JP6322883B2 - Optical deflection element - Google Patents

Description

  The present invention relates to an optical deflection element utilizing an electro-optic effect.

  A light beam scanner that scans a laser beam is used in various fields such as a laser printer, laser processing, display, measurement, and optical communication. The light beam scanner generally includes a laser oscillation device serving as a light source and a light deflection element that scans the laser beam. Furthermore, if necessary, an optical coupling optical system that shapes the light beam generated from the laser light source into a beam shape suitable for the optical deflection element, an optical beam profile output from the optical deflection element, or an optical deflection element An output optical system that enlarges or reduces a given deflection angle may be provided.

  Conventionally used as an optical deflection element are optical scanning by controlling the reflection angle of a light beam such as a rotating polygon mirror, a galvanometer mirror, and a MEMS (Micro-Electro-Mechanical Systems) mirror. In addition to the above, there are known materials that use a change in the refractive index of a material due to an acousto-optic (AO) effect or an electro-optic (EO) effect.

  In particular, the electro-optic effect has a very high response speed in principle, and an optical deflection element utilizing this is effective for high-speed beam scanning. In addition, since the scanning angle can be arbitrarily controlled according to the voltage applied to the electro-optic material, a desired deflection angle can be instantaneously given to the light beam.

  Light sources of light beam scanners having various wavelengths and light intensities are used depending on applications. In particular, it is known that a high-intensity laser light source of several hundred mW to 1 W or more is used for a light source for distance measurement by laser processing or laser radar, or for laser projection.

  A commonly known problem with electro-optic materials is photorefraction. This is a phenomenon in which the beam shape is greatly disturbed (beam distortion) when a material is irradiated with a high energy light beam. Photo refraction is caused by the fact that free electrons generated inside the material due to light energy excitation drift and a random internal electric field is formed in the material. It is thought to cause irregular refractive index changes. In the above-described light source for laser projection, this problem appears remarkably.

  Attempts have been made to optimize the composition of the material in order to solve the above photorefractive problem. For example, in contrast to common electro-optic materials and non-linear optical materials lithium niobate and lithium tantalate, magnesium is doped to increase the photoconductivity of the material, thereby generating an internal electric field formed by carriers excited by light. A technique is known in which the resistance of photorefractive is increased by lowering (Non-patent Document 1).

  This suppresses beam distortion when used for wavelength conversion elements that do not require the application of an external voltage or light modulation elements that require a low external voltage (the external electric field value required for operation is approximately 0.1 kV / mm). It is possible to do.

  However, when used for an optical deflecting element, the external electric field value required for operation is as high as about 1 kV / mm to 10 kV / mm, so that a phenomenon that the beam is distorted occurs even with magnesium-doped lithium niobate and lithium tantalate. . This is because the drift of photoexcited carriers is greatly promoted by the external electric field accompanying voltage application, and the formation of a space charge electric field by the carriers is promoted.

  This beam distortion is more noticeable when operating with a DC voltage than with an AC voltage. The reason is that when voltage application is always continued with the same polarity, carrier drift due to an external electric field advances and a local electric field distribution is formed. This beam distortion causes a problem that the performance such as the number of resolution points in the optical deflecting element is remarkably lowered.

  The present invention has been made to solve such a problem, and the object of the present invention is to suppress beam distortion generated when a high voltage is applied in an optical deflection element utilizing the electro-optic effect, and to To obtain an output beam having a profile.

An optical deflection element according to the present invention includes an optical waveguide having a core layer made of an electro-optic material, and a first electrode to which a predetermined voltage is applied, disposed on a pair of surfaces of the optical waveguide sandwiching the core layer A first electrode pair, and a second electrode pair disposed on the pair of surfaces and to which a voltage having a polarity opposite to a voltage applied to the first electrode pair is applied. The electrodes having a plurality of protrusions extending toward the electrodes constituting the second electrode pair on the same surface, and the electrodes constituting the second electrode pair are the first on the same surface. Each of the plurality of protrusions included in the first electrode pair and the plurality of protrusions included in the second electrode pair have a plurality of protrusions extending toward the electrode pair on the same surface. And the plurality of protrusions are alternately arranged in the light incident direction. Each trapezoidal der is, the same distance is greater than the thickness of the core layer of the electrode constituting the the electrodes constituting the first electrode to said second electrode pair in said surface, the light deflecting It is an element.

  According to the present invention, in an optical deflection element using the electro-optic effect, beam distortion generated when a high voltage is applied is suppressed, and an output beam having a good profile can be obtained.

It is a perspective view which shows schematic structure of the optical deflection | deviation element concerning embodiment of this invention. It is a perspective view which shows the optical waveguide and electrode in FIG. It is a figure for demonstrating the principle of the optical deflection | deviation in the optical deflection | deviation element concerning embodiment of this invention. It is a schematic diagram for demonstrating the physical mechanism in which a beam distortion generate | occur | produces in an electro-optic element. It is a figure for demonstrating the physical mechanism which beam distortion generate | occur | produces in an electro-optical element with an energy band. It is a figure which shows the waveform of the voltage applied to the optical deflection element which concerns on embodiment of this invention. It is a figure for demonstrating the principle by which beam distortion is suppressed by the optical deflection | deviation element which concerns on embodiment of this invention. It is a top view which shows the modification of the upper electrode in the optical deflection | deviation element concerning embodiment of this invention.

Embodiments of the present invention will be described below with reference to the drawings.
<Schematic configuration of light deflection element>
FIG. 1 is a perspective view showing a schematic configuration of an optical deflection element according to an embodiment of the present invention, and FIG. 2 is a perspective view showing an optical waveguide and electrodes in FIG. 2A is a perspective view of the upper surface side, and FIG. 2B is a perspective view of the lower surface side. In FIG. 2A, the extraction electrode 18 in FIG. 1 is omitted.

  As shown in FIG. 1, this optical deflecting element is an optical waveguide type optical deflecting element, and includes an optical waveguide 10, an upper electrode 14, a lower electrode 15, an adhesive layer 16, a support substrate 17, and an extraction electrode. It is comprised from 18.

  This light deflection element has a rectangular parallelepiped shape as a whole. In the XYZ orthogonal coordinate system in the figure, the + Y direction is the light propagation direction, the + Z direction is a direction perpendicular to the upper surface of the optical waveguide 10 from the lower surface of the support substrate 17, and the + X direction is orthogonal to the YZ plane and above the paper surface. It is the direction to go.

  The optical waveguide 10 includes a core layer 11, a pair of clad layers sandwiching the core layer 11, that is, an upper clad layer 12 formed on the upper surface of the core layer 11, and a lower clad layer 13 formed on the lower surface of the core layer 11. Become.

  An upper electrode 14 and an extraction electrode 18 are disposed on the upper surface of the upper cladding layer 12, which is one of a pair of surfaces of the optical waveguide 10 sandwiching the core layer 11, and the pair of surfaces of the optical waveguide 10 sandwiching the core layer 11. A lower electrode 15 is disposed on the lower surface of the lower cladding layer 13 which is the other of the above.

  The core layer 11 is an electro-optic material whose refractive index changes when a voltage is applied, and the refractive index can be changed by applying a voltage between the upper electrode 14 and the lower electrode 15. Since the optical deflection element according to this embodiment has a waveguide structure, the refractive index of the core layer 11 is selected to be higher than the refractive indexes of the upper cladding layer 12 and the lower cladding layer 13.

Examples of the material used for the core layer 11 include electro-optic materials such as lithium niobate (LiNbO ₃ ), lithium tantalate (LiTaO ₃ ), KTP (KTiOPO ₄ ), SBN, and KTN. It is preferable to use LN and LT from the viewpoint of suppressing beam distortion. In this embodiment, LN (Yamaju Ceramics opt grade Z-cut thickness 0.3 mm) is used. In this LN, spontaneous polarization is formed in the + Z direction.

Examples of the material of the upper cladding layer 12 and the lower cladding layer 13 include dielectrics such as SiO ₂ , Ta ₂ O ₅ , TIO ₂ , Si ₃ N ₄ , Al ₂ O ₃ , and HfO ₂ . In this embodiment, Ta ₂ O ₅ (thickness 1.5 μm) is used.

  Examples of the material of the upper electrode 14 and the lower electrode 15 include Au, Pt, Ti, Al, Ni, and Cr. In this embodiment, Ti (thickness 200 nm) is formed by sputtering.

  The material of the support substrate 17 is preferably a material having the same thermal expansion coefficient as that used for the core layer 11. The reason is that if the thermal expansion coefficients of the material of the support substrate 17 and the material of the core layer 11 are different, the core layer 11 is distorted due to internal stress during the thermal expansion due to the temperature change after bonding. This is because cracks are caused. In this embodiment, a lithium niobate substrate is used.

  This optical deflection element is manufactured by the following process. In order to reduce the thickness of the core layer 11 by polishing, the lower electrode 15 and the lower cladding layer 13 are formed on the core layer 11, and then the support substrate 17 is bonded to the core layer 11 with the adhesive layer 16. Thereafter, the core layer 11 is thinned by polishing, and the upper electrode 14 and the upper cladding layer 12 are formed. In order to take out the lower electrode 15, a hole penetrating from the lower clad layer 13 to the upper clad layer 12 is formed, and the lead electrode 18 is connected by a conductor. By reducing the thickness of the core layer 11 to an optical waveguide type, it is possible to provide an optical deflection element with a reduced driving voltage.

  The upper electrode 14 and the lower electrode 15 formed above and below the optical waveguide 10 have a triangular shape in order to add an optical deflection function to the optical waveguide 10. When a voltage is applied between the upper electrode 14 and the lower electrode 15, the refractive index of the electro-optic material changes due to the electro-optic effect, and as a result, a prism structure can be formed in the core layer 11. The light propagating in the core layer 11 is refracted at the boundary of the prism, and the light is deflected as a result of being emitted with different traveling directions of the incident light.

  The upper electrode 14 includes a first upper electrode 14-1 and a second upper electrode 14-2, and the lower electrode 15 includes a first lower electrode 15-1 and a second lower electrode 15-2. Each of the first upper electrode 14-1 and the second upper electrode 14-2 constituting the upper electrode 14 has a shape in which a plurality of (here, five) triangles are connected in the Y-axis direction (light propagation direction). It has a part. More specifically, it has a shape in which the end points of the bases of adjacent triangles are connected to each other and a rectangular lead portion is added to the outside of the base. Further, the triangular portions of the first upper electrode 14-1 and the triangular portions of the second upper electrode 14-2 are alternately arranged. The same applies to the first lower electrode 15-1 and the second lower electrode 15-2 constituting the lower electrode 15.

  This triangle is preferably an isosceles triangle in order to reduce the element size and maximize the deflection angle, but it may be a triangle having three different sides. In addition, since the electrode outline (peripheral line) need not be perpendicular to the light incident direction (the direction in which the alternate long and short dash line extends in FIG. 3B), it may have a shape other than a triangle, for example, a trapezoid as shown in FIG. In short, it is only necessary that the upper electrodes and the lower electrodes have a plurality of protrusions extending toward each other, and each of the plurality of protrusions is alternately arranged in the light incident direction.

  Further, the first upper electrode 14-1 and the first lower electrode 15-1 have the same shape and the same size, and have the same two-dimensional position (the positions of the same X coordinate and Y coordinate in the XYZ orthogonal coordinate system in FIG. 2). ). Similarly, the second upper electrode 14-2 and the second lower electrode 15-2 have the same shape and the same size, and are formed at the same two-dimensional position. The reason for this configuration is to form a vertical (Z-axis direction) electric field in the core layer 11 when a voltage is applied. If this positional accuracy is poor, the formed electric field is not perpendicular to the Z-axis direction, so that the refractive index generated by the first-order Pockels effect generated when a voltage is applied is lowered, resulting in deterioration of the optical deflection performance. Will be connected.

  In order to reduce the probability of occurrence of a short circuit between the electrodes, the distance between the first upper electrode 14-1 and the second upper electrode 14-2, and the first lower electrode 15-1 and the second lower electrode 15- 2 is preferably larger than the thickness of the core layer 11.

  The extraction electrode 18 includes a first extraction electrode 18-1 and a second extraction electrode 18-2, and vertically penetrates the first lower electrode 15-1, the second lower electrode 15-2, and the optical waveguide 10, respectively. Connected through a hole.

<Principle of light deflection>
FIG. 3 is a view for explaining the principle of light deflection in the light deflection element according to the embodiment of the present invention. Here, in FIG. 3A, a voltage is applied from the AC power source 21 between the upper electrode 14 and the lower electrode 15 (shown here is a case where only the first upper electrode 14-1 is applied), and incident light is deflected. FIG. 3B is a plan view of the first upper electrode 14-1 and the second upper electrode 14-2.

  As shown in FIG. 3A, when a voltage signal is applied from the external AC power source 21 to the optical deflection element, an electric field parallel to the polarization axis of the electro-optic material (electric field in the Z-axis direction) is formed, and the electro-optic effect The refractive index changes. When the refractive index change due to the Pockels effect is used, the refractive index change amount Δn of the electro-optic material is given by the following formula [1].

Δn = − (1/2) n ³ r _ij (V / d) Formula [1]

In equation [1], n is the refractive index of the electro-optic material, r _ij is the electro-optic constant, V is the applied voltage, and d is the thickness of the electro-optic material. This equation is formed when the polarization P and the external electric field E are in the same direction, that is, when E is in the + Z direction. On the other hand, when the external electric field E is in the −Z direction, the sign of the formula [1] is inverted.

  As described above, the first upper electrode 14-1, the second upper electrode 14-2, the first lower electrode 15-1, and the second lower electrode 15-2 are formed of a plurality of triangular portions (hereinafter referred to as “parts”). , A triangular electrode). When a voltage is applied between the upper and lower electrodes, the refractive index change region also has a triangular shape, and functions as a prism for the light beams that propagate through each region. For this reason, the propagating light beam is given a deflection angle in the X-axis direction as it propagates through the refractive index changing region. Since the deflection angle of the emitted light is the sum of the refraction angles at the respective triangular electrodes, the deflection angle of the emitted light can be increased.

  As will be described later, the voltage applied between the first upper electrode 14-1 and the first lower electrode 15-1, the second upper electrode 14-2 and the second lower electrode 15-2 Since the polarity is opposite to the voltage applied between them, the polarization angle equivalent to the structure in which the polarization direction is alternately reversed along the optical path can be obtained without forming the domain-inverted region in the core layer 11. it can.

  The deflection angle θ given to the emitted light is proportional to the amount of change in the refractive index of the electro-optic material, and is expressed by the following equation [2].

  θ = Δn (L / D) (2)

  In Expression [2], L is the length of the entire plurality of prism portions, and D is the prism width (length in the X-axis direction). In this embodiment, L is the length of the portion where the triangular electrode and the one-dot chain line overlap in FIG. 3B, and D is the height of the triangle in the triangular electrode.

  From equations [1] and [2], it can be seen that the deflection angle emitted from the optical deflection element is proportional to the applied voltage. Therefore, by controlling the voltage applied to the light deflection element, the light beam can be emitted at an arbitrary deflection angle.

  In the present embodiment, the light deflection element is manufactured in a size of X: 10 mm, Y: 24 mm, and Z: 0.3 mm, and a light beam (He—Ne 633 nm) is emitted from the X as shown by a one-dot chain line in FIG. Incident at the center position in the axial direction. This light deflection element is configured such that the transmission part of the triangular electrode at the center position in the X-axis direction (the part where the alternate long and short dash line and the triangle overlap in FIG. 3B) is equally spaced.

<Mechanism of beam distortion>
FIG. 4 is a schematic diagram for explaining a physical mechanism in which beam distortion occurs in the electro-optic element, and FIG. 5 is a diagram for explaining a physical mechanism in which beam distortion occurs in the electro-optic element by an energy band. is there.

  As shown in FIGS. 4 and 5, when a visible laser beam 101 is incident on an electro-optic crystal 100 typified by lithium niobate, photoexcited carriers are generated in a light irradiation portion in the crystal ((i in FIG. 5). )). This is because an impurity level 34 is formed between the conduction band 31 and the valence band 32 in the energy band due to lattice defects and impurities such as Fe and Cr in the crystal, and carriers are introduced into the impurity state by photoexcitation. This is because the transition from the position 34 to the conduction band 31 occurs.

  Further, when a voltage of an external DC power supply 104 is applied to the upper and lower electrodes 102 and 103 of the electro-optic crystal 100, photoexcited carriers drift inside the electro-optic crystal 100 by an external electric field formed by this voltage (FIG. 5). (Ii)). The drifted carriers are trapped by the trap level 33 formed in the crystal ((iii) in FIG. 5). As a result, the impurity level 34 that has contributed to photoexcitation functions as an ionization donor, and forms a local space charge electric field Ei with the electrons that are carriers ((iv) in FIG. 5).

  The electro-optic effect generated from the local space charge electric field Ei is considered to give an uneven distribution to the refractive index in the refractive index modulation region to which a voltage is applied. This non-uniform refractive index distribution causes beam distortion.

<Suppression of beam distortion>
Next, means for suppressing beam distortion in the optical deflection element according to the embodiment of the present invention will be described.

  FIG. 6 is a diagram showing a waveform of a voltage applied to the optical deflection element according to the embodiment of the present invention. Here, FIG. 6A shows a waveform of an alternating voltage, a waveform 41 is a voltage applied between the first upper electrode 14-1 and the first lower electrode 15-1, and a waveform 42 is a second waveform. The voltage applied between the upper electrode 14-2 and the second lower electrode 15-2. FIG. 6B shows a waveform of a DC voltage, a waveform 43 is a voltage applied between the first upper electrode 14-1 and the first lower electrode 15-1, and a waveform 44 is a second voltage. This voltage is applied between the upper electrode 14-2 and the second lower electrode 15-2.

  That is, the voltage applied between the first upper electrode 14-1 and the first lower electrode 15-1, the second upper electrode 14-2, and the second voltage both when applying the AC voltage and when applying the DC voltage. The voltage applied between the lower electrode 15-2 and the lower electrode 15-2 has the same amplitude and the opposite polarity.

  The principle of suppressing the beam distortion by applying the reverse polarity voltage will be described with reference to FIG. Here, FIG. 7A shows that the voltage level applied between the first upper electrode 14-1 and the first lower electrode 15-1 is −Va (Va ≠ 0), and the second upper electrode FIG. 7B is a diagram showing that the level of the voltage applied between the electrode 14-2 and the second lower electrode 15-2 is Va, and FIG. 7B is a cross-sectional view taken along the line FF ′ in FIG. It is sectional drawing cut | disconnected perpendicularly to XY plane in the center position of a X-axis direction.

  When the voltage having the polarity shown in FIG. 7A is applied, as shown in FIG. 7B, the first lower electrode 15-1 is interposed between the first upper electrode 14-1 and the first lower electrode 15-1. An electric field (electric field in the + Z direction) E1 from the first upper electrode 14-1 to the second upper electrode 14-2 is formed between the second upper electrode 14-2 and the second lower electrode 15-2. An electric field (an electric field in the −Z direction) E <b> 2 from 14-2 toward the second lower electrode 15-2 is formed.

  Since the electric field E1 and the electric field E2 are alternately formed in the Y-axis direction, the drifting photoexcited carriers can be electrically neutralized to suppress the formation of the space charge electric field Ei (FIG. 5). It becomes possible to operate as an optical deflection element having an optical deflection function with suppressed beam distortion.

  DESCRIPTION OF SYMBOLS 10 ... Optical waveguide, 14 ... Upper electrode, 14-1 ... 1st upper electrode, 14-2 ... 2nd upper electrode, 15 ... Lower electrode, 15-1 ... 1st lower electrode, 15-2 ... 1st 2 lower electrodes.

Applied.Physics.Letters.44 (9), pp.847-849,1May 1984

Claims (5)

An optical waveguide having a core layer made of an electro-optic material;
A first electrode pair disposed on a pair of surfaces of the optical waveguide sandwiching the core layer, to which a predetermined voltage is applied;
A second electrode pair disposed on the pair of surfaces, to which a voltage having a polarity opposite to a voltage applied to the first electrode pair is applied;
The electrodes constituting the first electrode pair have a plurality of protrusions extending toward the electrodes constituting the second electrode pair on the same surface,
The electrodes constituting the second electrode pair have a plurality of protrusions extending toward the first electrode pair on the same surface,
On the same surface, each of the plurality of protrusions included in the first electrode pair and each of the plurality of protrusions included in the second electrode pair are alternately arranged in the light incident direction. And
Each of said plurality of projections, Ri trapezoidal der,
The distance between the electrode constituting the first electrode pair and the electrode constituting the second electrode pair on the same surface is larger than the thickness of the core layer;
Optical deflection element. The light deflection element according to claim 1,
The optical deflection elements in which the electrodes constituting the first electrode pair and the electrodes constituting the second electrode pair have the same shape and are arranged at the same two-dimensional position. In the light deflection element according to claim 1 or 2,
An optical deflection element in which an interval between the pair of surfaces of the electrode constituting the first electrode pair and the electrode constituting the second electrode pair is larger than the thickness of the core layer. In the optical deflection element given in any 1 paragraph of Claims 1 thru / or 3,
The electro-optic material is an optical deflection element that is one lithium compound selected from the group consisting of lithium niobate, magnesium oxide-added lithium niobate, and lithium tantalate. In the light polarizing element according to any one of claims 1 to 4,
A support substrate that supports the other of the pair of surfaces;
A light polarizing element in which a thermal expansion coefficient of the support substrate is equal to a thermal expansion coefficient of the core layer.

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