Patent Publication Number: US-2023139572-A1

Title: Optical scanning device and method of driving micromirror device

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation application of International Application No. PCT/JP2021/025840, filed Jul. 8, 2021, the disclosure of which is incorporated herein by reference in its entirety. Further, this application claims priority from Japanese Patent Application No. 2020-122263 filed on Jul. 16, 2020, the disclosure of which is incorporated herein by reference in its entirety. 
    
    
     BACKGROUND 
     1. Technical Field 
     The technique of the present disclosure relates to an optical scanning device and a method of driving a micromirror device. 
     2. Description of the Related Art 
     A micromirror device (also referred to as a microscanner) is known as one of micro electro mechanical systems (MEMS) devices manufactured using the silicon (Si) microfabrication technique. The micromirror device is driven by a driving controller provided in an optical scanning device. By driving a mirror portion of the micromirror device, the driving controller two-dimensionally scans an object with a light beam reflected by the mirror portion. 
     An optical scanning method using the micromirror device is superior to an optical scanning method using a polygon mirror in the related art in terms of small size, light weight, and low power consumption. Therefore, application of the micromirror device to a light detection and ranging (LiDAR) device, a scanning beam display, and the like is attracting attention. 
     Examples of a drive method of the micromirror device include an electrostatic drive method, an electromagnetic drive method, and a piezoelectric drive method. In the piezoelectric drive method, the device has a small size and has a large scan angle because a device structure and a drive circuit are simple while torque is large. The micromirror device resonates at a natural vibration frequency determined by a mass, structure, and spring constant. By driving the micromirror device at the resonance frequency, a larger scan angle is obtained. The scan angle corresponds to a deflection angle of the mirror portion. 
     WO2018/230065A proposes a piezoelectric biaxial drive type micromirror device that enables precession of a mirror portion. The precession is a motion in which a central axis orthogonal to a reflecting surface of the mirror portion is deflected such that a circle is drawn. In order to cause the mirror portion to perform precession, it is necessary to allow the mirror portion to swing around each of a first axis and a second axis orthogonal to each other at the same frequency. Therefore, WO2018/230065A proposes matching a resonance frequency around the first axis (hereinafter, referred to as a first resonance frequency) with a resonance frequency around the second axis (hereinafter, referred to as a second resonance frequency). 
     By the precession of the mirror portion, an object to be scanned is scanned with a light beam reflected by the mirror portion such that a circle is drawn. The circular light beam is used, for example, in the LiDAR device. 
     JP2019-144497A discloses that crosstalk occurs between a first actuator that resonantly driving a mirror portion around a first axis and a second actuator that resonantly driving the mirror portion around a second axis in a piezoelectric biaxial drive type micromirror device. This crosstalk is caused by the fact that propagation of vibration generated in one actuator to the other actuator excites resonance vibration. 
     SUMMARY 
     As disclosed in WO2018/230065A, by matching the first resonance frequency with the second resonance frequency and matching frequencies of driving signals provided to first and second actuators with the first resonance frequency and the second resonance frequency, responsiveness of an operation of the mirror portion to the driving signals is improved. However, in a case where the first resonance frequency is matched with the second resonance frequency, it is considered that there is an adverse effect that the crosstalk disclosed in JP2019-144497A increases. 
     The present applicant has confirmed that crosstalk is likely to occur in a micromirror device in which the first actuator allows the mirror portion to swing around the first axis and the second actuator allows the first actuator to swing around the second axis together with the mirror portion. In a case where the first resonance frequency is matched with the second resonance frequency, an influence of the crosstalk is greatly exerted, particularly in a case where the deflection angle of the mirror portion is small, that is, in a case where the driving signal is small. 
     In order to cause the mirror portion to perform precession, it is necessary to accurately match a deflection angle around the first axis with a deflection angle around the second axis. However, in a case where the above-mentioned crosstalk occurs, it is difficult to set the driving signal, and the precession of the mirror portion is disturbed. 
     An object of the technique of the present disclosure is to provide an optical scanning device which can reduce crosstalk and improve an accuracy of precession of a mirror portion, and a method of driving a micromirror device. 
     In order to achieve the above-described object, an optical scanning device of the present disclosure comprises: a micromirror device including a mirror portion that has a reflecting surface for reflecting incident light, a first support portion that swingably supports the mirror portion around a first axis located in a plane including the reflecting surface in a case where the mirror portion is stationary, a first actuator that is connected to the mirror portion via the first support portion and allows the mirror portion to swing around the first axis, a second support portion that swingably supports the mirror portion around a second axis orthogonal to the first axis in the plane, and a second actuator that is connected to the first actuator via the second support portion and allows the mirror portion to swing around the second axis; and a processor that causes the mirror portion to perform precession by providing a first driving signal and a second driving signal each having the same driving frequency to the first actuator and the second actuator, respectively. In the micromirror device, in a case where a resonance frequency around the first axis is denoted by f 1  and a resonance frequency around the second axis is denoted by f 2 , a relationship of f 1 &lt;f 2  is satisfied, in a case where the mirror portion is driven around the first axis and the second axis simultaneously, the resonance frequency around the first axis changes by Δf from f 1 , and in a case where the driving frequency is denoted by f d , a relationship of f 1 −Δf&lt;f d  is satisfied. 
     It is preferable that a relationship of f 1 −Δf&lt;f d &lt;f 2  is satisfied. 
     It is preferable that a relationship of Δf&gt;0 is satisfied. 
     It is preferable that a relationship of f 1 −Δf&lt;f 2 &lt;1.008(f 1 −Δf) is satisfied. 
     It is preferable that the first actuator and the second actuator are piezoelectric actuators each including a piezoelectric element. 
     It is preferable that each of the first support portion and the second support portion is a torsion bar. 
     It is preferable that the optical scanning device further comprises a light source that emits a light beam perpendicularly to the reflecting surface in a case where the mirror portion is stationary. 
     A method of driving a micromirror device of the present disclosure is a method of driving a micromirror device including a mirror portion that has a reflecting surface for reflecting incident light, a first support portion that swingably supports the mirror portion around a first axis located in a plane including the reflecting surface in a case where the mirror portion is stationary, a first actuator that is connected to the mirror portion via the first support portion and allows the mirror portion to swing around the first axis, a second support portion that swingably supports the mirror portion around a second axis orthogonal to the first axis in the plane, and a second actuator that is connected to the first actuator via the second support portion and allows the mirror portion to swing around the second axis. In the micromirror device, in a case where a resonance frequency around the first axis is denoted by f 1  and a resonance frequency around the second axis is denoted by f 2 , a relationship of f 1 &lt;f 2  is satisfied, in a case where the mirror portion is driven around the first axis and the second axis simultaneously, the resonance frequency around the first axis changes by Δf from f 1 , and a first driving signal and a second driving signal each having a driving frequency f d  satisfying a relationship of f 1 −Δf&lt;f d  are provided to the first actuator and the second actuator, respectively, to cause the mirror portion to perform precession. 
     According to the technique of the present disclosure, it is possible to provide an optical scanning device which can reduce crosstalk and improve an accuracy of precession of a mirror portion, and a method of driving a micromirror device. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Exemplary embodiments according to the technique of the present disclosure will be described in detail based on the following figures, wherein: 
         FIG.  1    is a schematic view of an optical scanning device, 
         FIG.  2    is a block diagram showing an example of a hardware configuration of a driving controller, 
         FIG.  3    is an external perspective view of a micromirror device, 
         FIG.  4    is a plan view of the micromirror device as viewed from a light incident side, 
         FIG.  5    is a cross-sectional view taken along the line A-A of  FIG.  4   , 
         FIG.  6    is a cross-sectional view taken along the line B-B of  FIG.  4   , 
         FIG.  7    is a diagram showing an example in which a first actuator is driven in an anti-phase resonance mode, 
         FIG.  8    is a diagram showing an example in which a second actuator is driven in an anti-phase resonance mode, 
         FIG.  9    is a diagram showing an example of a driving signal provided to the first actuator and the second actuator, 
         FIG.  10    is a diagram illustrating a time change of a maximum deflection angle, 
         FIG.  11    is a diagram illustrating precession of a mirror portion, 
         FIG.  12    is a diagram schematically showing a relationship between a deflection angle and a driving frequency, 
         FIG.  13    is a diagram showing a measurement result of a change in a shift amount with respect to a maximum deflection angle, 
         FIG.  14    is a diagram illustrating a relationship between a first deflection angle and a second maximum deflection angle and an amplitude voltage, 
         FIG.  15    is a diagram showing a measurement result of a maximum deflection angle in a case where a driving frequency f d  is set in a range of f d &lt;f 1 −Δf, 
         FIG.  16    is a diagram showing a measurement result of a maximum deflection angle in a case where the driving frequency f d  is set in a range of f 1 −Δf&lt;f d &lt;f 1 , 
         FIG.  17    is a diagram showing a measurement result of a maximum deflection angle in a case where the driving frequency f d  is set in a range of f 1 &lt;f d &lt;f 2 , 
         FIG.  18    is a diagram showing a measurement result of a maximum deflection angle in a case where the driving frequency f d  is set in a range of f 2 &lt;f d , and 
         FIG.  19    is a diagram showing a measurement result of a relationship between a maximum deflection angle and a driving frequency. 
     
    
    
     DETAILED DESCRIPTION 
     An example of an embodiment relating to the technology of the present disclosure will be described with reference to the accompanying drawings. 
       FIG.  1    schematically shows an optical scanning device  10  according to an embodiment. The optical scanning device  10  includes a micromirror device (hereinafter, referred to as micromirror device (MMD))  2 , a light source  3 , and a driving controller  4 . The optical scanning device  10  optically scans a surface to be scanned  5  by reflecting a light beam L emitted from the light source  3  by the MMD  2  under the control of the driving controller  4 . The surface to be scanned  5  is, for example, a screen. 
     The MMD  2  is a piezoelectric biaxial drive type micromirror device capable of allowing a mirror portion  20  (see  FIG.  3   ) to swing around a first axis a 1  and a second axis a 2  orthogonal to the first axis a 1 . Hereinafter, the direction parallel to the first axis a 1  is referred to as an X direction, the direction parallel to the second axis a 2  is a Y direction, and the direction orthogonal to the first axis a 1  and the second axis a 2  is referred to as a Z direction. 
     The light source  3  is a laser device that emits, for example, laser light as the light beam L. It is preferable that the light source  3  emits the light beam L perpendicularly to a reflecting surface  20 A (see  FIG.  3   ) included in the mirror portion  20  in a state where the mirror portion  20  of the MMD  2  is stationary. 
     The driving controller  4  outputs a driving signal to the light source  3  and the MMD  2  based on optical scanning information. The light source  3  generates the light beam L based on the input driving signal and emits the light beam L to the MMD  2 . The MMD  2  allows the mirror portion  20  to swing around the first axis a 1  and the second axis a 2  based on the input driving signal. 
     As will be described in detail below, the driving controller  4  causes the mirror portion  20  to perform precession. By the precession of the mirror portion  20 , the surface to be scanned  5  is scanned with the light beam L reflected by the mirror portion  20  such that a circle is drawn on the surface to be scanned  5 . The circular light beam L is used, for example, in the LiDAR device. 
       FIG.  2    shows an example of a hardware configuration of the driving controller  4 . The driving controller  4  has a central processing unit (CPU)  40 , a read only memory (ROM)  41 , a random access memory (RAM)  42 , a light source driver  43 , and an MMD driver  44 . The CPU  40  is an arithmetic unit that realizes the entire function of the driving controller  4  by reading out a program and data from a storage device such as the ROM  41  into the RAM  42  and executing processing. The CPU  40  is an example of a “processor” according to the technology of the present disclosure. 
     The ROM  41  is a non-volatile storage device and stores a program for the CPU  40  to execute processing and data such as the optical scanning information described above. The RAM  42  is a non-volatile storage device that temporarily holds a program and data. 
     The light source driver  43  is an electric circuit that outputs a driving signal to the light source  3  under the control of the CPU  40 . In the light source driver  43 , the driving signal is a driving voltage for controlling the irradiation timing and the irradiation intensity of the light source  3 . 
     The MMD driver  44  is an electric circuit that outputs a driving signal to the MMD  2  under the control of the CPU  40 . In the MMD driver  44 , the driving signal is a driving voltage for controlling the timing, cycle, and deflection angle for allowing the mirror portion  20  of the MMD  2  to swing. 
     The CPU  40  controls the light source driver  43  and the MMD driver  44  based on the optical scanning information. The optical scanning information is information for indicating how the surface to be scanned  5  is scanned with the light beam L. In the present embodiment, the optical scanning information is information for indicating that the surface to be scanned  5  is scanned with the light beam L such that a circle is drawn on the surface to be scanned  5 . For example, in a case where the optical scanning device  10  is incorporated in the LiDAR device, the optical scanning information includes a time at which the light beam L for distance measurement is emitted, an irradiation range, and the like. 
     Next, an example of the MMD  2  will be described with reference to  FIGS.  3  to  6   .  FIG.  3    is an external perspective view of the MMD  2 .  FIG.  4    is a plan view of the MMD  2  as viewed from the light incident side.  FIG.  5    is a cross-sectional view taken along the line A-A in  FIG.  4   .  FIG.  6    is a cross-sectional view taken along the line B-B in  FIG.  4   . 
     As shown in  FIGS.  3  and  4   , the MMD  2  has a mirror portion  20 , a first actuator  21 , a second actuator  22 , a support frame  23 , a first support portion  24 , a second support portion  25 , and a fixed portion  26 . The MMD  2  is a so-called MEMS device. 
     The mirror portion  20  has a reflecting surface  20 A for reflecting incident light. The reflecting surface  20 A is formed of a metal thin film such as gold (Au) and aluminum (Al) provided on one surface of the mirror portion  20 . The reflecting surface  20 A is, for example, circular. 
     The first actuator  21  is disposed to surround the mirror portion  20 . The support frame  23  is disposed to surround the mirror portion  20  and the first actuator  21 . The second actuator  22  is disposed to surround the mirror portion  20 , the first actuator  21 , and the support frame  23 . The support frame  23  is not an essential component of the technology of the present disclosure. 
     The first support portion  24  connects the mirror portion  20  and the first actuator  21  on the first axis a 1 , and swingably supports the mirror portion  20  around the first axis a 1 . The first axis a 1  is located in a plane including the reflecting surface  20 A in a case where the mirror portion  20  is stationary. For example, the first support portion  24  is a torsion bar stretched along the first axis a 1 . In addition, the first support portion  24  is connected to the support frame  23  on the first axis a 1 . 
     The second support portion  25  connects the first actuator  21  and the second actuator  22  on the second axis a 2 , and swingably supports the mirror portion  20  and the first actuator  21  around the second axis a z . The second axis a z  is orthogonal to the first axis a 1  in the plane including the reflecting surface  20 A in a case where the mirror portion  20  is stationary. The second support portion  25  is connected to the support frame  23  and the fixed portion  26  on the second axis a 2 . 
     The fixed portion  26  is connected to the second actuator  22  by the second support portion  25 . The fixed portion  26  has a rectangular outer shape and surrounds the second actuator  22 . Lengths of the fixed portion  26  in the X direction and the Y direction are, for example, about 1 mm to 10 mm, respectively. A thickness of the fixed portion  26  in the Z direction is, for example, about 5 μm to 0.2 mm. 
     The first actuator  21  and the second actuator  22  are piezoelectric actuators each comprising a piezoelectric element. The first actuator  21  applies rotational torque around the first axis a 1  to the mirror portion  20 . The second actuator  22  applies rotational torque around the second axis a 2  to the mirror portion  20  and the first actuator  21 . Thereby, the mirror portion  20  swings around the first axis a 1  and the second axis a 2 . 
     The first actuator  21  is an annular thin plate member that surrounds the mirror portion  20  in an XY plane. The first actuator  21  is composed of a pair of a first movable portion  21 A and a second movable portion  21 B. Each of the first movable portion  21 A and the second movable portion  21 B is semi-annular. The first movable portion  21 A and the second movable portion  21 B have a shape that is line-symmetrical with respect to the first axis a 1 , and are connected on the first axis a 1 . 
     The support frame  23  is an annular thin plate member that surrounds the mirror portion  20  and the first actuator  21  in the XY plane. 
     The second actuator  22  is an annular thin plate member that surrounds the mirror portion  20 , the first actuator  21 , and the support frame  23  in the XY plane. The second actuator  22  is composed of a pair of a first movable portion  22 A and a second movable portion  22 B. Each of the first movable portion  22 A and the second movable portion  22 B is semi-annular. The first movable portion  22 A and the second movable portion  22 B have a shape that is line-symmetrical with respect to the second axis a 2 , and are connected on the second axis a 2 . 
     In the first actuator  21 , the first movable portion  21 A and the second movable portion  21 B are provided with a piezoelectric element  27 A and a piezoelectric element  27 B, respectively. In addition, in the second actuator  22 , the first movable portion  22 A and the second movable portion  22 B are provided with a piezoelectric element  28 A and a piezoelectric element  28 B, respectively. 
     In  FIGS.  3  and  4   , a wiring line and an electrode pad for providing the driving signal to the piezoelectric elements  27 A,  27 B,  28 A, and  28 B are not shown. A plurality of the electrode pads are provided on the fixed portion  26 . 
     As shown in  FIGS.  5  and  6   , the MMD  2  is formed, for example, by performing an etching treatment on a silicon on insulator (SOI) substrate  30 . The SOI substrate  30  is a substrate in which a silicon oxide layer  32  is provided on a first silicon active layer  31  made of single crystal silicon, and a second silicon active layer  33  made of single crystal silicon is provided on the silicon oxide layer  32 . 
     The mirror portion  20 , the first actuator  21 , the second actuator  22 , the support frame  23 , the first support portion  24 , and the second support portion  25  are formed of the second silicon active layer  33  remaining by removing the first silicon active layer  31  and the silicon oxide layer  32  from the SOI substrate  30  by an etching treatment. The second silicon active layer  33  functions as an elastic portion having elasticity. The fixed portion  26  is formed of three layers of the first silicon active layer  31 , the silicon oxide layer  32 , and the second silicon active layer  33 . 
     The piezoelectric elements  27 A,  27 B,  28 A, and  28 B have a laminated structure in which a lower electrode  51 , a piezoelectric film  52 , and an upper electrode  53  are sequentially laminated on the second silicon active layer  33 . An insulating film is provided on the upper electrode  53 , but is not shown. 
     The upper electrode  53  and the lower electrode  51  are formed of, for example, gold (Au) or platinum (Pt). The piezoelectric film  52  is formed of, for example, lead zirconate titanate (PZT), which is a piezoelectric material. The upper electrode  53  and the lower electrode  51  are electrically connected to the driving controller  4  described above via the wiring line and the electrode pad. 
     A driving voltage is applied to the upper electrode  53  from the driving controller  4 . The lower electrode  51  is connected to the driving controller  4  via the wiring line and the electrode pad, and a reference potential (for example, a ground potential) is applied thereto. 
     In a case where a positive or negative voltage is applied to the piezoelectric film  52  in the polarization direction, deformation (for example, expansion and contraction) proportional to the applied voltage occurs. That is, the piezoelectric film  52  exerts a so-called inverse piezoelectric effect. The piezoelectric film  52  exerts an inverse piezoelectric effect by applying a driving voltage from the driving controller  4  to the upper electrode  53 , and displaces the first actuator  21  and the second actuator  22 . 
       FIG.  7    shows a state in which the first actuator  21  is driven by extending one of the piezoelectric films  52  of the first movable portion  21 A and the second movable portion  21 B and contracting the other piezoelectric film  52 . In this way, the first movable portion  21 A and the second movable portion  21 B are displaced in opposite directions to each other, whereby the mirror portion  20  rotates around the first axis a 1 . 
       FIG.  7    shows an example in which the first actuator  21  is driven in an anti-phase resonance mode in which the displacement direction of the first movable portion  21 A and the second movable portion  21 B and the rotation direction of the mirror portion  20  are opposite to each other. In  FIG.  7   , the first movable portion  21 A is displaced in the −Z direction and the second movable portion  21 B is displaced in the +Z direction, so that the mirror portion  20  rotates in the +Y direction. The first actuator  21  may be driven in an in-phase resonance mode in which the displacement direction of the first movable portion  21 A and the second movable portion  21 B and the rotation direction of the mirror portion  20  are the same direction. 
     An angle at which a normal line N of the reflecting surface  20 A of the mirror portion  20  is inclined in the YZ plane is called a first deflection angle θ 1 . In a case where the normal line N of the reflecting surface  20 A is inclined in the +Y direction, the first deflection angle θ 1  takes a positive value, and in a case where it is inclined in the −Y direction, the first deflection angle θ 1  takes a negative value. 
     The first deflection angle θ 1  is controlled by the driving signal (hereinafter, referred to as a first driving signal) provided to the first actuator  21  by the driving controller  4 . The first driving signal is, for example, a sinusoidal AC voltage. The first driving signal includes a driving voltage waveform V 1A  (t) applied to the first movable portion  21 A and a driving voltage waveform V 1B  (t) applied to the second movable portion  21 B. The driving voltage waveform V 1A  (t) and the driving voltage waveform V 1B  (t) are in an anti-phase with each other (that is, the phase difference is 180°). 
       FIG.  8    shows an example in which the second actuator  22  is driven in an anti-phase resonance mode in which the displacement direction of the first movable portion  22 A and the second movable portion  22 B and the rotation direction of the mirror portion  20  are opposite to each other. In  FIG.  8   , the first movable portion  22 A is displaced in the −Z direction and the second movable portion  22 B is displaced in the +Z direction, so that the mirror portion  20  rotates in the +X direction. The second actuator  22  may be driven in an in-phase resonance mode in which the displacement direction of the first movable portion  22 A and the second movable portion  22 B and the rotation direction of the mirror portion  20  are the same direction. 
     An angle at which the normal line N of the reflecting surface  20 A of the mirror portion  20  is inclined in the XZ plane is called a second deflection angle θ 2 . In a case where the normal line N of the reflecting surface  20 A is inclined in the +X direction, the second deflection angle θ 2  takes a positive value, and in a case where it is inclined in the −X direction, the second deflection angle θ 2  takes a negative value. 
     The second deflection angle θ 2  is controlled by the driving signal (hereinafter, referred to as a second driving signal) provided to the second actuator  22  by the driving controller  4 . The second driving signal is, for example, a sinusoidal AC voltage. The second driving signal includes a driving voltage waveform V 2A  (t) applied to the first movable portion  22 A and a driving voltage waveform V 2B  (t) applied to the second movable portion  22 B. The driving voltage waveform V 2A  (t) and the driving voltage waveform V 2B  (t) are in an anti-phase with each other (that is, the phase difference is 180°). 
       FIG.  9    shows an example of a driving signal provided to the first actuator  21  and the second actuator  22 . (A) of  FIG.  9    shows the driving voltage waveforms V 1A  (t) and V 1B  (t) included in the first driving signal. (B) of  FIG.  9    shows the driving voltage waveforms V 2A  (t) and V 2B  (t) included in the second driving signal. 
     The driving voltage waveforms V 1A  (t) and V 1B  (t) are represented as follows, respectively. 
         V   1A ( t )= V   off1   +V   1  sin(2π f   d   t )
 
         V   1B ( t )= V   off1   +V   1  sin(2π f   d   t +α)
 
     Here, V 1  is the amplitude voltage. V off1  is the bias voltage. f d  is the driving frequency. t is time. α is the phase difference between the driving voltage waveforms V 1A  (t) and V 1B  (t). In the present embodiment, for example, α=180°. 
     By applying the driving voltage waveforms V 1A  (t) and V 1B  (t) to the first movable portion  21 A and the second movable portion  21 B, the mirror portion  20  swings around the first axis a 1  (see  FIG.  7   ). 
     The driving voltage waveforms V 2A  (t) and V 2B  (t) are represented as follows, respectively. 
         V   2A ( t )= V   off2   +V   2  sin(2π f   d   t +φ)
 
         V   2B ( t )= V   off2   +V   2  sin(2π f   d   t +β+φ)
 
     Here, V 2  is the amplitude voltage. V off2  is the bias voltage. f d  is the driving frequency. t is time. β is the phase difference between the driving voltage waveforms V 2A  (t) and V 2B  (t). In the present embodiment, for example, 0=180°. In addition, φ is the phase difference between the driving voltage waveforms V 1A  (t) and V 1B  (t) and the driving voltage waveforms V 2A  (t) and V 2B  (t). In the present embodiment, φ=90° is set in order to cause the mirror portion  20  to perform precession. 
     The bias voltages V off1  and V off2  are DC voltages for determining a state where the mirror portion  20  is stationary. In a state where the mirror portion  20  is stationary, a plane including the reflecting surface  20 A may not be parallel to an upper surface of the fixed portion  26  and may be inclined with respect to the upper surface of the fixed portion  26 . 
     By applying the driving voltage waveforms V 2A  (t) and V 2B  (t) to the first movable portion  22 A and the second movable portion  22 B, the mirror portion  20  swings around the second axis a 2  (see  FIG.  8   ). 
     As described above, the first driving signal and the second driving signal have the same driving frequency f d  and a phase difference of 90°. In order to cause the mirror portion  20  to perform precession, as shown in  FIG.  10   , it is necessary to set the amplitude voltages V 1  and V 2  appropriately such that the maximum deflection angle θ m1  of the first deflection angle θ 1  matches the maximum deflection angle θ m2  of the second deflection angle θ 2 . This is because a relationship between the amplitude voltage V 1  and the first deflection angle θ 1  and a relationship between the amplitude voltage V 2  and the second deflection angle θ 2  are not the same. In the description of the present specification, the meaning of “match” includes not only the meaning of perfect match but also the meaning of substantial match including allowable errors in design and manufacturing. 
     Hereinafter, the maximum deflection angle θ m1  of the first deflection angle θ 1  is referred to as a first maximum deflection angle θ m1 . The maximum deflection angle θ m2  of the second deflection angle θ 2  is referred to as a second maximum deflection angle θ m2 . Further, in a case where the first maximum deflection angle θ m1  and the second maximum deflection angle θ m2  are not distinguished, it is simply referred to as a maximum deflection angle θ m . 
     In order to cause the mirror portion  20  to perform precession with high accuracy, it is necessary to appropriately set the driving frequency f d .  FIG.  11    shows the precession of the mirror portion  20 . The precession is a motion in which the normal line N of the reflecting surface  20 A of the mirror portion  20  is deflected such that a circle is drawn. By irradiating the mirror portion  20  performing the precession with the light beam L from the light source  3 , the surface to be scanned  5  can be scanned with the light beam L such that a circle is drawn on the surface to be scanned  5 . 
       FIG.  12    schematically shows a relationship between the deflection angle of the mirror portion  20  and the driving frequency f d . The mirror portion  20  vibrates around the first axis a 1  and the second axis a 2  at a natural vibration frequency. The mirror portion  20  resonates in a case where the driving frequency f d  matches the natural vibration frequency. 
     In  FIG.  12   , f 1  represents a resonance frequency (hereinafter, referred to as a first resonance frequency) around the first axis a 1  of the mirror portion  20 . f 2  represents a resonance frequency (hereinafter, referred to as a second resonance frequency) around the second axis a 2  of the mirror portion  20 . The first resonance frequency f 1  is a resonance frequency in a case where the mirror portion  20  swings only around the first axis a 1  without swinging around the second axis a 2 . The second resonance frequency f 2  is a resonance frequency in a case where the mirror portion  20  swings only around the second axis a 2  without swinging around the first axis a 1 . 
     A plurality of resonance modes exist for the swing of the mirror portion  20  in addition to the phase (in-phase or anti-phase) with the movable portion described above. For example, the first resonance frequency f 1  and the second resonance frequency f 2  are in an anti-phase and are resonance frequencies in the lowest order (that is, the lowest frequency) resonance mode. 
     The closer the driving frequency f d  is to the first resonance frequency f 1 , the larger the first deflection angle θ 1  is. In addition, the closer the driving frequency f d  is to the second resonance frequency f 2 , the larger the second deflection angle θ 2  is. Therefore, in general, by matching the first resonance frequency f 1  with the second resonance frequency f 2  and matching the driving frequency f d  with the first resonance frequency f 1  and the second resonance frequency f 2 , the responsiveness of the deflection angle to the driving signal is improved. The first resonance frequency f 1  and the second resonance frequency f 2  can be set by adjusting an inertial moment, a spring constant, and the like of the components of the MMD  2  in terms of design. 
     However, in a case where the first resonance frequency f 1  is matched with the second resonance frequency f 2 , it is considered that there is an adverse effect that the crosstalk increases. The crosstalk is caused by the fact that propagation of vibration generated in one of the first actuator  21  and the second actuator  22  to the other excites resonance vibration. In a case where the first resonance frequency f 1  is matched with the second resonance frequency f 2 , an influence of the crosstalk is greatly exerted, particularly in a case where the maximum deflection angle θ m  of the mirror portion  20  is small, that is, in a case where the driving signal is small. 
     In the MMD  2  of the present embodiment, the first actuator  21  allows the mirror portion  20  to swing around the first axis a 1  and the second actuator  22  allows the first actuator  21  to swing around the second axis a 2  together with the mirror portion  20 . In this way, in a case where the mirror portion  20  is allowed to swing around the second axis a 2 , the first actuator  21  also swings around the second axis a 2 , so that a vibration component around the second axis a 2  propagates to the first axis a 1 , which affects a change in voltage characteristics of the first actuator  21  and the first resonance frequency f 1 . The present applicant confirmed that the first resonance frequency f 1  shifts in a case where the mirror portion  20  swings around the first axis a 1  and around the second axis a 2  simultaneously. Hereinafter, this shift amount will be denoted by Δf. 
     In order to avoid the crosstalk, it is preferable that the first resonance frequency f 1  is not matched with the second resonance frequency f 2 . In addition, a magnitude relationship between the first resonance frequency f 1  and the second resonance frequency f 2  needs to be determined in consideration of the shift amount Δf. This is because the shift amount Δf changes depending on a magnitude of the maximum deflection angle θ m  of the mirror portion  20 . 
       FIG.  13    shows an example of a measurement result of a change in the shift amount Δf with respect to the maximum deflection angle θ m  of the mirror portion  20 . The present applicant measured the shift amount Δf of the first resonance frequency f 1  with respect to the maximum deflection angle θ m  in a case where the surface to be scanned  5  was scanned with the light beam L such that a positive circle is drawn on the surface to be scanned  5  by causing the mirror portion  20  to perform precession. A size of the circle scanned on the surface to be scanned  5  is proportional to the maximum deflection angle θ m . 
     As shown in  FIG.  13   , it was confirmed that the shift amount Δf of the first resonance frequency f 1  increased as the maximum deflection angle θ m  increased. Here, the shift amount Δf was a positive value (that is, Δf&gt;0). Therefore, even in a case where f 1 ≠f 2 , the shifted first resonance frequency f 1 -Δf may be matched with the second resonance frequency f 2  because of a change in the shift amount Δf with a change in the maximum deflection angle θ m . In a case where the shifted first resonance frequency f 1 -Δf is matched with the second resonance frequency f 2 , the crosstalk may occur. 
     For example, in a case where a relationship of f 1 &gt;f 2  is satisfied, f 1 -Δf=f 2  and the crosstalk occurs because of the change in the shift amount Δf with the change of the maximum deflection angle θ m . On the other hand, in a case where a relationship of f 1 &lt;f 2  is satisfied, a relationship of f 1 -Δf&lt;f 2  is always be satisfied instead of f 1 -Δf=f 2 , even though the shift amount Δf changes with the change of the maximum deflection angle θ m . 
     From the above, the present applicant found that the crosstalk is reduced by setting the driving frequency f d  so as to satisfy a relationship of f 1 &lt;f 2  and a relationship of f 1 -Δf&lt;f d . 
     For example, in a case where the optical scanning device  10  is applied to the LiDAR device, a radius of a circle scanned on the surface to be scanned  5  by the light beam L (hereinafter, referred to as a scanning radius) is controlled. This scanning radius corresponds to the maximum deflection angle θ m  of the mirror portion  20 . The first maximum deflection angle θ m1  depends on the amplitude voltage V 1  of the first driving signal. The second maximum deflection angle θ m2  depends on the amplitude voltage V 2  of the second driving signal. Therefore, in order to control the scanning radius by the driving controller  4 , it is preferable that a relationship between the first maximum deflection angle θ m1  and the amplitude voltage V 1  and a relationship between the second maximum deflection angle θ m2  and the amplitude voltage V 2  are each represented by a single function. 
       FIG.  14    shows an example in which the relationship between the first maximum deflection angle θ m1  and the amplitude voltage V 1  is represented by a logarithmic function, and the relationship between the second maximum deflection angle θ m2  and the amplitude voltage V 2  is represented by a linear function. In this case, in order to determine the amplitude voltages V 1  and V 2  satisfying θ m1 =θ m2 , the amplitude voltage V 1  need only be determined based on a logarithmic function, and the amplitude voltage V 2  need only be determined based on a linear function. 
     In a case where the above-described crosstalk occurs, the relationship between the first maximum deflection angle θ m1  and the amplitude voltage V 1  is disturbed in a region where the maximum deflection angle θ m  of the mirror portion  20  is small, and the driving control becomes difficult. The present applicant confirmed that, in a case where f 1 -Δf&lt;f d  is satisfied, the relationship between the first maximum deflection angle θ m1  and the amplitude voltage V 1  is represented by a single function even in a region where the maximum deflection angle θ m  of the mirror portion  20  is small, and that the drive control of the MMD  2  becomes easy. 
     The measurement results of the maximum deflection angle θ m  of the mirror portion  20  and the amplitude voltages V 1  and V 2  are shown below. The present applicant measured the maximum deflection angle θ m  for each of the following four cases: a case where the driving frequency f d  is set in a range of f d &lt;f 1 -Δf; a case where the driving frequency f d  is set in a range of f 1 -Δf&lt;f d &lt;f 1 ; a case where the driving frequency f d  is set in a range of f 1 &lt;f d &lt;f 2 ; and a case where the driving frequency f d  is set in a range of f 2 &lt;f d . 
     Specifically, the present applicant measured a relationship between the amplitude voltages V 1  and V 2  with respect to the radius of the circle (scanning radius) in a case where the surface to be scanned  5  was scanned with the light beam L such that a positive circle is drawn on the surface to be scanned  5  by causing the mirror portion  20  to perform precession. The surface to be scanned  5  was provided with gradations at intervals of 1 mm, and the maximum deflection angle θ m  was measured based on a measured value of a shape and size of the circle using the gradations. The MMD  2  used in this measurement has f 1 =1228 Hz and f 2 =1237 Hz, and satisfies the relationship of f 1 &lt;f 2 . 
       FIG.  15    shows a measurement result of the maximum deflection angle θ m  in a case where the driving frequency f d  is set in a range of f d &lt;f 1 -Δf. In  FIG.  15   , a relationship between the first maximum deflection angle θ m1  and the amplitude voltage V 1  is different in each region of a first region of 0≤V 1 &lt;3 V, a second region of 3 V≤V 1 &lt;4.2 V, and a third region of 4.2 V≤V 1 &lt;9.2 V. The first region has a relationship of θ m1 =0.8 V 1 , the second region has a relationship of θ m1 =3.0 V 1 , and the third region has a relationship of θ m1 =0.2 V 1 . In this way, in a case where f d &lt;f 1 -Δf, the voltage characteristics differ for each region, and the relationship between the first maximum deflection angle θ m1  and the amplitude voltage V 1  cannot be represented by a single function, so that the drive control of the MMD  2  is difficult. 
       FIG.  16    shows a measurement result of the maximum deflection angle θ m  in a case where the driving frequency f d  is set in a range of f 1 -Δf&lt;f d &lt;f 1 . In  FIG.  16   , the first maximum deflection angle θ m1  and the amplitude voltage V 1  have a relationship of θ m1 =2.2 ln (V 1 )+1.7 and can be represented by a single function (logarithmic function). 
       FIG.  17    shows a measurement result of the maximum deflection angle θ m  in a case where the driving frequency f d  is set in a range of f 1 &lt;f d &lt;f 2 . In  FIG.  17   , the first maximum deflection angle θ m1  and the amplitude voltage V 1  have a relationship of θ m1 =1.8 ln (V 1 )+1.7 and can be represented by a single function (logarithmic function). 
       FIG.  18    shows a measurement result of the maximum deflection angle θ m  in a case where the driving frequency f d  is set in a range of f 2 &lt;f d . In  FIG.  18   , the first maximum deflection angle θ m1  and the amplitude voltage V 1  have a relationship of θ m1 =2.8 ln (V 1 )−2.9 and can be represented by a single function (logarithmic function). 
     In any case of  FIGS.  15  to  18   , the relationship between the second maximum deflection angle θ m2  and the amplitude voltage V 2  can be represented by a single function (linear function). 
     As described above, in a case where f 1 -Δf&lt;f d  is satisfied, the relationship between the first maximum deflection angle θ m1  and the amplitude voltage V 1  and the relationship between the second maximum deflection angle θ m2  and the amplitude voltage V 2  are represented by a single function. Since the driving controller  4  can determine the amplitude voltages V 1  and V 2  based on a single function, the driving control of the MMD  2  can be easily performed. 
     As described above, in a case where f 1 -Δf&lt;f d  is satisfied, the driving control of the MMD  2  can be easily performed. However, in a case where the driving frequency f d  satisfies a relationship of f 2 &lt;f d , the responsiveness of the first maximum deflection angle θ m1  to the amplitude voltage V 1  is reduced. As shown in  FIG.  18   , even in a case where V 1 =50 V, the first maximum deflection angle θ m1  is less than 10°. Therefore, in order to improve the responsiveness of the first maximum deflection angle θ m1  to the amplitude voltage V 1 , it is more preferable that the driving frequency f d  satisfies a relationship of f 1 -Δf&lt;f d &lt;f 2 . As shown in  FIGS.  16  and  17   , in a case where the relationship of f 1 -Δf&lt;f d &lt;f 2  is satisfied, the first maximum deflection angle θ m1  can be set to 10° or more. 
     As described above, the crosstalk is reduced by setting the driving frequency f d  so as to satisfy the relationship of f 1 &lt;f 2  and the relationship of f 1 -Δf&lt;f d &lt;f 2 . However, in a case where the first resonance frequency f 1  and the second resonance frequency f 2  are significantly different from each other, it is difficult to allow the mirror portion  20  to swing around the first axis a 1  and around the second axis a 2  simultaneously with a single driving frequency f d . 
     Therefore, the present applicant examined an upper limit of the second resonance frequency f 2  with respect to the first resonance frequency f 1 . Specifically, the present applicant examined an upper limit of the second resonance frequency f 2  for allowing the first maximum deflection angle θ m1  and the second maximum deflection angle θ m2  to reach 10° within a range of f 1 &lt;f d &lt;f 2 . 
       FIG.  19    shows a measurement result of a relationship between the maximum deflection angle of the mirror portion  20  and the driving frequency f d . In  FIG.  19   , the first maximum deflection angle θ m1  is the maximum value of the first deflection angle θ 1  in a case where V 1 =5 V and the mirror portion  20  is allowed to swing around the first axis a 1 . The second maximum deflection angle θ m2  is the maximum value of the second deflection angle θ 2  in a case where V 2 =5 V and the mirror portion  20  is allowed to swing around the second axis a 2 . The MMD  2  used in this measurement had f 1=1228  Hz and f 2=1237  Hz. 
     In  FIG.  17   , the first maximum deflection angle θ m1  at V 1 =5 V is 4.8°, and the second maximum deflection angle θ m2  at V 2 =5V is 1.8°. That is, in a case where it is possible to satisfy θ m1 ≥4.8° and θ m2 ≥1.8° in one-axis drive, the first maximum deflection angle θ m1  and the second maximum deflection angle θ m2  can be set to 10° or more in two-axis drive by increasing the amplitude voltages V 1  and V 2 . Therefore, in  FIG.  19   , the second resonance frequency f 2  in a case where the driving frequency f d1  on the high frequency side where θ m1 =4.8° and the driving frequency flu on the low frequency side where θ m2 =1.8° are equal to each other is the upper limit of the second resonance frequency f 2 . The driving frequency f d1  has a relationship of f d1 =1.002(f 1 -Δf) in consideration of the shift amount Δf. The driving frequency f d2  has a relationship of f d2 =0.9939f 2 . 
     In a case where 1.002(f 1 -Δf)=0.9939f 2  and this is modified, a relationship of f 2 =1.008(f 1 −Δf). Therefore, the second resonance frequency f 2  that satisfies the relationship of f 2 =1.008(f 1 −Δf) is the upper limit of the second resonance frequency f 2  with respect to the first resonance frequency f 1 . From this, it can be said that it is preferable that the first resonance frequency f 1  and the second resonance frequency f 2  satisfy a relationship of f 1 −Δf&lt;f 2 &lt;1.008(f 1 -Δf). 
     The configuration of the MMD  2  shown in the above embodiment can be changed as appropriate. For example, in the above embodiment, although the first actuator  21  and the second actuator  22  have an annular shape, one or both of the first actuator  21  and the second actuator  22  may have a meander structure. In addition, it is possible to use a support member having a configuration other than a torsion bar as the first support portion  24  and the second support portion  25 . 
     The hardware configuration of the driving controller  4  can be variously modified. A processing unit of the driving controller  4  may be configured of one processor, or may be configured of a combination of two or more processors of the same type or different types (for example, a combination of a plurality of field programmable gate arrays (FPGAs) and/or a combination of a CPU and an FPGA). 
     All documents, patent applications, and technical standards mentioned in this specification are incorporated herein by reference to the same extent as in a case where each document, each patent application, and each technical standard are specifically and individually described by being incorporated by reference.