Patent Publication Number: US-2023163570-A1

Title: Optical device

Description:
TECHNICAL FIELD 
     Embodiments of the present disclosure relate to an optical device. 
     BACKGROUND ART 
     In recent years, research on automatic driving and driving assistance systems for automobiles has progressed, and an in-vehicle radar as a sensing means has attracted attention. 
     In the in-vehicle radar, a frequency modulated continuous wave (FMCW) radar is widely used as a radar that can measure the distance and speed relative to an object, and as a radar that uses a heterodyne method that can detect weak reflected waves. Developments of directionality of electromagnetic wave and miniaturization of an antenna are in progress. 
     In these developments, research on an FMCW light detection and ranging (LiDAR), which aims to improve directionality, reduce size, and reduce power consumption by replacing electromagnetic waves with laser light, is also underway. 
     The light source of such an FMCW LiDAR needs to sweep wavelength with respect to time, and a micro-electromechanical systems (MEMS) tunable laser that changes the wavelength by directly modulating a cavity length is known. 
     For example, Japanese Patent No. 6328112 describes a tunable laser that emits tunable radiation with an output power spectrum over a radiation wavelength band having a central wavelength and an average radiated power. 
     This tunable laser includes a MEMS drive mechanism that includes an optical resonator provided with a first mirror and a second mirror, a gain region between the first mirror and the second mirror, a gap tuning region, and a deformable dielectric membrane mounted on a rigid support structure that is translucent over a wavelength band. The MEMS drive mechanism modulates a gap. 
     The MEMS drive mechanism has a membrane stress value in the range of 100 to 1000 MPa. The frequency response of the MEMS drive mechanism has substantially increased damping by the effect of squeeze film damping controlled by a central plate having a diameter greater than 50 μm and less than an actuator diameter. 
     A free spectral range (FSR) of the optical resonator exceeds 5% of the center wavelength. The tunable laser operates in a substantially single vertical and horizontal mode over the wavelength band. 
     The MEMS drive mechanism has a frequency response of wavelength tuning with a bandwidth of 6 dB over about 1 kHz. 
     CITATION LIST 
     Patent Literature 
     
         
         [PTL 1] 
         Japanese Patent No. 6328112 
       
    
     SUMMARY OF INVENTION 
     Technical Problem 
     Since a conventional MEMS tunable laser uses electrostatic attraction to modulate a cavity length (i.e., distance between two parallel reflecting mirrors), the linearity of a drive voltage and a displacement of the mirrors becomes insufficient to obtain a desired wavelength. Furthermore, since an initial value of the cavity length cannot be accurately adjusted to the interval of an integral multiple of an oscillation wavelength in manufacturing, undesirably, a high voltage has to be applied to a movable portion of the mirror to move the mirrors to obtain the cavity length that satisfies an oscillation condition at the time of oscillation. 
     It is an object of the disclosure to provide an optical device that improves the linearity of an oscillation wavelength and the drive voltage and achieves light having a desired oscillation wavelength. 
     Solution to Problem 
     An optical device according to an embodiment of the disclosed technology includes a first reflector; a second reflector; an elastic support unit supporting the second reflector; a piezoelectric element on the elastic support unit; a light emitter configured to emit light having an oscillation wavelength; and circuitry configured to output a signal to apply drive voltage to the piezoelectric element to elastically deform the elastic support unit. The deformation of the elastic support unit changes a distance between the first reflector and the second reflector to change the oscillation wavelength of the light emitted from the light emitter. 
     An optical device according to another embodiment of the disclosed technology includes a first reflector; a second reflector; at least three elastic support units supporting the second reflector; piezoelectric elements on the at least three elastic support units, respectively; circuitry configured to output drive signals independent of each other to the piezoelectric elements of the at least three elastic support units, respectively, to deform the at least three elastic support units elastically and independently of each other. The deformation of the at least three elastic support units changes plane parallelism between the first reflector and the second reflector. 
     Advantageous Effects of Invention 
     The embodiments of the present disclosure provide an optical device that achieves light with a desired oscillation wavelength by improving the linearity of the oscillation wavelength with respect to the drive voltage. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The accompanying drawings are intended to depict example embodiments of the present invention and should not be interpreted to limit the scope thereof. The accompanying drawings are not to be considered as drawn to scale unless explicitly noted. Also, identical or similar reference numerals designate identical or similar components throughout the several views. 
         FIG.  1    is a diagram of the overall configuration of the tunable laser according to an embodiment. 
         FIG.  2    is a diagram of an example of a detailed structure of a half-VCSEL element. 
         FIG.  3    is a plan view of a first example of a support structure of a second reflector. 
         FIG.  4    is a cross-sectional view taken along the line AA of  FIG.  3   . 
         FIG.  5    is a plan view of a second example of a support structure of a second reflector. 
         FIG.  6    is a perspective view illustrating a driving state of a second reflector in the support structure of  FIG.  5   . 
         FIG.  7    is a side view illustrating a driving state of the second reflector in the support structure of  FIG.  5   . 
         FIG.  8    is a graph of a relation between a drive voltage and a cavity length in the tunable laser in  FIG.  1   . 
         FIG.  9    is a graph of a relation between a drive voltage and a cavity length in a MEMS tunable laser using electrostatic attraction according to a comparative example. 
         FIG.  10    is a plan view of a third example of the support structure of the second reflector. 
         FIG.  11 A  and  FIG.  11 B  are illustrations for describing wavelength sweeping by the tunable laser in  FIG.  1   , according to an embodiment of the present disclosure. 
         FIG.  12    is a graph of the simulation results of a relation between the length of a gap between the half-VCSEL element and the second reflector and the oscillation wavelength. 
         FIG.  13    is a diagram of a tunable laser according to a modification of an embodiment of the present disclosure, corresponding to  FIG.  1   . 
         FIG.  14    is a diagram of a modified example of the tunable laser according to a modification of an embodiment of the present disclosure, corresponding to  FIG.  6   . 
         FIG.  15 A  is a graph of a relation between a temperature and a cavity length in the tunable laser according to an embodiment of the present disclosure. 
         FIG.  15 B  is a graph of the relation between a temperature and a wavelength in the tunable laser according to an embodiment of the present disclosure. 
         FIG.  16    is a plan view of an example of the support structure of a second reflector. 
         FIG.  17    is a cross-sectional view taken along the line AA in  FIG.  16   . 
         FIG.  18    is a side view illustrating a driving state of the second reflector in the support structure of  FIGS.  15  and  16   . 
         FIG.  19    is a graph of a relation between the drive voltage and a cavity length in the tunable laser according to an embodiment of the present disclosure. 
         FIG.  20    is a graph of a relation between a drive voltage and a cavity length in a MEMS tunable laser using electrostatic attraction according to a comparative example. 
         FIG.  21    is a functional block diagram of a configuration for supplying independent drive signals to piezoelectric elements of the three elastic support units. 
         FIG.  22    is an illustration for describing a state accompanied by an operation that applies different voltages to two connecting portions to tilt the reflector. 
         FIG.  23    is a diagram of an example of the relation between the displacement amount of the connecting portion and the tilt obtained when the distance between the connecting ends is changed. 
         FIG.  24 A  and  FIG.  24 B  are illustrations for describing wavelength sweeping by the tunable laser in  FIG.  1   , according to an embodiment of the present disclosure. 
         FIG.  25    is a graph of the simulation results of a relation between the length of the gap between the half-VCSEL element and the second reflector and the oscillation wavelength. 
         FIG.  26    is a diagram of a tunable laser according to a first modification of an embodiment of the present disclosure. 
         FIG.  27    is a diagram of a tunable laser according to a second modification of an embodiment of the present disclosure. 
         FIG.  28 A  is a cross-sectional view of a half-VCSEL element bonded with a movable reflector element with a tilt uncorrected. 
         FIG.  28 B  is a cross-sectional view of a half-VCSEL element bonded with a movable reflector element with a second reflector and a movable reflector structure for high-speed driving. 
         FIG.  29 A  is an illustration for describing adjustment of a tilt of the second reflector. 
         FIG.  29 B  is a graph of signals of voltage to be applied to the piezoelectric element. 
         FIG.  30    is a block diagram of calibration of plane parallelism between the movable reflector element and the half-VCSEL element. 
         FIG.  31    is a graph of the relation between a phase of a drive-signal source during calibration and optical power of the tunable laser according to an embodiment of the present disclosure. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the present invention. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. 
     In describing embodiments illustrated in the drawings, specific terminology is employed for the sake of clarity. However, the disclosure of this specification is not intended to be limited to the specific terminology so selected and it is to be understood that each specific element includes all technical equivalents that have a similar function, operate in a similar manner, and achieve a similar result. 
     An embodiment is described below referring to the drawings. Like reference signs are applied to identical or corresponding components throughout the drawings and redundant description thereof may be omitted. 
     The tunable laser (optical device)  10  according to the present embodiment will be described in detail with reference to  FIGS.  1  to  31   . 
     The X, Y, and Z directions in the following description are based on the arrow line directions drawn in the drawing. Notably, in the present disclosure, a “drive voltage” may be read as a “drive signal.” 
       FIG.  1    is a diagram of the overall configuration of the tunable laser  10  according to the present embodiment. 
     The tunable laser  10  includes a half-vertical-cavity surface emitting laser (VCSEL) element  100  as a light emitter, a movable reflector element  200 , and a bonding layer  300  to maintain or restrict the relative position of the half-VCSEL element  100  and the movable reflector element  200 . 
     The half-VCSEL element  100  and the movable reflector element  200  have a rectangular shape that is spread in the XY plane, and the bonding layer  300  has a rectangular frame shape extending in Z direction. The boding layer  300  connects the lower-end face of the movable reflector element  200  in the Z direction with the upper-end face of the half-VCSEL element  100  in the Z direction. The lower-end face of the movable reflector element  200  and the upper-end face of the half-VCSEL element  100  constitute XY planes of the movable reflector element  200  and the half-VCSEL element  100 , respectively. 
     The half-VCSEL element  100  has a first reflector  400  (a first reflecting mirror), a semiconductor substrate  500 , and an antireflection film  600 , laminated in this order from an upper layer to a lower layer in the Z direction 
     The half-VCSEL element  100  has an electrode  101 , an electrode  102 , and a wiring  103  that electrically connects the electrode  101  and the electrode  102 . 
     The movable reflector element  200  includes a stationary support unit  700  having a rectangular frame shape extending over the XY plane, and a second rectangular reflector  800  (a second reflecting mirror) supported at the center of the XY plane of the stationary support unit  700 . The second reflector  800  is supported by the stationary support unit  700  in such a manner that it can approach and separate from the first reflector  400  from a reference facing position with the first reflector  400  defined by the bonding layer  300 . (Details will be described later). 
     There is a space between the first reflector  400  and the second reflector  800 , and the space is evacuated or filled with gas. 
     An active layer  105  (described later with reference to  FIG.  2   ), which is the light emitting source of the half-VCSEL element  100 , is disposed between the first reflector  400  and the second reflector  800 . 
     By sweeping the wavelength of the light from the half-VCSEL element  100  and changing the distance (i.e., gap) G between the first reflector  400  and the second reflector  800  in the Z direction, the tunable laser  10  changes the oscillation wavelength of light from the half-VCSEL element  100 . 
     Though, in  FIG.  1   , the arrow indicating the distance G between the first reflector  400  and the second reflector  800  extends from the second reflector  800  to slightly above the first reflector  400  for convenience sake, the lower end of the arrow is assumed to be drawn out from the first reflector  400 . 
     The structure for changing the distance G from the first reflector  400  to the second reflector  800  in the Z direction will be described in detail later. 
       FIG.  2    is a diagram of an example of the detailed structure of the half-VCSEL element  100   FIG.  1   . 
     The half-VCSEL element  100  has the semiconductor substrate  500  laminated on the upper surface of the antireflection film  600 . 
     The semiconductor substrate  500  includes a semiconductor substrate such as an n-GaAs substrate. 
     The first reflector  400  is laminated on the upper surface of the semiconductor substrate  500 . 
     The first reflector  400  constitutes a semiconductor multilayer film reflector having a first semiconductor layer  410 , a second semiconductor layer  420 , and a third semiconductor layer  430  laminated in this order from the upper layer of the semiconductor multilayer film reflector to the lower layer. 
     The number and arrangement of the semiconductor layers constituting the first reflector  400  are flexible (not limited to those illustrated here), and various design changes are possible. 
     The semiconductor layers (for example, the first semiconductor layer  410 , the second semiconductor layer  420 , and the third semiconductor layer  430 ) constituting the first reflector  400  includes, for example, a low refractive index layer of n-Al0.9Ga0.1As and a high refractive index layer of n-Al0.2Ga0.8As. 
     In order to reduce the electric resistance, a composition gradient layer in which the composition gradually changes from one composition to the other may be disposed between the semiconductor layers (i.e., refractive index layers) constituting the first reflector  400 . The thickness of the composition gradient layer is, for example, 20 nm. 
     To set the optical film thickness of each semiconductor layer, or each refractive index layer, to λ/4, where λ is an oscillation wavelength, including ½ of the adjacent composition gradient layers is preferable. 
     In this case, when the optical thickness of the layer is λ/4, the actual film thickness D is λ/4n where n is the refractive index of the medium of the layer. 
     On the upper surface of the semiconductor substrate  500 , in addition to the first reflector  400 , or as a part of the components of the first reflector  400 , the electrode  101 , the electrode  102 , the wiring  103 , and a spacer layer  104 , the active layer  105 , a selective oxide layer  106 , a contact layer  107 , an insulating layer  108 , a mesa  109 , a groove  110 , an opening  111 , and an opening  112  are formed. 
     The spacer layer  104  includes, for example, a non-doped AlGaInP layer, and is formed on the boundary layer between the first reflector  400  of the semiconductor multilayer and the active layer  105 . 
     A portion including the spacer layer  104  and the active layer  105  is also referred to as a resonator structure or a resonator region, includes ½ of the adjacent composition gradient layer, and has an optical thickness of one wavelength (λ). 
     Two spacer layers  104  are disposed with an active layer  105  between the two spacer layers  104  in the Z direction. 
     The active layer  105  has, for example, a triple quantum well structure having three quantum well layers and four barrier layers. 
     For example, each quantum well layer is an InGaAs layer and each barrier layer is an AlGaAs layer. 
     The selective oxide layer  106  includes an oxidized region  106 A and a non-oxidized region  106 B. 
     For example, the selective oxide layer  106  composed of p-AlAs with a thickness of 30 nm is disposed the first reflector  400  of the semiconductor multilayer. 
     The selective oxide layer  106  is disposed, for example, within the second pair of the high refractive index layer and the low refractive index layer counted from the spacer layer  104 . 
     The selective oxide layer  106  may be disposed between layers such as the composition gradient layer and, an intermediate layer. In the present embodiment, the selective oxide layer  106  includes layers actually oxidized. 
     The contact layer  107  includes, for example, a p-GaAs layer, and is formed on the first reflector  400  of the semiconductor multilayer. 
     The mesa  109  and the groove  110  are formed by partially etching the first reflector  400  of the semiconductor multilayer (for example, the first semiconductor layer  410 , the second semiconductor layer  420 , and the third semiconductor layer  430 ), the spacer layer  104 , the active layer  105 , and the contact layer  107 . 
     The insulating layer  108  contains, for example, SiN, SiON, or SiO 2 , and covers the mesa  109 . The opening  111  that exposes a part of the contact layer  107  of the mesa  109  is formed in the insulating layer  108 . 
     The opening  111  is formed to overlap the non-oxidized region  106 B of the selective oxide layer  106  in a planar view. 
     An electrode  101  electrically connected to the contact layer  107  through the opening  111  is formed on the insulating layer  108  of the mesa  109 . 
     As the electrode  101 , for example, a laminated film in which titanium (Ti)/platinum (Pt)/gold (Au) are laminated in that order from the insulating layer side can be used. 
     The insulating layer  108  covers the groove  110 . 
     The opening  112  that exposes a part of the semiconductor substrate  500  in the insulating layer  108 . 
     An electrode  102  electrically connected to the contact layer  107  through the opening  112  is formed on the insulating layer  108  of the groove  110 . 
     As the electrode  102 , for example, a laminated film in which germanium alloy (AuGe)/nickel (Ni)/gold (Au) are laminated in order from the semiconductor substrate  500  side can be used. 
     The wiring  103  electrically connects the electrode  101  and the electrode  102 . 
     As the wiring  103 , for example, a laminated film in which titanium (Ti)/platinum (Pt)/gold (Au) are laminated in order from the side of the semiconductor substrate  500  can be used. 
     In the present embodiment, the half-VCSEL element  100  has been described as an example of the “light emitter”. Alternatively, a laser diode (LD) or a light-emitting diode (LED) can be used as the “light emitter”. 
     An edge emitting laser (EEL) may be used as the “light emitter”. 
     Furthermore, as the “light emitter”, a single light source is applicable, and a plurality of light sources that emits light simultaneously (for example, a VCSEL array light source) is also applicable. 
     There is a latitude in the specific aspect of “the light emitter”, and various design changes are possible. 
     In a conventional MEMS tunable laser, since electrostatic attraction is used to modulate a cavity length, that is, the distance between the two mirrors facing each other in parallel, the linearity of a drive voltage (a drive signal) and the amount of displacement becomes insufficient to obtain a desired wavelength. 
     Furthermore, since an initial value of the cavity length cannot be accurately adjusted to the interval of an integral multiple of an oscillation wavelength in manufacturing, undesirably, a high voltage has to be applied to move a movable portion of the mirror to the cavity length that satisfies an oscillation condition at the time of oscillation. 
     The drive voltage (drive signal) and the cavity length do not have the linearity in a MEMS tunable laser using the electrostatic attraction because the position of the mirror on the MEMS side is proportional to the square of the drive voltage. 
     The position of the mirror on the MEMS side is determined by an initial gap between the two mirrors without applied voltage, and a balanced condition between the electrostatic attraction generated by the potential difference, and the restoring force of the spring that constitutes the driving structure of the MEMS. 
     When the drive voltage is increased, the electrostatic attraction, which is a force to cause the two mirrors to attract each other, increases, and the cavity length becomes shorter. 
     When the voltage range is wide, the displacement amount of the reflector deviates from the linearity. 
     To avoid such a situation, a method involves distorting the drive voltage in advance such that the wavelength changes linearly with respect to the drive voltage is conceivable. 
     However, since an initial gap of 1 to 2 μm between the mirrors changes with variations in a device manufacturing, and temperature changes, an initial gap control with ultra-high-precision is difficult to achieve. 
     Such a method that involves distorting the drive voltage preliminarily under the static conditions fails to have the linearity in the change of the cavity length. 
     Furthermore, when the gap between the two mirrors changes to ⅓ or more of the initial gap, the electrostatic attraction generated by the potential difference exceeds the restoring force of a spring of the driving structure of the MEMS, which causes a pull-in effect that attracts films to each other. As a result, the MEMS tunable laser does not work. 
     In other words, the movable range of the mirror in the MEMS tunable laser using the electrostatic attraction is limited to less than ⅓ of the initial gap. 
     In view of the above-described circumstances, the present embodiment employs a structure in which a piezoelectric element is formed as a film on a spring as a driving source of a reflector on the MEMS side. 
     In this structure, the cavity length is linearly modulated with respect to the voltage by using the linearity of an applied voltage and the amount of volume reduction of the piezoelectric element. 
     When an electrode is formed on the spring having a meandering structure connected to the reflector on the MEMS side so as to have the piezoelectric element sandwiched between the spring and the electrode, and the voltage is applied; the volume of the piezoelectric element linearly decreased with the applied voltage. 
     The spring of the meandering structure deforms in the direction perpendicular to the plane of the film of the piezoelectric element according to a stress generated by this volume reduction in the in-plane direction of the film. 
     At this time, the linearity is maintained between the stress and the amount of deformation of the piezoelectric element. 
     Furthermore, since the spring is connected to the reflector, the relation between the amount of deformation of the spring and the amount of displacement of the reflector is linearly maintained. As a result, the reflector can be linearly moved with respect to the voltage. 
     In addition, since this relationship does not depend on the initial value of the cavity length, the cavity length can be controlled with high accuracy in an electrostatically driven MEMS. 
     More specifically, in the present embodiment, an elastic support unit  900  that supports the second reflector  800  and the piezoelectric element  1000  disposed in the elastic support unit  900  are formed in the stationary support unit  700  of the movable reflector element  200 . The oscillation wavelength of light emitted from the half-VCSEL element is changed by changing the distance between the first reflector  400  and the second reflector  800  by changing the elastic support unit by applying the drive voltage to the piezoelectric element  1000 . 
     The first reflector  400  and the half-VCSEL element  100  are formed as a single unit, that is, a first substrate, and the second reflector  800 , the elastic support unit  900 , and the piezoelectric element  1000  are formed as a single unit, that is, a second substrate. This allows a simple structure of the tunable laser  10 . 
     For example, the half-VCSEL element  100  constitutes the first substrate and the movable reflector element  200  constitutes the second substrate. 
       FIG.  3    is a plan view of a first example of the support structure of the second reflector  800  (i.e., support structure of the MEMS side reflector). 
       FIG.  4    is a cross-sectional view taken along a line AA in  FIG.  3   . 
     As illustrated in  FIG.  3   , the second reflector  800  is supported in a floating state with a through hole  700 X interposed in the center of the stationary support unit  700  that spreads in the XY plane. 
     A connecting end  810  and a connecting end  820  are formed to protrude from the upper and lower sides of the second reflector  800 , respectively, and a connecting portion  710  and a connecting portion  720  are formed to protrude from the upper and lower sides of the stationary support unit  700 , respectively. 
     The connecting end  810  and the connecting portion  710  are connected by an elastic support unit  910 , and the connecting end  820  and the connecting portion  720  are connected by an elastic support unit  920 . 
     The elastic support units  910  and  920  are disposed apart from each other around the second reflector  800 . 
     The elastic support unit  910  includes a first arm  911  extending to the right from the connecting portion  710 , a folded portion  912  folded downward from the first arm  911 , and a second arm  913  extending to the left from the folded portion  912  and connected to the connecting end  810 . In other words, the elastic support unit  910  forms a meandering structure. 
     A piezoelectric element  1011  is disposed on the left half of the first arm  911 , and is a displacement portion  911 A that elastically deforms when the drive voltage is applied to the piezoelectric element  1011 . 
     The piezoelectric element  1011  is not disposed on the right half of the first arm  911 , and is a displacement defining portion  911 B that does not elastically deform when the drive voltage is applied to the piezoelectric element  1011 . 
     A piezoelectric element  1013  is disposed on the right half of the second arm  913 , and is a displacement portion  913 A that elastically deforms when a drive voltage is applied to the piezoelectric element  1013 . 
     The piezoelectric element  1013  is not disposed on the left half of the second arm  913 , and is a non-displacement defining portion  913 B that does not elastically deform when the drive voltage is applied to the piezoelectric element  1013 . 
     As described above, the elastic support unit  910  includes the first arm  911  and the second arm  913  extending adjacent to each other, and the folded portion  912  connecting the ends on the same side of the first arm  911  and the second arm  913  in the extending direction. The piezoelectric element  1011  is disposed on the first arm  911  and the piezoelectric element  1013  is disposed on the second arm  913 . 
     The piezoelectric element  1011  and the piezoelectric element  1013  are arranged at different positions, which are displaced from each other in the extending direction of the first arm  911  and the second arm  913 . In other words, the piezoelectric element  1011  and the piezoelectric element  1013  are not aligned along the direction orthogonal to the extending direction of the first arm  911  and the second arm  913 . 
     The elastic support unit  920  includes a first arm  921  extending to the right from the connecting portion  720 , a folded portion  922  folded upward from the first arm  921 , and a second arm  923  extending to the left from the folded portion  922  and connected to the connecting end  820 . In other words, the elastic support unit  920  forms the meandering structure. 
     A piezoelectric element  1021  is disposed on the left half of the first arm  921 , and is a displacement portion  921 A that elastically deforms when the drive voltage is applied to the piezoelectric element  1021 . 
     The piezoelectric element  1021  is not disposed on the right half of the first arm  921 , and is a displacement defining portion  921 B that does not elastically deform when the drive voltage is applied to the piezoelectric element  1021 . 
     A piezoelectric element  1023  is disposed on the right half of the second arm  923 , and is a displacement portion  923 A that elastically deforms when the drive voltage is applied to the piezoelectric element  1023 . 
     The piezoelectric element  1023  is not disposed on the left half of the second arm  923 , and is a displacement defining portion  923 B that does not elastically deform when the drive voltage is applied to the piezoelectric element  1023 . 
     As described above, the elastic support unit  920  includes the first arm  921  and the second arm  923  extending adjacent to each other, and the folded portion  922  connects the ends on the same side of the first arm  921  and the second arm  923  in the extending direction. The piezoelectric element  1021  is disposed on the first arm  921 , and the piezoelectric element  1023  is disposed on the second arm  923 . 
     The piezoelectric element  1021  and the piezoelectric element  1023  are arranged at different positions, which are displaced from each other in the extending direction of the first arm  921  and the second arm  923 . In other words, the piezoelectric element  1021  and the piezoelectric element  1023  are not aligned along the direction orthogonal to the extending direction of the first arm  921  and the second arm  923 . 
     As illustrated in  FIG.  4   , the stationary support unit  700  includes a support layer  730 , an oxide insulating layer  740 , a silicon active layer  750 , and an insulating layer  760 , which are laminated in this order from the lower layer side to the upper layer side in the Z direction. 
     A displacement defining portion  911 B and a displacement defining portion  921 B include the silicon active layer  750  and the insulating layer  760 , which are laminated in this order from the lower layer side to the upper layer side in the Z direction (the displacement defining portion  911 B and the displacement defining portion  921 B are a laminated structure which partially share the stationary support unit  700 ). 
     A displacement portion  913 A (including the piezoelectric element  1013 ) and a displacement portion  923 A (including the piezoelectric element  1023 ) include an actuator in which the piezoelectric element  1000  is formed on the upper surface of the silicon active layer  750 . The piezoelectric element  1000  include a lower electrode  1000 A, a piezoelectric material  1000 B, and an upper electrode  1000 C laminated in this order from the lower layer side to the upper layer side in the Z direction. 
     By applying a drive voltage to a lower electrode  1000 A and an upper electrode  1000 C, the displacement portion  913 A and the displacement portion  923 A are elastically deformed. The insulating layer  760  and a protective film  1100  are disposed on the upper surface of the piezoelectric element  1000  (the upper electrode  1000 C). 
     In the present embodiment, by applying the drive voltage to the piezoelectric elements  1011 ,  1013 ,  1021 , and  1023  to elastically deform the elastic support units  910  and  920  and to change the distance between the first reflector  400  and the second reflector  800 , the oscillation wavelength of the light by the half-VCSEL element  100  is changed. 
     With displacement generated by combinations of the actuators (the piezoelectric elements  1011 ,  1013 ,  1021 , and  1023 ) on the meandering structure, the displacement defining portions  911 B,  913 B,  921 B, and  923 B, and the folded portions  912  and  922  via the connecting ends  810  and  820 , the second reflector  800  as a movable reflector translationally moves in the Z direction while maintaining the parallelism between the stationary support unit  700  and the second reflector  800 . 
     This translational movement in the Z direction is caused by the potential difference generated by applying the drive voltage to the lower electrode  1000 A and the upper electrode  1000 C of the actuator. 
       FIG.  5    is a plan view illustrating a second example of the support structure (the support structure of the MEMS side reflector) of the second reflector  800 . 
       FIG.  6    is a perspective view illustrating a driving state of the second reflector  800  of the support structure in  FIG.  5   .  FIG.  7    is a side view of a driving state of the second reflector  800  of the support structure in  FIG.  5   . 
     The second example includes, in addition to the elastic support units  910  and  920  disposed on the right half of the stationary support unit  700  extending in the XY plane, the elastic support units  930  and  940  on the left half of the stationary support unit  700  extending in the XY plane. 
     The elastic support units  910 ,  920 ,  930 , and  940  are apart from each other around the second reflector  800 . 
     As described above, by providing the four elastic support units  910 ,  920 ,  930 , and  940  symmetrically at the four corners of the stationary support unit  700  extending in the XY plane, a parallelism between the stationary support unit  700  and the second reflector  800  as a movable reflector can be maintained more steadily. 
     The elastic support unit  930  includes a first arm  931  extending to the left from the connecting portion  710 , a folded portion  932  folded downward from the first arm  931 , and a second arm  933  extending to the right from the folded portion  932  and to the connecting end  810 . In other words, the elastic support unit  930  forms the meandering structure. 
     A piezoelectric element  1031  is disposed on the right half of the first arm  931 , and is a displacement portion  931 A that elastically deforms when the drive voltage is applied to the piezoelectric element  1031 . 
     The piezoelectric element  1031  is not disposed on the left half of the first arm  931 , and is a displacement defining portion  931 B that does not elastically deform when the drive voltage is applied to the piezoelectric element  1031 . 
     A piezoelectric element  1033  is disposed on the left half of the second arm  933 , and is a displacement portion  933 A that elastically deforms when the drive voltage is applied to the piezoelectric element  1033 . 
     The piezoelectric element  1033  is not disposed on the right half of the second arm  933 , and is a displacement defining portion  933 B that does not elastically deform when a drive voltage is applied to the piezoelectric element  1033 . 
     As described above, the elastic support unit  930  includes the first arm  931  and the second arm  933  extending adjacent to each other, and the folded portion  932  connecting the ends on the same side of the first arm  931  and the second arm  933  in the extending direction. The piezoelectric element  1031  is disposed on the first arm  931 , and the piezoelectric element  1033  is disposed on the second arm  933 . 
     The piezoelectric element  1031  and the piezoelectric element  1033  are arranged at different positions, which are displaced from each other in the extending direction of the first arm  931  and the second arm  933 . In other words, the piezoelectric element  1031  and the piezoelectric element  1033  are not aligned along the direction orthogonal to the extending direction of the first arm  931  and the second arm  933 . 
     The elastic support unit  940  includes a first arm  941  extending to the left from the connecting portion  720 , a folded portion  942  folded upward from the first arm  941 , and a second arm  943  extending to the right from the folded portion  942  and connected to the connecting end  820 . In other words, the elastic support unit  940  forms a meandering structure. 
     A piezoelectric element  1041  is disposed on the right half of the first arm  941 , and is a displacement portion  941 A that elastically deforms when the drive voltage is applied to the piezoelectric element  1041 . 
     The piezoelectric element  1041  is not disposed on the left half of the first arm  941 , and is a displacement defining portion  941 B that does not elastically deform when the drive voltage is applied to the piezoelectric element  1041 . 
     A piezoelectric element  1043  is disposed the left half of the second arm  943 , and is a displacement portion  943 A that elastically deforms when the drive voltage is applied to the piezoelectric element  1043 . 
     The piezoelectric element  1043  is disposed on the right half of the second arm  943 , and is a displacement defining portion  943 B that does not elastically deform when the drive voltage is applied to the piezoelectric element  1043 . 
     As described above, the elastic support unit  940  includes the first arm  941  and the second arm  943  extending adjacent to each other, and the folded portion  942  connecting the ends on the same side of the first arm  941  and the second arm  943  in the extending direction. The piezoelectric element  1041  is disposed on the first arm  941  and the piezoelectric element  1043  is disposed on the second arm  943 . 
     The piezoelectric element  1041  and the piezoelectric element  1043  are disposed not to be in parallel with a direction (i.e., the vertical direction of the drawing) orthogonal to the extending directions of the first arm  941  and the second arm  943 . 
     The piezoelectric element  1041  and the piezoelectric element  1043  are arranged at different positions, which are displaced from each other in the extending direction of the first arm  941  and the second arm  943 . In other words, the piezoelectric element  1041  and the piezoelectric element  1043  are not aligned along the direction orthogonal to the extending direction of the first arm  941  and the second arm  943 . 
     In this present embodiment, by applying the drive voltage to the piezoelectric elements  1011 ,  1013 ,  1021 ,  1023 ,  1031 ,  1033 ,  1041 , and  1043  to the elastic support units  910 ,  920 ,  930 , and  940  to change the distance between the first reflector  400  and the second reflector  800 , the oscillation wavelength of the light by the half-VCSEL element  100  is changed. 
     When a drive voltage is applied to a piezoelectric element, the volume of the piezoelectric element changes. 
     Since the piezoelectric element and the arm are physically connected, an in-plane stress is generated in the vicinity of the piezoelectric element of the arm. 
     The arm stressed elastically deforms to relieve the stress. 
     At this time, adding an anisotropy to a dimension ratio of the arm in the XY direction to shorten the thickness of the arm in the Z direction causes the arm to warp dominantly in the Z direction, and the piezoelectric element also warps in the Z direction with a predetermined radius of curvature maintained. 
     The warp of the arm generated by the piezoelectric element is transmitted to the displacement defining portion without the piezoelectric element. 
     The warp of the displacement defining portion in the Z direction due to gravity, which is much smaller than the warp of the piezoelectric element, can be ignored. In other words, the warp of the displacement defining portion is equivalent to a negligibly small elastic deformation. 
       FIG.  7    is an enlarged view of the elastic support unit  920  as an example, and the elastic support units  910 ,  930 , and  940  have the similar configuration. 
     As illustrated in  FIG.  7   , the displacement Z1 of the folded portion  922  of the elastic support unit  920  in the Z direction at least increases the warp (reducing the radius of curvature) generated by the piezoelectric elements  1021  and  1023 , and lengthens the displacement defining portion  921 B of the first arm  921 . 
     The first arm  921  tilts toward the folded portion  922  at an angle θ 1  from a free state, and after the folded portion  922  is folded back, the second arm  923  tilts at an angle θ 2  from the free state (the difference between the angle θ 1  and the angle θ 2  is an tilt angle between the first arm  921  and the second arm  923 ). 
     When the second arm  923  has a piezoelectric element with the same dimensions of the piezoelectric element of the first arm  921  drives, the tilt of the first arm  921  is canceled out. Therefore, at the connecting end  820  disposing at the end of the second arm  923 , the second reflector  800  as a movable reflector can be moved in the Z direction with maintaining the parallelism between the XY plane of the stationary support unit  700  and the second reflector  800  as a movable reflector from before driving. 
     The dimensions of the piezoelectric elements formed on the first arm  921  and the second arm  923  on the XY plane may or may not be the same. 
     When the dimensions of the piezoelectric elements formed on the first arm  921  and the second arm  923  on the XY plane is not the same, the voltage applied to each piezoelectric element is adjusted such that the warpage generated in each piezoelectric element is the same. 
     The drive voltage applied to the piezoelectric element is a DC voltage and may alternatively be a sine waveform, a cosine waveform, or a triangular waveform whose voltage changes with time. 
     Under a condition that the modulation frequency of the voltage and the resonance frequency of the movable reflector element are separated from each other, the linearity of the displacement amount with respect to voltage is maintained as described above. 
     Piezoelectric elements are driven by applying a voltage, and there is a linearity between a voltage and a generated stress. 
     Since the generated stress and the amount of deformation of the arm also have the linearity, the applied voltage and the amount of displacement have the linearity. 
       FIG.  8    is a graph illustrating the relation between the drive voltage of the tunable laser  10  and the cavity length in the present embodiment.  FIG.  9    is a graph illustrating the relation between the driving voltage and the cavity length of the conventional MEMS tunable laser with the electrostatic attraction. 
     As illustrated in  FIG.  8   , according to the tunable laser  10  of the present embodiment, a high linearity between the drive voltage and the cavity length is realized. 
     On the other hand, as illustrated in  FIG.  9   , since the position of the mirror on the MEMS side in the conventional MEMS tunable laser using the electrostatic attraction is proportional to the square of the drive voltage, and the pull-in effect in which the films are attracted to each other when the gap between the two mirrors displaced to ⅓ or more of the initial gap large, the linearity between the drive voltage and the cavity length is insufficient to obtain. 
       FIG.  10    is a plan view illustrating a third example of the support structure of the second reflector  800  (support structure of the MEMS side reflector). 
     In the third example, the piezoelectric element  1011 X is disposed at the region of the displacement defining portion  911 B of the first arm  911  in which the piezoelectric element is not disposed in the first example, and the piezoelectric element  1013 X is disposed at the region of the displacement defining portion  913 B of the second arm  913 , and the piezoelectric element  1021 X is disposed at the region of the displacement defining portion  921 B of the first arm  921 , and the piezoelectric element  1023 X is disposed at the region of the displacement defining portion  923 B of the second arm  923 . 
     Piezoelectric elements  1011 ,  1013 ,  1021 ,  1023 ,  1011 X,  1013 X,  1021 X,  1023 X are arranged along the direction (i.e., the vertical direction of the drawing sheet) orthogonal to the extending direction of the first arm  911  and the second arm  913  and also orthogonal to the extending direction of the first arm  921  and the second arm  923 . 
     In the third example, the piezoelectric element includes a first set of piezoelectric elements  1011 ,  1013 ,  1021 , and  1023  and a second set of piezoelectric elements  1011 X,  1013 X,  1021 X, and  1023 X. 
     When a drive voltage is applied to the first set of piezoelectric elements, the drive voltage is not applied to the second set of piezoelectric elements. Conversely, when the drive voltage is applied to the second set of piezoelectric elements, the drive voltage is not applied to the first set of piezoelectric elements. 
     In the third example, when voltage is applied to the piezoelectric element  1011 , the region of the displacement portion  911 A of the first arm  911  serves as the displacement portion, and the region of the displacement portion  911 B serves as the displacement defining portion. When voltage is applied to the piezoelectric element  1011 X, the region of the displacement defining portion  911 B of the first arm  911  serves as the displacement portion, and the region of the displacement portion  913 A serves as the displacement defining portion. 
     When voltage is applied to the piezoelectric element  1013 , the region of the displacement portion  913 A of the second arm  913  serves as the displacement portion, and the region of the displacement portion  913 B serves as the displacement defining portion. When voltage is applied to the piezoelectric element  1013 X, the region of the displacement portion  913 B of the second arm  913  serves as the displacement portion, and the region of the displacement portion  913 A of the second arm  913  serves as the displacement defining portion. 
     When voltage is applied to the piezoelectric element  1021 , the region of the displacement portion  921 A of the first arm  921  serves as the displacement portion, and the region of the displacement defining portion  921 B of the first arm  921  serves as the displacement defining portion. When voltage is applied to the piezoelectric element  1021 X, the region of the displacement portion  921 B of the first arm  921  serves as the displacement portion, and the region of the displacement portion  921 A of the first arm  921  serves as the displacement defining portion. 
     When voltage is applied to the piezoelectric element  1023 , the region of the displacement portion  923 A of the second arm  923  serves as the displacement portion, and the region of the displacement defining portion  923 B of the second arm  923  serves as the displacement defining portion. When voltage is applied to the piezoelectric element  1023 X, the region of the displacement defining portion  923 B of the second arm  923  serves as the displacement portion, and the region of the displacement portion  923 A of the second arm  923  serves as the displacement defining portion. 
     In the first example, the displacement defining portion is the portion without the piezoelectric element among the plurality of arms. On the other hand, in the third example, the displacement defining portion is the portion with the piezoelectric element and without the applied voltage among the plurality of arms. 
     In the third example, when a failure or malfunction accidentally occurs in the piezoelectric elements included in one set of the first set and the second set of the piezoelectric elements, the tunable laser  10  can be operated by applying the voltage to the other set without being affected by the failure or malfunction of the piezoelectric element. 
       FIGS.  11 A and  11 B  are diagrams of an example of wavelength sweeping by the tunable laser  10  of the present embodiment. 
     The laser oscillation wavelength of the tunable laser  10  is defined by an emission spectrum (i.e., a wavelength distribution) characteristic in materials constituting the active layer  105  and the resonator structure. 
     The resonator structure is characterized by the distance G in the Z direction between the first reflector  400  and the second reflector  800 , and the refractive index and dimensions of the material existing between the first reflector  400  and the second reflector  800 . 
     The active layer  105  emits light with the current injected into the half-VCSEL element  100  through the electrodes, and the wavelength of the resonant light (i.e., the oscillation light) changes with variation of the distance G between the first reflector  400  and the second reflector  800  in the Z direction. 
     In  FIG.  11 A  is a diagram of the resonant light (i.e., the oscillation light) emitted upward in the Z direction. In  FIG.  11 B  is a diagram of the resonant light (i.e., the oscillating light) emitted upward in the Z direction. 
     Since the distance G between the first reflector  400  and the second reflector  800  in the Z direction can be changed by changing the position of the second reflector  800  as a movable reflector element in the Z direction, the wavelength of the resonant light (i.e., oscillating light) can be modulated. 
     In particular, in the present embodiment, the distance G between the first reflector  400  and the second reflector  800  in Z the direction can be controlled with high accuracy by devising the support structure of the second reflector  800  by using the elastic support unit  900  and the piezoelectric element  1000 . 
     As a result, the linearity of the oscillation wavelength with respect to the drive voltage is improved and the light having a desired oscillation wavelength is obtained. 
       FIG.  12    is a graph of a simulation result illustrating the relationship between the length of the gap between the first reflector  400  and the second reflector  800  of the half-VCSEL element  100  and the oscillation wavelength. 
     The oscillating light emitted by the half-VCSEL element  100  is emitted to the first reflector  400  or the second reflector  800 , whichever has a lower reflectance. 
     Therefore, by adjusting the magnitude of the reflectance between the first reflector  400  and the second reflector  800 , the emission direction of the oscillating light by the half-VCSEL element  100  can be adjusted. 
     When the reflectance of the first reflector  400  is set to be lower than the reflectance of the second reflector  800 , the oscillating light is emitted downward in the Z direction ( FIG.  11 A ). When the reflectance of the first reflector  400  is set to be higher than the reflectance of the second reflector  800 , the oscillating light is emitted upward in the Z direction ( FIG.  11 B ). 
     A bonding layer including multilayer metal films containing a plurality of metals is formed on each of the half-VCSEL element  100  (first reflector  400 ) and the second reflector  800 . 
     For example, by bonding the bonding layers on each element together with an atomic diffusion bonding method, a space can be obtained between the first reflector  400  and the second reflector  800 . 
     The active layer  105  of the half-VCSEL element  100  is disposed in the center of the resonator structure, which is a position corresponding to the antinode in the standing wave of the electric field to obtain a highly stimulated emission probability. 
       FIG.  13    is a diagram of a modified example of the tunable laser  10  of the present embodiment, corresponding to  FIG.  1   . 
     As illustrated in  FIG.  13   , one second reflector  800  is provided with a plurality of half-VCSEL elements  100 . 
     Specifically, one second reflector  800  is provided with three half-VCSEL elements  100 X,  100 Y, and  100 Z adjacent to each other in the X direction on the XY plane. 
     The first reflector  400 , the semiconductor substrate  500 , and the antireflection film  600  may be shared by these three half-VCSEL elements  100 X,  100 Y, and  100 Z. 
     There is a degree of latitude in the number and arrangement of half-VCSEL elements, and various design changes are possible. For example, the second reflector is provided with two or four or more half-VCSEL elements. In other words, the number of half-VCSEL elements is not limited to three, and may be any plural number (i.e., the second substrate includes a plurality of light emitters.) Alternatively, the second reflector is provided with a plurality of half-VCSEL elements arranged in the Z direction, instead of or in addition to the plurality of half-VCSEL elements arranged in the X direction or the Y direction within the XY plane. 
       FIG.  14    is a diagram of a modified example of the tunable laser  10  of the present embodiment, corresponding to  FIG.  6   . 
     In  FIG.  14   , a movable reflector structure  800 X for high-speed driving inside the second reflector  800  as a movable reflector is additionally illustrated. 
     The length of the gap between the half-VCSEL element  100  (first reflector  400 ) and the second reflector  800 , which are bonded together with the bonding layer  300  between the half-VCSEL element  100  and the second reflector  800 , depends on the usage environment (e.g., temperature) of the tunable laser  10 . 
     The change length of this gap is several micrometers. A center of the oscillation wavelength is fixed by statically driving the meandering structure with a voltage prepared (preset) according to the usage environment (e.g., temperature) of the tunable laser  10 . 
     With the center wavelength fixed, the movable reflector structure  800 X for high-speed driving is displaced to several tens of nm by a modulation signal in the Z direction so as to sweep the oscillation wavelength within a certain bandwidth of the wavelength with the center wavelength maintained constant regardless of environmental changes. 
     In this way, by providing the second reflector  800  that handles a relatively large drive range and the movable reflector structure  800 X that handles a relatively small drive range (by dividing the functions), the gap between the half-VCSEL element  100  (first reflector  400 ) and the second reflector  800  can be controlled with higher accuracy. 
     This configuration further improves the linearity of the oscillation wavelength with respect to the drive voltage and enables light having a desired oscillation wavelength. 
     The movable reflector structure  800 X for high-speed driving is provided with piezoelectric elements. Applying the voltage that continuously fluctuates with time and a modulation signal that periodically changes into the piezoelectric elements enables the movable reflector structure to displace in the Z direction at a drive speed of tens to hundreds of nanometers on the order of MHz. 
       FIG.  15 A  is a graph of the relation between the optical path length of the resonator and the temperature in the tunable laser according to an embodiment of the present disclosure.  FIG.  15 B  is a graph of the relation between the oscillation wavelength and the temperature. With a change in the temperature of the tunable laser due to, for example, environmental temperature changes or heat generated by a module of the laser, a difference in thermal expansion coefficient between the material of the movable reflector element and the material of the half-VCSEL element changes the length of the gap tuning region between the movable reflector element and the half-VCSEL element. 
     Further, the change in the temperature of the tunable laser also changes the refractive index of the material (e.g., semiconductor material) forming the resonator of the movable reflector element and the half-VCSEL element, causing the optical path length of the resonator of the tunable laser to vary. Using recorded data on the variation in the wavelength with changes in the temperature, a voltage to be applied to the piezoelectric elements of the meandering structure connected to the second reflector is distorted in advance to keep the wavelength constant with the temperature, thus reducing the variation in wavelength with the temperature.  FIG.  30    is a functional block diagram of wavelength correction. For example, a temperature measuring device is disposed near a light source to measure the temperature of or near the tunable laser during its oscillation. A storage unit in  FIG.  30    stores, as a correlation formula or a table, the relation between the wavelength, the output power, and the temperature of the tunable laser during oscillation, and also stores the relation between the drive voltage and the oscillation wavelength of the piezoelectric elements. The processor determines a correction voltage for which the oscillation wavelength is constant with temperature, or the output power becomes maximum, based on the relation between the temperature and the wavelength measured and stored in the storage unit. According to that voltage, a drive signal source drives a piezoelectric element. 
     Such an adjustment method is applicable when the length of the gap tuning region changes by a large amount of, for example, several hundreds of nanometers. To deal with the change of the above-described order, a sufficient amount of displacement cannot be obtained to cancel out the amount of the change with the temperature by using a static voltage to actuate the movable reflector structure  800 X that handles a relatively small drive range. 
     In view of such a situation, with the position of the second reflector being statically adjusted with the temperature, the movable reflector structure  800 X that handles a relatively small drive range is actuated at a high speed by a modulation signal of, for example, a sine wave. This can sweep the wavelength at a high speed while maintaining the central wavelength with changes in temperature. 
     As described above, the optical device of the present embodiment includes the first reflector, the second reflector, the elastic support unit that supports the second reflector, and the piezoelectric element provided in the elastic support unit. 
     By applying a drive voltage to the piezoelectric element to elastically deform the elastic support unit, the distance between the first reflector the second reflector is changed, and the oscillation wavelength of light emitted by the light emitting portion is changed. 
     The reflector on the MEMS side to control the cavity length that determines the wavelength of light is connected to the elastic meandering structure in which the piezoelectric element is formed. 
     As a result, the linearity of the oscillation wavelength with respect to the drive voltage is improved and the light with a desired oscillation wavelength is obtained. 
     The present invention is not limited to the above embodiments, and can be modified in various ways. 
     The size, the shape, the function, and the like of the components illustrated in the accompanying drawings are not limited to the above embodiments, and can be appropriately changed within the effects of the present invention. 
     In addition, the size, the shape, the function, and the like of the components illustrated in the accompanying drawings can be appropriately modified and implemented as long as the size, the shape, the function, and the like of the components illustrated in the accompanying drawings does not deviate from the purpose of the present invention. 
     In the above embodiment, the support unit is formed as the meandering structure that includes the two arms extending adjacent to each other and one folded portion connecting the ends on the same side of the two arms in the extending direction of the arms, and the piezoelectric elements are formed on the two arms. 
     In the above embodiment, the support unit is formed as the meandering structure that may include three or more arms extending adjacent to each other and two or more of folded portions each connecting the ends on the same side of two adjacent arms of the three or more of arms in the extending direction, and the piezoelectric elements are formed on the three or more of arms. 
     The elastic support unit includes a plurality of arms extending adjacent to each other and a folded portion for connecting the ends on the same side of the plurality of arms in the extending direction, and the piezoelectric element is displaced on the plurality of arms. 
     In the above embodiment, the case where two or four elastic support units are provided apart from each other around the second reflector has been described as an example, but there is a latitude in the number of the elastic support units separate from each other around the second reflector, and various design changes are possible. 
     For example, at least two elastic support units may be provided or at least three elastic support units may be provided so as to be apart from each other around the second reflector. 
     In the conventional MEMS tunable laser, when the plane parallelism between the light emitter (e.g., the half-VCSEL element) and the movable reflector element (e.g., the MEMS mirror) is low, that is, the plane parallelism between the two reflectors (i.e., the first mirror and the second mirror) is low, the threshold current for laser emission becomes high. 
     In particular, in the MEMS tunable laser using electrostatic attraction, the parallelism between two mirrors is tend to be low and the threshold current for laser emission is tend to be high, and since the accuracy of the plane parallelism is fixed at the time of joining, an sufficient emission intensity cannot be obtained. 
     In the modified example, the plane parallelism of two reflectors, in other words, the plane parallelism between the emitting unit and the reflector can be improved. 
     According to the modified example, the plane parallelism of two reflectors, in other words, the plane parallelism between the emitting unit and the reflector can be improved. 
     In the conventional MEMS tunable laser, the plane parallelism between the light emitter (e.g., the half-VCSEL element) and the movable reflector element (e.g., the MEMS mirror) is low, that is, the two reflectors (first and second) is low. The parallelism between the two reflectors causes a high threshold current of laser emission. 
     In particular, in the MEMS tunable laser using electrostatic attraction, the parallelism between two mirrors is tend to be low and the threshold current for laser emission is tend to be high, and since the accuracy of the plane parallelism is fixed at the time of joining, an sufficient emission intensity cannot be obtained. 
     A cause of degrading the plane parallelism between the light emitter (e.g., the half-VCSEL element) and the movable reflector element (e.g., the MEMS mirror) comes from mounting conditions at the time of device integration. 
     In the MEMS tunable laser, the light emitter (e.g., the half-VCSEL element) and the movable reflector element (e.g., the MEMS mirror) are integrated in a single device, and a gap is disposed between the light emitter and the movable reflector element. 
     Among the ultra-high reflectors possessed by each element, a wavelength sweep function is added to the oscillating light by injecting a current into the half-VCSEL while slightly driving the reflector on the MEMS side in the direction perpendicular to the element surface. 
     The integration method is classified into a laminated method and a bonding method. The bonding method uses a bonding layer for bonding the two elements, and the plane parallelism of the two elements changes depending on the film thickness variation, the bonding temperature, and pressure. 
     A film formation method such as sputtering or evaporation is used to form the bonding layer. In the film formation, the film thickness varies depending on the film formation position. Heating and pressurizing are required to join the bonding layers formed on each element. There are positional variations on heating and pressurizing conditions. 
     The bonding layer bonding the light emitter and the movable reflector element does not uniformly deform and varies in thickness. This reduces the plane parallelism between the light emitter (e.g., the half-VCSEL element) and the movable reflector element (e.g., the MEMS mirror). 
     In a laser element such as the half-VCSEL that has a resonator in the direction perpendicular to the plane, a low threshold current, a low power consumption, and a high output are achieved by reducing the loss of light that oscillates between the resonator includes two reflectors. 
     Therefore, in the tunable laser containing the device structure as described above, a tilt in the parallelism between the elements, for example, two reflectors, increases the loss of light by that amount of tilt and the threshold current increases. 
     In the present embodiment, at least three elastic support units (e.g.,  910 ,  920 , and  930 ) are included as the elastic support unit  900  for supporting the second reflector  800 , and piezoelectric elements (e.g.,  1010 ,  1020 , and  1030 ) are disposed on at least three elastic support units (e.g.,  910 ,  920 ,  930 ), respectively. 
     Then, an independent drive signal is applied to at least three piezoelectric elements (e.g.,  1010 ,  1020 , and  1030 ) of three elastic support units (e.g.,  910 ,  920 , and  930 ), and by independently elastically deforming at least three elastic support units (e.g.,  910 ,  920 , and  930 ), the plane parallelism between the first reflector  400  and the second reflector  800 , in other words, the plane parallelism between the half-VCSEL element  100  and the second reflector  800 , is changed. 
     In the present embodiment, the number of elastic bodies connecting the MEMS-side reflector and the chip support unit is three or more, and each elastic body is driven by an independent signal. 
     In this structure of the present embodiment, the driving amount of each elastic body is independently changed according to the tilt of a chip, so that the positions of the reflector and the plurality of connecting portions of the elastic body in the direction perpendicular to the chip surface are changed. 
     By forming the reflector with a highly rigid material, the positional difference in the connecting portion can be changed to the tilt of the reflector. 
     By using this change to drive the elastic body so as to cancel the tilt between the elements, the loss of light in the resonator structure can be reduced. 
       FIG.  16    is a plan view of the support structure of the second reflector  800  (i.e., support structure of the MEMS side reflector). 
       FIG.  17    is a cross-sectional view of the support structure of the second reflector  800  taken along the line AA of  FIG.  16   . 
     As illustrated in  FIG.  16   , the second reflector  800  is supported in a floating state with a through hole  700 X in the central portion of the stationary support unit  700  extending in the XY plane. 
     The connecting end  810 , the connecting end  820  and the connecting end  830  are formed to protrude around the second reflector  800 , and in the stationary support unit  700 , a connecting portion  710 , a connecting portion  720  and a connecting portion  730  are formed to protrude around the second reflector  800 . 
     The connecting end  810  and the connecting portion  710  are connected by the elastic support unit  910 , the connecting end  820  and the connecting portion  720  are connected by the elastic support unit  920 , and the connecting end  830  and the connecting portion  730  are connected by the elastic support unit  930 . 
     A set of the elastic support unit  910 , the connection end  810 , and the connecting portion  710 , a set of the elastic support unit  920 , the connection end  820 , and the connecting portion  720 , and a set of the elastic support unit  930 , the connection end  830 , and the connecting part  730  are disposed at equal or nearly equal intervals (180° intervals) in the circumferential direction. 
     The elastic support unit  910  includes the first arm  911  extending to the upper right diagonal from the connecting portion  710 , the folded portion  912  folded from the first arm  911 , and the second arm  913  extending to the lower left diagonal from the folded portion  912  and connecting to the connecting end  810 . In other words, the elastic support unit  930  forms a meandering structure. 
     A piezoelectric element  1011  is disposed on the left half of the first arm  911 , and is a displacement portion  911 A that elastically deforms when the drive voltage is applied to the piezoelectric element  1011 . 
     The piezoelectric element  1011  is not disposed on the right half of the first arm  911 , and is a displacement defining portion  911 B that does not elastically deform when the drive voltage is applied to the piezoelectric element  1011 . 
     The second arm  913  is provided with a piezoelectric element  1013 , and is a displacement portion  913 A that elastically deforms when a drive voltage is applied to the piezoelectric element  1013 . 
     As described above, the elastic support unit  910  includes the first arm  911  and the second arm  913  extending adjacent to each other, and the folded portion  912  connecting the ends of the first arm  911  and the second arm  913  in the extending direction. The piezoelectric element  1011  is disposed on the first arm  911  and the piezoelectric element  1013  is disposed on the second arm  913 . 
     The piezoelectric element  1011  and the piezoelectric element  1013  are provided so as not to be arranged in a direction orthogonal to the extending direction of the first arm  911  and the second arm  913 . 
     The piezoelectric element  1010  contains a set of the piezoelectric element  1011  and the piezoelectric element  1013 . 
     The elastic support unit  920  includes the first arm  921  extending to the lower right diagonal from the connecting portion  720 , the folded portion  922  folded from the first arm  921 , and the second arm  923  extending to the upper right diagonal from the folded portion  922  and connecting to the connecting end  820 . In other words, the elastic support unit  920  forms a meandering structure. 
     A piezoelectric element  1021  is disposed on the left half of the first arm  921 , and is a displacement portion  921 A that elastically deforms when the drive voltage is applied to the piezoelectric element  1021 . 
     The piezoelectric element  1021  is not disposed on the right half of the first arm  921 , and is the displacement defining portion  921 B that does not elastically deform when the drive voltage is applied to the piezoelectric element  1021 . 
     The second arm  923  is provided with a piezoelectric element  1023 , and is a displacement portion  923 A that elastically deforms when the drive voltage is applied to the piezoelectric element  1023 . 
     As described above, the elastic support unit  920  includes the first arm  921  and the second arm  923  extending adjacent to each other, and the folded portion  922  connects the ends on the same side of the first arm  921  and the second arm  923  in the extending direction. The piezoelectric element  1021  is disposed on the first arm  921  and the piezoelectric element  1023  is disposed on the second arm  923 . 
     The piezoelectric element  1021  and the piezoelectric element  1023  are provided so as not to be arranged in a direction orthogonal to the extending direction of the first arm  921  and the second arm  923 . 
     The piezoelectric element  1020  contains a set of the piezoelectric element  1021  and the piezoelectric element  1023 . 
     The elastic support unit  930  includes the first arm  931  extending to the left from the connecting portion  730 , the folded portion  932  folded downward from the first arm  931 , and the second arm  933  extending to the right from the folded portion  932  and to the connecting end  830 . In other words, the elastic support unit  930  forms a meandering structure. 
     A piezoelectric element  1031  is disposed on the right half of the first arm  931 , and is a displacement portion  931 A that elastically deforms when the drive voltage is applied to the piezoelectric element  1031 . 
     The piezoelectric element  1031  is not disposed on the left half of the first arm  931 , and is the displacement defining portion  931 B that does not elastically deform when the drive voltage is applied to the piezoelectric element  1031 . 
     The second arm  933  is provided with a piezoelectric element  1033 , and is a displacement portion  933 A that elastically deforms when the drive voltage is applied to the piezoelectric element  1033 . 
     As described above, the elastic support unit  930  includes the first arm  931  and the second arm  933  extending adjacent to each other, and the folded portion  932  connecting the ends on the same side of the first arm  931  and the second arm  933  in the extending direction. The piezoelectric element  1031  is disposed on the first arm  931 , and the piezoelectric element  1033  is disposed on the second arm  933 . 
     The piezoelectric element  1031  and the piezoelectric element  1033  are provided so as not to be arranged in a direction orthogonal to the extending direction of the first arm  931  and the second arm  933 . 
     The piezoelectric element  1030  contains a set of the piezoelectric element  1031  and the piezoelectric element  1033 . 
     As illustrated in  FIG.  17   , the stationary support unit  700  includes a support layer  730 , an oxide insulating layer  740 , a silicon active layer  750 , and an insulating layer  760  laminated in this order from the lower layer side to the upper layer side in the Z direction. 
     The displacement defining portion  921 B and the displacement defining portion  931 B include the silicon active layer  750  and the insulating layer  760  laminated in this order from the lower layer side to the upper layer side in the Z direction. The laminated structure of the displacement defining portion  912 B and the displacement defining portion  931 B is partially common with the stationary support unit  700 . 
     The displacement portion  923 A (including the piezoelectric element  1023 ) and the displacement portion  933 A (including the piezoelectric element  1033 ) includes an actuator in which the piezoelectric element  1000  is formed on the upper surface of the silicon active layer  750 . 
     The piezoelectric element  1000  includes a lower electrode  1000 A, a piezoelectric material  1000 B, and an upper electrode  1000 C laminated in this order from the lower layer side to the upper layer side in the Z direction. 
     By applying the drive voltage to the lower electrode  1000 A and the upper electrode  1000 C, the displacement portion  923 A and the displacement portion  933 A are elastically deformed. The insulating layer  760  and a protective film  1100  are disposed on the upper surface of the piezoelectric element  1000  (the upper electrode  1000 C). 
     In the present embodiment, by applying a drive voltage to the piezoelectric elements  1011 ,  1013 ,  1021 ,  1023 ,  1031 , and  1033  to elastically deform the elastic support units  910 ,  920 , and  930 , the distance between the first reflector  400  and the second reflector  800  (in other words, the half-VCSEL element  100  and the second reflector  800 ) is changed to change the oscillation wavelength of light emitted by the half-VCSEL. 
     With displacement generated by combinations of the actuators (the piezoelectric elements  1011 ,  1013 ,  1021 ,  1023 ,  1031 , and  1033 ) on the meandering structure, the displacement defining portions  911 B,  931 B, and  921 B, and the folded portions  912 ,  922 , and  932  via the connecting ends  810 ,  820 , and  830 , the second reflector  800  as a movable reflector translationally moves in the Z direction while maintaining the parallelism between the stationary support unit  700  and the second reflector  800 . 
     This translation in the Z direction is executed by generating the potential difference by applying the drive voltage to the lower electrode  1000 A and the upper electrode  1000 C of the actuator. 
     When the drive voltage (e.g., the same voltage) is applied to the piezoelectric element, the volume of the piezoelectric element changes. 
     Since the piezoelectric element and the arm are physically connected, an in-plane stress is generated in the vicinity of the piezoelectric element of the arm. 
     The arm stressed elastically deforms to relieve the stress. 
     At this time, to add an anisotropy to a dimension ratio of the arm in the XY direction, and to shorten the thickness of the arm in the Z direction make the arm warp dominantly in the Z direction, and the piezoelectric element also warps in the Z direction with keeping a predetermined radius of curvature. 
     The warp of the arm generated by the piezoelectric element is transmitted to the displacement defining portion without the piezoelectric element. 
     Thought the displacement defining portion warps in the Z direction due to gravity, this warp can be ignored because the warp by gravity is sufficiently small with respect to the warp of the piezoelectric element (equivalent to a negligibly small elastic deformation). 
       FIG.  18    is a side view illustrating a driving state of the second reflector  800  in the support structure of  FIG.  16    and  FIG.  17   . 
       FIG.  18    is an enlarged view of the elastic support unit  910  as an example, and the elastic support units  920  and  930  have the similar configuration. 
     In  FIG.  18   , the displacement Z1 of the folded portion  912  of the elastic support unit  910  in the Z direction increases the deflection generated by the piezoelectric elements  1011  and  1013 , or reduces the radius of curvature of the first arm  911 . The displacement defining portion  911 B of the first arm  911  may be lengthened, or both may be implemented. 
     The first arm  911  tilts toward the folded portion  912  at an angle θ1 from a free state, and after the folded portion  912  is hold back, the second arm  913  tilts at an angle θ2 from the free state (the difference between the angle θ1 and the angle θ2 is the tilt angle between the first arm  911  and the second arm  913 ). 
     When a piezoelectric element with the same dimension as the piezoelectric element of the first arm  911  formed on the second arm  913  drives, the tilt of the first arm  911  is canceled. Therefore, at the connection end  810  disposed at the end of the second arm  913 , the second reflector  800  as a movable reflector can be moved in the Z direction with maintaining the parallelism between the XY plane of the stationary support unit  700  and the second reflector  800  as a movable reflector. 
     The dimensions of the piezoelectric elements formed on the first arm  911  and the second arm  913  on the XY plane may or may not be the same. 
     When the dimensions of the piezoelectric elements formed on the first arm  921  and the second arm  923  on the XY plane is not the same, the voltage applied to each piezoelectric element is adjusted such that the warpage generated in each piezoelectric element is the same. 
     The drive voltage that applies to the piezoelectric element is a DC voltage and may alternatively be a sine waveform, a cosine waveform, or a triangular waveform whose voltage changes with time. 
     Under a condition that the modulation frequency of the voltage and the resonance frequency of the movable reflector element are separated from each other, the linearity of the displacement amount with respect to voltage is maintained as described above. 
     Piezoelectric elements are driven by applying a voltage, and there is a linearity between a voltage and a generated stress. 
     Since the generated stress and the amount of deformation of the arm also have the linearity, the applied voltage and the amount of displacement have the linearity. 
       FIG.  19    is a graph of the relation between the drive voltage and the cavity length in the tunable laser  10  of the present embodiment.  FIG.  20    is a graph of the relation between the drive voltage and the cavity length in the conventional MEMS tunable laser using the electrostatic attraction. 
     As illustrated in  FIG.  19   , the tunable laser  10  according to the present embodiment achieves a high linearity between the drive voltage and the cavity length. 
     As illustrated in  FIG.  20   , in a MEMS tunable laser using the electrostatic attraction according to a comparative example, the position of the reflector on the MEMS side is proportional to the square of the drive voltage, and the films attract to each other when the gap between two reflectors displaces to ⅓ or more of the initial gap (i.e., pull-in effect). Such a pull-in effect causes an insufficient linearity between the driving voltage and the cavity length. 
       FIG.  21    is a functional block diagram of a configuration to supply independent drive signals (drive voltages) to the piezoelectric elements  1010  to  1030  of the elastic support units  910  to  930 . 
     As illustrated in  FIG.  21   , by applying an independent drive signal (a drive voltage signal) to the piezoelectric element  1010  (piezoelectric element  1011 ) of the elastic support unit  910 .  1013 , the piezoelectric element  1020  (piezoelectric element  1021 ,  1023 ) of the elastic support unit  920 , and the piezoelectric element  1030  (piezoelectric element  1031 ,  1033 ) of the elastic support unit  930  by the drive signal supply unit (drive voltage supply unit)  1200  which is a functional component of the central processing unit (CPU), the elastic support units  910 ,  920 , and  930  deform independently and the plane parallelism between the first reflector  400  and the second reflector  800  (i.e., the plane parallelism between the half-VCSEL element  100  and the second reflector  800 ) is changed. 
     The drive signal supply unit  1200  may further supply the drive signal (drive voltage signal) to the piezoelectric elements  1011  and  1013 , which is independent of the drive signal (drive voltage signal) supplied to the piezoelectric element  1010  of the elastic support unit  910 . 
     The drive signal supply unit  1200  may further supply the drive signal (drive voltage signal) to the piezoelectric elements  1021  and  1023 , which is independent of the drive signal (drive voltage signal) supplied to the piezoelectric element  1020  of the elastic support unit  920 . 
     The drive signal supply unit  1200  may further make the drive signal (drive voltage signal) supplied to the piezoelectric elements  1031  and  1033  independent of the drive signal (drive voltage signal) supplied to the piezoelectric element  1030  of the elastic support unit  930 . 
     For example, when a part of the elastic support units  910 ,  920 , and  930  is much closer to or much farther from the second reflector  800  (the half-VCSEL element  100 ) than the elastic support unit of the other portion, a part of the elastic support unit is used. By making the drive signal (drive voltage signal) supplied to the piezoelectric element of the elastic support unit different from the drive signal (drive voltage signal) supplied to the piezoelectric element of other parts, the postures (elasticity) of the elastic support units  910 ,  920 ,  930  are made different from each other. Adjusting whether or not to deform, the degree and direction of elastic deformation, etc.), the plane parallelism of the first reflector  400  and the second reflector  800  (the half-VCSEL element  100  and the second The plane parallelism of the reflector  800 ) can be optimally set. 
     The drive signal supply unit  1200  can improve the plane parallelism of the first reflector  400  and the second reflector  800  (the plane parallelism of the half-VCSEL element  100  and the second reflector  800 ) based on the tilt of the first reflector  400  to the second reflector  800  by supplying independent drive signals (drive voltage signals) to the piezoelectric element  1010  (piezoelectric elements  1011  and  1013 ), the piezoelectric element  1020  (piezoelectric elements  1021  and  1023 ), and piezoelectric element  1030  (piezoelectric elements  1031  and  1033 ) of the elastic support units  910 ,  920 , and  930 , respectively, to elastically deform the elastic support unit  910 ,  920 , and  930 . 
     The drive signal supply unit  1200  can improve the half-VCSEL element (light emitter)  100  based on the intensity of light from the half-VCSEL element (light emitter)  100  by supplying independent drive signals (drive voltage signals) to the piezoelectric element  1010  (piezoelectric elements  1011  and  1013 ), the piezoelectric element  1020  (piezoelectric elements  1021  and  1023 ), and piezoelectric element  1030  (piezoelectric elements  1031  and  1033 ) of the elastic support units  910 ,  920 , and  930 , respectively, to elastically deform the elastic support units  910 ,  920 , and  930 . 
     There is a latitude in the method of detecting the intensity of light emitted by the half-VCSEL element (light emitter)  100 . For example, an external detection device (e.g., photodiode) that detects the reflectance of the first reflector  400  and the second reflector  800  can be used. 
     The tilt adjustment of the second reflector  800  by the drive signal supply unit  1200  can be performed at the time of manufacturing or mounting the tunable laser  10 . 
     Alternatively, after manufacturing or mounting, the tilt of the second reflector  800  can be adjusted by the drive signal supply unit  1200  at startup or at predetermined time interval according to surrounding environment (e.g., material or temperature). 
     The drive signal supply unit  1200  may be either an internal component or an external component of the tunable laser  10 , and there is a latitude in the specific aspect. The drive signal supply unit is not limited to the example of  FIG.  21   . 
       FIG.  22    is a diagram of an example of an operation in which different voltages are applied to the two connecting portions to give the reflector (e.g., the second reflector  800 ) a tilt of θ. 
     As illustrated in  FIG.  22   , when one connecting portion is displaced in the +Z direction and the other connecting portion is displaced in the −Z direction, the reflector (the second reflector  800 ) tilts by θ with respect to a tilt with non-driving state. 
     The generated title depends on the displacement amount Δz of the connecting portion and the distance between the connecting ends, and the relation is θ=arcsine(2·Δz/l). 
       FIG.  23    is a graph of the relation between the displacement amount of the connecting portion and the title angle obtained when the distance between the connecting ends is changed. 
     By appropriately setting the displacement amount of the connecting portion and the distance between the connecting ends, a tilt (of the first reflector  400  and the second reflector  800 ) actually generated in mounting process can be absorbed. 
     When the same offset voltage is further applied while different voltages are applied to a plurality of connecting portions, the mirror portion can be driven in the Z direction with maintaining its tile according to the linearity of the voltage and the displacement amount described above. 
     Therefore, sweeping of the wavelength with the appropriate tilt of the reflector for the laser oscillation maintained is possible. 
       FIG.  24 A  and  FIG.  24 B  are diagrams of an example of wavelength sweeping by the tunable laser  10  of the present embodiment. 
     The laser oscillation wavelength of the tunable laser  10  is defined by an emission spectrum (i.e., a wavelength distribution) characteristic in materials constituting the active layer  105  and the resonator structure. 
     The resonator structure is characterized by the distance G in the Z direction between the first reflector  400  and the second reflector  800 , and the refractive index and dimensions of the material existing between the first reflector  400  and the second reflector  800 . 
     The active layer  105  emits light with the current injected into the half-VCSEL element  100  through the electrodes, and the wavelength of the resonant light (i.e., the oscillation light) changes with variation of the distance G between the first reflector  400  and the second reflector  800  in the Z direction. 
       FIG.  24 A  is a diagram illustrating a state in which the resonance light (i.e., the oscillation light) is directed downward in the Z direction, and  FIG.  24 B  is a diagram illustrating a state in which the resonance light (i.e., the oscillation light) is directed upward in the Z direction. 
     Since the distance G between the first reflector  400  and the second reflector  800  in the Z direction can be changed by changing the position of the second reflector  800  as a movable reflector element in the Z direction, the wavelength of the resonant light (i.e., the oscillating light) can be modulated. 
     In particular, in the present embodiment, the distance G between the first reflector  400  and the second reflector  800  in Z the direction can be controlled with high accuracy by devising the support structure of the second reflector  800  by using the elastic support unit  900  and the piezoelectric element  1000 . 
     As a result, the linearity of the oscillation wavelength with respect to the drive voltage is improved and the light with a desired oscillation wavelength is obtained. 
     Furthermore, by applying independent drive signal (drive voltage signal) to the piezoelectric element  1010  (piezoelectric element  1011 ,  1013 ) of the elastic support unit  910 , the piezoelectric element  1020  (piezoelectric element  1021 ,  1023 ) of the elastic support unit  920 , and the piezoelectric element  1030  (piezoelectric elements  1031 ,  1033 ) of the elastic support unit  930 , the elastic support units  910 ,  920 , and  930  are elastically deformed, and the plane parallelism of the first reflector  400  and the second reflector  800  (i.e., the plane parallelism of the half-VCSEL element  100  and the second reflector  800 ) can be improved. 
       FIG.  25    is a graph of simulation results of the relation of the length of the gap (i.e., the distance) between the half-VCSEL element  100  (first reflector  400 ) and the second reflector  800 , and the oscillation wavelength. 
     The oscillation light emitted by the half-VCSEL element  100  is emitted to the first reflector  400  or the second reflector  800 , whichever has a lower reflectance. 
     Therefore, by adjusting the magnitude of the reflectance between the first reflector  400  and the second reflector  800 , the emission direction of the oscillating light by the half-VCSEL element  100  can be adjusted. 
     For example, when the reflectance of the first reflector  400  is lower than the reflectance of the second reflector  800 , the oscillating light is emitted downward in the Z direction ( FIG.  24 A ). When the reflectance of the first reflector  400  is higher than the reflectance of the second reflector  800 , the oscillating light is emitted upward in the Z direction ( FIG.  24 B ). 
     A bonding layer including multilayer metal films containing a plurality of metals is formed on the half-VCSEL element  100  (the first reflector  400 ) and the second reflector  800 , respectively. 
     For example, by bonding the bonding layers on each element with an atomic diffusion bonding method, a space can be obtained between the first reflector  400  and the second reflector  800 . 
     The active layer  105  of the half-VCSEL element  100  is displaced in the center of the resonator structure, which is a position corresponding to the antinode in the standing wave of electric field to obtain a highly stimulated emission probability. 
       FIG.  26    is a diagram of a tunable laser  10  according to a first modification of an embodiment of the present disclosure. 
     As illustrated in  FIG.  25   , one second reflector  800  is provided with a plurality of the half-VCSEL elements  100 . 
     Specifically, three half-VCSEL elements  100 X,  100 Y, and  100 Z adjacent to each other in the X direction on the XY plane are provided with one second reflector  800 . 
     The semiconductor substrate  500 , and the antireflection film  600  may be shared these three half-VCSEL elements  100 X,  100 Y, and  100 Z. 
     In the configuration of  FIG.  26   , when a current is injected into the three half-VCSEL elements  100 X,  100 Y, and  100 Z with the reflector (e.g., the second reflector  800 ) tilted, cavity length for each element is inclined, and the oscillation wavelength is dispersed. 
     Further, by controlling amount of tilt of the reflector, the magnitude of wavelength dispersion can be controlled. 
       FIG.  27    is a diagram of a tunable laser  10  according to a second modification of an embodiment of the present disclosure. 
     As illustrated in  FIG.  27   , a movable reflector structure  800 X for high-speed driving is added inside of the second reflector  800  as a movable reflector. 
     The length of the gap between the half-VCSEL element  100  (the first reflector  400 ) and the second reflector  800 , which are bonded together with the bonding layer  300  therebetween, is changed with the usage environment (e.g., temperature) of the tunable laser  10 . 
     The length change of this gap is several μm. A center of the oscillation wavelength is fixed by statically driving the meandering structure with a voltage prepared (preset) according to the usage environment (e.g., temperature) of the tunable laser  10 . 
     Furthermore, after fixing the center wavelength, the movable reflector structure  800 X for high-speed driving is displaced in the Z direction by several tens of nm in accordance with a modulation signal so as to sweep the oscillation wavelength within a certain bandwidth of the wavelength with keeping the center wavelength regardless of environmental changes. 
     As described above, by providing the second reflector  800  that handles a relatively large drive range and the movable reflector structure  800 X that handles a relatively small drive range (i.e., by using two ranges), the gap between the half-VCSEL element  100  (the first reflector  400 ) and the second reflector  800  can be controlled with higher accuracy. 
     This configuration can further improve the linearity of the oscillation wavelength and the drive voltage, and thus enables light having a desired oscillation wavelength. 
     As illustrated in  FIG.  22   , if the reflector (e.g., the second reflector  800 ) is driven in the Z direction without correction of its tilt, the loss of light in the resonator may increase because only part of the reflected light is incident on the active layer. This makes it difficult to produce laser oscillation in a wavelength region with a small gain, thus narrowing the wavelength sweep width. To deal with such an issue, the tilt of the reflector is corrected to minimize the loss of light caused during the light reflection, so as to increase both the wavelength sweep width and the light intensity. 
       FIG.  28 A  is a cross-sectional view of a movable reflector element and a half-VCSEL element, which are bonded together with the tilt uncorrected. The plane parallelism between the movable reflector element and the half-VCSEL element decreases partly because of an unevenness of the thickness of the bonding layer bonding the movable reflector element and the half-VCSEL element together. Such an unevenness in thickness occurs because the bonding layer already has an unevenness in thickness before bonding the movable reflector element and the half-VCSEL element together; or because heat or pressure applied to the movable reflector element, the half-VCSEL element, and the bonding layer during the bonding operation is unevenly distributed within a plane. 
       FIG.  28 B  is a cross-sectional view of a half-VCSEL element and a movable reflector element provided with a second reflector and a movable reflector structure for high-speed driving, which are bonded together. As illustrated in  FIG.  28 A , driving the elastic support units, which support the second reflector, and the piezoelectric elements on the elastic support units using the independent drive signals improves the plane parallelism that has been degraded. With the voltage maintained, the movable reflector structure for high-speed driving is temporally driven using a modulation signal in a continuous manner. This reduces or eliminates the loss of light during the light reflection and also achieves a high-speed wavelength sweep. 
       FIG.  29 A  is an illustration for describing adjustment of the tilt of the second reflector. A piezoelectric element on an elastic body connected to the second reflector is driven in accordance with a certain voltage signal, with respect to the normal to the second reflector (i.e., non-driving second reflector) that is not driven. In this case, the tilt a is created between the normal to the non-driving second reflector and the normal to the driving second reflector that is driven. Further, a point P on the straight line starting from the surface of the second reflector and along the direction of the normal to the non-driving second reflector changes its position according to the drive voltage applied to the piezoelectric element. For example, multiple piezoelectric elements are driven with independent voltage signals, and the tilt a and the position of the point P are controlled by changing the difference between relative or absolute values of the signals. 
       FIG.  29 B  is a graph of the relation between the piezoelectric elements and the voltage signals applied to the piezoelectric elements. In this example, three piezoelectric elements are driven in accordance with signals A, B, and C, which are independent of each other. The three voltage values change with the phase in a sinusoidal manner. For example, the voltage signals A, B, and C are given by the following equations where phase_ini_A, phase_ini_B, and phase_ini_C are the initial phases for the signals A, B, and C, respectively, and Amp denotes an amplitude: 
       Voltage_ A =Amp*sin(phase+phase_ini_ A ) 
       Voltage_ B =Amp*sin(phase+phase_ini_ B ) 
       Voltage_ C =Amp*sin(phase+phase_ini_ C ) 
     By giving a certain phase using the equations above, the voltage values of all the signals can be obtained. As the voltage value to be applied to a piezoelectric element is proportional to the displacement of the elastic body, the displacements of the elastic bodies can be unequal to each other by setting a certain phase. Accordingly, the tilt a and the position of the point P as illustrated in  FIG.  29 A  can be controlled, and thus the tilt of the second reflector can be adjusted at a higher accuracy. 
       FIG.  30    is a block diagram of a configuration of calibration of the plane parallelism between the movable reflector element and the half-VCSEL element, according to an embodiment.  FIG.  31    is a graph of the relation between a phase of a drive-signal source during calibration and optical power of the tunable laser, according to an embodiment. 
     The drive-signal source includes a voltage-signal source for driving the piezoelectric element in the movable reflector element constituting the tunable laser, and a current-signal source for injecting a current into the half-VCSEL element. When the above-described phase in the voltage signal source is swept in the voltage signal source while injecting a certain current into the half-VCSEL element, the tilt of the second reflector changes. At a certain tilt, the surfaces between the movable reflector element and the half-VCSEL element become infinitely close to parallel, and the loss of light during light reflection is reduced. When the loss of light is small, the intensity of naturally emitted or oscillation light emitted from the tunable laser increases. Accordingly, the light output intensity is maximized in a certain phase of the drive signal source. 
     Further, a photoelectric convertor generates a voltage according to the intensity of the incident light. The relation between the phase value and the optical output intensity is plotted using the phase value and voltage value of the drive source or the conversion constant (i.e., voltage generated with the unit optical output value) of the photoelectric convertor. Then, a phase condition for a high plane parallelism is determined by obtaining a phase value for which a voltage value or optical output value is maximum. Further, by adopting not only the optical output value but also the oscillation threshold current or the slope efficiency value used as the performance index of the tunable laser, calibration that can optimize each performance is possible. In addition, such a calibration is performed not only when the device using the tunable laser starts up, but also when external factors such as temperature, humidity, and atmospheric pressure change significantly, to drive the device while maintaining its performance. 
     For example, a temperature measuring device is disposed near a light source to measure the temperature of or near the tunable laser during its oscillation. A storage unit in  FIG.  30    stores, as a correlation formula or a table, the relation between the wavelength, the output power, and the temperature of the tunable laser during oscillation, and also stores the relation between the drive voltage and the oscillation wavelength of the piezoelectric elements. The processor determines a correction voltage for which the oscillation wavelength is constant with temperature, or the output power becomes maximum, based on the relation between the temperature and the wavelength measured and stored in the storage unit. According to that voltage, a drive signal source drives a piezoelectric element. 
     As described above, the optical device of the present embodiment includes the first reflector, the second reflector, at least three elastic support units that support the second reflector, and piezoelectric elements provided the at least three elastic support units, respectively. 
     Then, by supplying independent drive signals to the piezoelectric elements of at least three elastic support units and elastically deforming at least three elastic support units independently, the plane parallelism of the first and second reflectors (alternatively, the plane parallelism between the light emitting portion and the second reflector) is changed. 
     By controlling the control portion using the piezoelectric drive independently, and by supporting the support unit and the mirror by the connecting portion provided with at least three or more of the drive portions, the tilt of the light emitting portion (e.g., the half-VCSEL element) and the mirror can be controlled freely. 
     This improves the plane parallelism of the two reflectors (the first and the second reflectors) or the plane parallelism of the light emitting portion (e.g., the half-VCSEL element) and the reflector (second reflector). 
     As a result, the threshold current of the light emitter decreases, and the light emitting intensity increases. 
     The present disclosure is not limited to the above-described embodiments, and can be modified in various ways. 
     The size, the shape, the function, and the like of the components illustrated in the accompanying drawings are not limited to the above embodiments, and can be appropriately changed within the effects of the present invention. 
     In addition, the size, the shape, the function, and the like of the components illustrated in the accompanying drawings can be appropriately modified and implemented as long as the size, the shape, the function, and the like of the components illustrated in the accompanying drawings does not deviate from the purpose of the present invention. 
     In the above embodiment, the case where three elastic support units for supporting the second reflector are provided apart from each other around the second reflector has been described as an example. There is a latitude in the number of elastic support units provided apart from each other, and various design changes are possible. 
     For example, four, five, or six or more elastic support units that support the second reflector may be provided around the second reflector (at least three may be provided). 
     In the above embodiment, at least three elastic support units are included, and each support unit has the meandering structure that includes two arms extending adjacent to each other and one folded portion connecting the ends on the same side of the two arms in the extending direction, and the piezoelectric elements are formed on the two arms. 
     In the above embodiment, at least three elastic support units are included, and each support unit has the meandering structure that may include three or more of arms extending adjacent to each other and two or more of folded portions each connecting the ends on the same side of two adjacent arms of the three or more of arms in the extending direction, and the piezoelectric elements may be formed on the three or more of arms. 
     In other words, each of at least three elastic support units has a plurality of the arms extending adjacent to each other and one or more folded portion each connecting the ends on the same side of two adjacent arms of the plurality of arms in the extending direction, and the piezoelectric elements are formed on at least three elastic support units, respectively. 
     In the above embodiment, the piezoelectric elements are provided so as not to be arranged in a direction orthogonal to the extending direction of the plurality of arms of at least three elastic support units. 
     The piezoelectric elements may be provided so as to be arranged in a direction orthogonal to the extending direction of the plurality of arms of at least three elastic support units. 
     In the above embodiment, the plurality of arms of at least three elastic support units include the displacement defining portions that do not elastically deform when a drive voltage is applied to the piezoelectric elements, wherein and the plurality of the displacement defining portions do not include the piezoelectric element among the plurality of arms. 
     The displacement defining portion with the piezoelectric element may be a portion where the drive voltage is not applied among the plurality of arms. 
     The present invention can be implemented in any convenient form, for example using dedicated hardware, or a mixture of dedicated hardware and software. The present invention may be implemented as computer software implemented by one or more networked processing apparatuses. The processing apparatuses include any suitably programmed apparatuses such as a general purpose computer, a personal digital assistant, a Wireless Application Protocol (WAP) or third-generation (3G)-compliant mobile telephone, and so on. Since the present invention can be implemented as software, each and every aspect of the present invention thus encompasses computer software implementable on a programmable device. The computer software can be provided to the programmable device using any conventional carrier medium (carrier means). The carrier medium includes a transient carrier medium such as an electrical, optical, microwave, acoustic or radio frequency signal carrying the computer code. An example of such a transient medium is a Transmission Control Protocol/Internet Protocol (TCP/IP) signal carrying computer code over an IP network, such as the Internet. The carrier medium also includes a storage medium for storing processor readable code such as a floppy disk, a hard disk, a compact disc read-only memory (CD-ROM), a magnetic tape device, or a solid state memory device. 
     Each of the functions of the described embodiments may be implemented by one or more processing circuits or circuitry. Processing circuitry includes a programmed processor, as a processor includes circuitry. A processing circuit also includes devices such as an application specific integrated circuit (ASIC), a digital signal processor (DSP), a field programmable gate array (FPGA), and conventional circuit components arranged to perform the recited functions. 
     This patent application is based on and claims priority to Japanese Patent Application No. 2020-093946, filed on May 29, 2020, Japanese Patent Application No. 2020-093949, filed on May 29, 2020, Japanese Patent Application No. 2021-079475, filed on May 10, 2021, and Japanese Patent Application No. 2021-079476, filed on May 10, 2021 in the Japan Patent Office, the entire disclosure of which are hereby incorporated by reference herein. 
     REFERENCE SIGNS LIST 
     
         
           10  Tunable laser (optical device) 
           100  Half-VCSEL element (light emitter, first substrate) 
           200  Movable reflector element (second substrate) 
           400  First reflector (first reflecting mirror) 
           800  Second reflector (second reflecting mirror) 
           900 ,  910 ,  920 ,  930  Elastic support unit 
           911 ,  921 ,  931  First arm 
           911 B,  921 B,  931 B Displacement defining portion 
           913 ,  923 ,  933  Second arm 
           913 B,  923 B,  933 B,  943 B Displacement defining portion 
           912 ,  922 ,  932 ,  942  Folded portion 
           1000 ,  1010 ,  1011 ,  1013 ,  1020 ,  1021 ,  1023 ,  1030 ,  1011 X,  1013 X,  1021 X,  1023 X,  1031 ,  1033 , 
           1041 ,  1043  Piezoelectric element 
           1200  Drive signal supply unit (drive voltage supply unit)