, Seiji Toyoda, Takashi Sakamoto, Yuzo Sasaki, Junya Kobayashi, “Coherence length extension of 200 kHz wavelength swept light source by widening KTN optical deflector”, Electronics, Information and Communication Electronics Society, 2014, C-3-26

  However, the conventional multi-path electro-optic light deflector has a problem that the trajectory of the deflected beam scan becomes a curved curve from a straight line. This is because when the light refraction at the boundary surface of the crystal is considered on the plane orthogonal to the scanning direction, the beam is not incident on the exit end surface of the crystal but is incident obliquely.

  FIG. 1 (a) shows a cross-sectional view perpendicular to the electric field application direction of a conventional electro-optic light deflector having a three-pass multi-path configuration, and FIG. 1 (b) shows the electro-optic light deflector from the Y-axis direction of FIG. The figure which saw is shown. The multi-path configuration is realized by tilting the incident beam with respect to the incident end face of the electro-optic light deflector 100, obliquely incident it, and reflecting it on the highly reflective film 103 installed on the crystal end face and turning the beam back within the crystal. ing. Further, an electric field is applied in the X-axis direction by applying a voltage to the electrode 104 for applying an electric field.

  A dotted line 101 represents a beam trajectory, a vector D ′ is a direction vector of the incident beam incident on the crystal, a vector S is a normal vector of the incident surface of the crystal, and a vector I is a direction vector of the beam incident on the crystal exit surface. , Vector D represents the direction vector of the outgoing beam emitted from the crystal. Each vector is a unit vector.

  The entrance and exit surfaces of the crystal are parallel, the Y component of vector I is 0 and in the XZ plane, and the X component of vector D 'is 0 and in the YZ plane. The X axis is perpendicular to the drawing, and the Z axis is parallel to the vector I. The angle formed by the Y axis and the crystal entrance plane and exit plane is θ.

  The electric field application direction of the electro-optic light deflector 100, that is, the deflection direction is the X-axis direction, and the X component of the direction vector of the beam propagating in the crystal is the refractive index distribution dn / dX in the X direction and the propagation length in the crystal. The deflection angle, that is, the angle formed by the vector I and the Z axis is defined as φ. From the above conditions, each direction vector is expressed as follows.

The refractive index of the space surrounding the electro-optic crystal is n1, and the average refractive index of the crystal is n2. On the exit surface, these vectors satisfy the following relationship according to Snell's law.

従って、

Therefore,

It becomes. From this result,

It becomes. However, N = n2 / n1, and A = cos θ / sin θ. Since vector D is a unit vector,

Satisfy the relationship. From equations (6), (7), (8),

Is obtained. Since this equation is a quadratic equation for Dy, the following can be obtained by solving using the solution formula.

Since Dy is the Y component of the propagation direction vector D of the refracted light, the angle θy formed by the Y axis and D takes the inner product of the unit vector of the Y axis,

It is expressed. Since the expression (11) includes cos φ as is clear from the expression (10), it indicates that the optical deflection is accompanied by not only the original angle change in the X-axis direction but also the angle change in the Y-axis direction. As a result, the conventional multi-pass electro-optic deflector has a problem that the trajectory of the beam scan is not a straight line but a curved curve.

  FIG. 2 shows a calculation example of the deflection angle dependence of θy of a conventional electro-optic light deflector. Numerical examples used in the calculation are θ = 5.2 °, n1 = 1, and n2 = 2.2. The relative angle change amount represents an angle change amount based on θy when the deflection angle φ = 0. This change in angle is a light deflection in a direction that is not originally intended, and therefore becomes a problem for the optical deflector.

  The present invention has been made in view of such problems, and an object thereof is to provide an electro-optic light deflector in which the locus of deflection of the emitted beam is a straight line.

  In order to solve the above-described problems, the present invention provides an electro-optic light deflector comprising: an electro-optic crystal whose refractive index changes when an electric field is applied; and an electrode for applying an electric field to the electro-optic crystal. The light incident on the exit surface of the electro-optic crystal from within the electro-optic crystal is perpendicular to the line of intersection between the surface perpendicular to the electric field application direction of the electro-optic crystal and the exit surface. An electro-optic crystal is arranged.

  According to a second aspect of the present invention, in the electro-optic light deflector according to the first aspect, a multi-path structure that reflects light propagating in the electro-optic crystal and returns a propagation optical path in the electro-optic crystal. It is characterized by having.

  According to a third aspect of the present invention, in the electro-optic light deflector according to the second aspect, the multipath structure has two parallel surfaces of the electro-optic crystal as reflection surfaces.

  According to a fourth aspect of the present invention, in the electro-optic light deflector according to the third aspect, the incident surface of the electro-optic crystal is parallel to the emission surface, and the incidence surface and the emission surface are the reflection surface. And non-parallel.

  The invention according to claim 5 is an electro-optic light deflector, wherein an electro-optic crystal whose refractive index changes by application of an electric field, an electrode for applying an electric field to the electro-optic crystal, and the electro-optic crystal Two prisms having the same refractive index as that of the electro-optic crystal, which are bonded to the entrance surface and the exit surface, respectively, and light incident on the exit surface of the prism from within the prism is The electro-optic crystal and the prism are arranged so as to be perpendicular to an intersection line between a surface perpendicular to an electric field application direction and an exit surface of the prism.

  According to a sixth aspect of the present invention, in the electro-optic light deflector according to the fifth aspect, a multi-path structure that reflects light propagating in the electro-optic crystal and folds a propagation optical path in the electro-optic crystal is provided. It is characterized by having.

  According to a seventh aspect of the present invention, in the electro-optic light deflector according to the sixth aspect, the multipath structure has two parallel surfaces of the electro-optic crystal as reflection surfaces.

  The invention according to claim 8 is the electro-optic light deflector according to claim 7, wherein the incident surface of the prism is parallel to the exit surface of the prism, and the entrance surface of the prism and the exit surface of the prism Is not parallel to the reflecting surface.

  By adopting the configuration of the present invention, it is possible to make the locus of deflection of the beam emitted from the electro-optic light deflector straight.

(A) is a cross-sectional view perpendicular to the electric field application direction of a conventional electro-optic optical deflector having a three-pass multipath configuration, and (b) is a view of the electro-optic optical deflector from the Y-axis direction of (a). It is a figure. It is a figure which shows the example of calculation of the deflection angle dependence of (theta) y of the conventional electro-optic light deflector. It is sectional drawing perpendicular | vertical to the electric field application direction of the electro-optic light deflector which concerns on Embodiment 1 of this invention. 1 is a perspective view of an electro-optic light deflector according to Embodiment 1 of the present invention. It is sectional drawing perpendicular | vertical with respect to the X-axis of the electro-optic light deflector which concerns on Embodiment 2 of this invention.

  Hereinafter, embodiments of the present invention will be described in detail.

(Embodiment 1)
FIG. 3 is a cross-sectional view perpendicular to the electric field application direction of the electro-optic light deflector according to Embodiment 1 of the present invention, and FIG. 4 is a perspective view thereof.

  The electro-optic light deflector 300 includes an electro-optic crystal 302, two antireflection films 303, two high reflection films 304, and an electric field application electrode 305. The electric field application direction is parallel to the X-axis direction.

  The beam passes through the trajectory 301 and passes through one antireflection film 303 when entering the electro-optic crystal 302. The beam 301 is reflected a plurality of times, in this case, twice between the two high reflection films 304, passes through the other antireflection film 303, and is emitted from the electro-optic crystal 302.

  The entrance surface and the exit surface of the electro-optic crystal 302 are parallel to each other so that the beams incident on the entrance surface and the exit surface are incident perpendicularly on the YZ plane with respect to the entrance surface and the exit surface, respectively. That is, at least the beam incident on the exit surface is perpendicular to the intersection line between the exit surface and the YZ plane. Further, the opposing surfaces of the electro-optic crystal 302 on which the highly reflective film 304 is disposed are angled with respect to the incident surface and the exit surface by cutting the electro-optic crystal 302 or the like, and are parallel to each other. In this case, the normal vector S of the exit surface is

As with the vector I, it is also perpendicular to the Y axis. Since the expressions (2) and (3) are also established in this embodiment, from the expressions (12) and (2) to (4),

Is obtained. This indicates that Dy = 0. In other words, since the Y-axis direction is not affected by Snell's law, the beam scan locus is not deflected in the Y-axis direction but only in the X-axis direction by the refractive index distribution depending on the electric field distribution. Becomes a straight line.

  The electro-optic crystal 302 is preferably a high dielectric constant crystal such as potassium tantalate niobate (KTN). The antireflection film 303 and the high reflection film 304 are preferably dielectric multilayer films.

  The number of reflections in the electro-optic crystal 302 is not limited to two in the present embodiment, but can be two or more, and a multi-pass structure of three or more passes can be adopted. .

(Embodiment 2)
FIG. 5 shows a cross-sectional view of an electro-optic light deflector 400 according to Embodiment 2 of the present invention. FIG. 5 shows a cross section perpendicular to the X-axis as in FIG. The electric field application direction is parallel to the X-axis direction, as in the first embodiment.

  The electro-optic light deflector 400 has a cube shape in which the electro-optic crystal 402 and the triangular prism 405 are combined and the thickness in the X-axis direction is thin. An anti-reflection film 403 and a high-reflection film 404 are provided on two parallel surfaces that are perpendicular to the YZ plane of the electro-optic crystal 402 and face each other, and the electro-optic crystal is in contact with the anti-reflection film 403 and the high-reflection film 404. Triangular prisms 405-1 and 405-2 having the same refractive index as 402 are fixed. Antireflection films 403 are respectively provided on the surfaces of the triangular prisms 405-1 and 405-2 that face the surfaces that are in contact with the antireflection film 403 and the high reflection film 404, and are parallel to each other.

  The beam 401 is perpendicularly incident on the surface of the triangular prism 405-1 on which the antireflection film 403 is installed. Since the electro-optic crystal 402 and the triangular prism 405 have the same refractive index, the beam 401 incident on the electro-optic crystal 402 from the triangular prism 405-1 goes straight without being refracted. Similarly, the beam incident on the triangular prism 405-2 from the electro-optic crystal 402 goes straight without being refracted. Therefore, the incident surface and the exit surface where refraction occurs are surfaces on which the antireflection films 403 of the triangular prisms 405-1 and 405-2 are installed, and each beam incident on the entrance surface and the exit surface is the entrance surface and the exit surface. Are perpendicularly incident on the YZ plane. That is, at least the beam incident on the exit surface is perpendicular to the intersection line between the exit surface and the YZ plane.

  For this reason, also in the electro-optic light deflector 400 according to the second embodiment, as in the first embodiment, the beam 401 is incident on the incidence surface and the emission surface perpendicularly in the YZ plane, so that the equation (13) Holds. For this reason, the electro-optic light deflector 400 according to the second embodiment is not deflected in the Y-axis direction but is deflected only in the X-axis direction due to the refractive index distribution depending on the electric field distribution, and the beam scan locus becomes a straight line. .

  In the first and second embodiments, the entrance surface and the exit surface are made parallel and the two highly reflective films are made parallel, but what is important is that the propagating beam is incident on the exit surface vertically. If the propagating beam is incident on the exit surface perpendicularly, the entrance surface and the exit surface need not be parallel and the two highly reflective films do not need to be parallel.

100, 300, 400 Electro-optic light deflector 102, 302, 402 Electro-optic crystal 103, 304, 404 High reflection film 303, 403 Anti-reflection film 104, 305 Electrode 405 Triangular prism

Claims (8)

An electro-optic crystal whose refractive index changes when an electric field is applied;
An electrode for applying an electric field to the electro-optic crystal;
So that light incident on the exit surface of the electro-optic crystal from within the electro-optic crystal is perpendicular to the line of intersection between the surface perpendicular to the electric field application direction of the electro-optic crystal and the exit surface. An electro-optic light deflector, wherein the electro-optic crystal is disposed.   2. The electro-optic light deflector according to claim 1, wherein the electro-optic light deflector has a multipath structure that reflects light propagating in the electro-optic crystal and turns a propagation light path in the electro-optic crystal.   3. The electro-optic light deflector according to claim 2, wherein the multipath structure has two parallel surfaces of the electro-optic crystal as reflection surfaces.   4. The electro-optic light deflector according to claim 3, wherein an incident surface of the electro-optic crystal is parallel to the exit surface, and the entrance surface and the exit surface are non-parallel to the reflection surface. An electro-optic crystal whose refractive index changes when an electric field is applied;
An electrode for applying an electric field to the electro-optic crystal;
Two prisms having the same refractive index as that of the electro-optic crystal, which are bonded to the entrance surface and the exit surface of the electro-optic crystal,
And the light incident on the exit surface of the prism from within the prism is perpendicular to the line of intersection of the surface perpendicular to the electric field application direction of the electro-optic crystal and the exit surface of the prism. An electro-optic light deflector comprising an optical crystal and the prism.   6. The electro-optic light deflector according to claim 5, wherein the electro-optic light deflector has a multipath structure that reflects light propagating in the electro-optic crystal and turns a propagation light path in the electro-optic crystal.   The electro-optic light deflector according to claim 6, wherein the multi-pass structure has two parallel surfaces of the electro-optic crystal as reflection surfaces.   8. The electro-optic according to claim 7, wherein an incident surface of the prism is parallel to an output surface of the prism, and an incident surface of the prism and an output surface of the prism are non-parallel to the reflecting surface. Optical deflector.

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