Patent Publication Number: US-2017357075-A1

Title: Optical element

Description:
TECHNICAL FIELD 
     A technique disclosed here relates to an optical element. 
     BACKGROUND ART 
     A typically known optical element has an actuator drive a mirror. A known optical filter device receives incident light, and let portion of the incident light exit such that the exiting light has a specific wavelength. 
     For example, PATENT DOCUMENT 1 discloses an optical filter device including two mirrors spaced away from each other, and having an actuator adjust the space between the two mirrors to change the wavelength of exiting light. One of the mirrors is driven by electrostatic force generated between a pair of electrodes arranged in parallel. This optical filter device previously obtains the relationship of a wavelength of the exiting light to a drive voltage for generating the electrostatic force, and stores the relationship. Based on the relationship, the optical filter device selects a drive voltage corresponding to a desired wavelength. In addition, this optical filter device corrects the drive voltage based on a wavelength of actually exiting light to output light having a desired wavelength. 
     CITATION LIST 
     Patent Documents 
     PATENT DOCUMENT 1: Japanese Unexamined Patent Publication No. 2013-152489 
     SUMMARY OF THE INVENTION 
     Technical Problem 
     An optical element having an actuator drive a mirror is required to accurately detect displacement of the mirror. For accurately controlling the wavelength of the exiting light in the above optical filter device, a possible option is to detect the displacement of the two mirrors and precisely control the space between the mirrors, other than to correct the drive voltage based on the wavelength of the actually exiting light as described above. Furthermore, not only for optical filter devices but also for optical elements in general, precision is required in detecting displacement of a moving unit driven by an actuator. 
     A technique disclosed here is conceived in view of the above issues, and attempts to precisely detect displacement of a moving unit in an optical element. 
     Solution to the Problem 
     An optical element disclosed here includes: a moving unit; an actuator driving the moving unit; and a detection electrode detecting displacement of the moving unit, the detection electrode including: a movable comb electrode including movable combs and connected to the moving unit; and a stationary comb electrode including stationary combs facing the movable combs in parallel with each other, and the movable combs being displaced in parallel with the stationary combs when the movable comb electrode is displaced together with the moving unit. 
     Such features make it possible to detect the displacement of the moving unit based on the change in the capacitance between movable comb electrode and the stationary comb electrode. 
     In detecting the change in capacitance between two electrodes, another possible option is to arrange two plate electrodes in parallel with each other, and detect the capacitance created due to the change in the space between the two plate electrodes. However, the capacitance between the plate electrodes is inversely proportional to the space, and the wider the space is, the less precise the detection of the capacitance is. 
     In contrast, the use of comb electrodes solves the problem of the plate electrodes. In the comb electrodes, the movable combs of the movable comb electrode and the stationary combs of the stationary comb electrode face each other without contact. In this state, the movable comb electrode is displaced such that the overlapping areas of the movable combs and the stationary combs change, followed by the change in the capacitance between the movable combs and the stationary combs. Since the capacitance of the comb electrodes is proportional to the overlapping areas, the change in capacitance may be precisely detected. 
     In addition, the movable combs are displaced in parallel with the stationary combs. Such a feature makes it possible to detect the change in the capacitance more precisely. 
     Specifically, the movable comb electrode tilts with respect to the stationary comb electrode when displacement of a member is detected based on the capacitance between the movable comb electrode and the stationary comb electrode. Here, an overlapping portion of a movable comb and a stationary comb is not always shaped into a rectangle. The overlapping area changes in shape such as a rectangle, a triangle, and a polygon having five angles or more, depending on a tilted state of the movable comb. Accordingly, the overlapping area does not always change in proportion to the displacement of the movable comb. As a result, the relationship of the displacement of the member corresponding to the change in the capacitance changes depending on a tilted state of the movable comb, making it difficult to control the displacement of the member. In addition, in the tilting of the movable comb electrode, the displacement with respect to the tilt angle becomes greater as the tilted portion is farther distant from a center of the tilt. If the displacement of the member becomes great, a portion, of the movable comb, distant from the center of the tilt does not face the stationary comb. Hence, the distance keeps the capacitance from changing. Specifically, the configuration in which the movable comb electrode tilts does not effectively utilize the overlapping area of the movable comb and the stationary comb for detecting the change of the capacitance. 
     Whereas, in the case of a configuration in which a movable comb is displaced in parallel with a stationary comb, an overlapping area of the movable comb and the stationary comb changes substantially in proportion to the displacement of the movable comb. Such a feature makes it possible to detect the displacement of the moving unit with uniform precision no matter how much the displacement is. Specifically, the precision in detecting the displacement of the moving unit may be substantially equal throughout an area in which the displacement of the moving unit is detectable. As a result, precision may improve in detecting the displacement of the moving unit throughout the displacement detectable area. Moreover, the relationship of a displacement of the moving unit to a change in the capacitance is uniform throughout the displacement detectable area. Such a feature allows the displacement of the moving unit to be more controllable. In addition, the displacement of movable combs is substantially the same as that of the moving unit. Such a feature makes it possible to effectively utilize the areas of the movable combs and the stationary combs to detect the change of the capacitance. 
     Advantages of the Invention 
     The optical element may precisely detect the displacement of a moving unit. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view of an optical filter device. 
         FIG. 2  is a plan view of a first unit. 
         FIG. 3  is an enlarged plan view of hinges and a detection electrode. 
         FIG. 4  is a perspective view of the detection electrode in an initial state. 
         FIG. 5  is a schematic view illustrating how movable combs and stationary combs face each other in the initial state. 
         FIG. 6  is a perspective view of the detection electrode when a first mirror is displaced. 
         FIG. 7  is a schematic view illustrating how the movable comb and the stationary comb face each other when the first mirror is displaced. 
         FIG. 8  is a plan view of a shutter device. 
     
    
    
     DESCRIPTION OF EMBODIMENT 
     An embodiment as an example is described in detail below with reference to the drawings. 
       FIG. 1  is a cross-sectional view of an optical filter device  1000 .  FIG. 2  is a plan view of a first unit  100 . Note that  FIG. 1  is a cross-sectional view taken along line A-A in  FIG. 2 . 
     The optical filter device  1000  includes: the first unit  100  having a first mirror  101 ; a second unit  200  having a second mirror  201  facing the first mirror  101 ; and a controller  900 . The first unit  100  and the second unit  200  lie on top of each other. Each of the first mirror  101  and the second mirror  201  lets portion of incident light transmit. Of the incident light into the second mirror  201 , the optical filter device  1000  outputs from the first mirror  101  light having a wavelength corresponding to a space between the first mirror  101  and the second mirror  201 . The optical filter device  1000  adjusts the space between the first mirror  101  and the second mirror  201  to adjust the wavelength of the exiting light. Specifically, the optical filter device  1000  is a variable wavelength filter device which employs the principle of a Fabry-Pérot resonator. The optical filter device  1000  is an example of an optical element. 
     The first unit  100  includes: the first mirror  101 ; two actuators  300  driving the first mirror  101  to change the space between the first mirror  101  and the second mirror  201 ; two detection electrodes  400  detecting displacement of the first mirror  101 ; and a frame  500 . 
     The first unit  100  is made of a Silicon on Insulator (SOI) substrate B. The SOI substrate B includes a first silicon layer b 1  formed of monocrystalline silicon, an oxide film layer b 2  formed of SiO 2 , and a second silicon layer b 3  formed of monocrystalline silicon. These layers are stacked on top of one another in the stated order. 
     The frame  500  is shaped into a substantially rectangular frame in a planar view. The frame  500  includes the first silicon layer b 1 , the oxide film layer b 2 , and the second silicon layer b 3 . Note that the frame  500  has a surface to the first silicon layer b 1 . On the surface, an SiO 2  film  318  is deposited. This SiO 2  film  318  is the same film as the SiO 2  film  318  of an actuator  300  to be described later. 
     The first mirror  101  includes: a mirror body  102 ; two attachments  103 ; and a cylinder  104  provided to the mirror body  102 . The mirror body  102  is shaped into a substantial rectangle in a planar view. The mirror body  102  is formed of the first silicon layer b 1  and a dielectric multilayer film  121  stacked on a surface of the first silicon layer b 1 . The dielectric multilayer film  121  includes high refractive index layers and low refractive index layers alternately stacked one on top of another. 
     For the sake of explanation, the following axes are defined: an X-axis passing through a center C of the mirror body  102  and lying in parallel with a pair of sides, of the mirror body  102 , facing each other; a Y-axis passing through the center C of the mirror body  102  and lying in parallel with another pair of sides, of the mirror body  102 , facing each other; and a Z-axis passing through the center C of the mirror body  102  and running perpendicular to both the X-axis and the Y-axis. Moreover, in the Z-axis direction, an upside in  FIG. 1  may be referred to as “the upside”, and a downside in  FIG. 1  may be referred to as “the downside.” 
     Each of the two attachments  103  is provided to a corresponding one of a pair of sides, of the mirror body  102 , facing each other. The pair of sides lies in parallel with the Y-axis. One of the attachments  103  extends in the X-axis direction from an end of a first side a 1  (an end to a second side a 2 ) which is in parallel with the Y-axis. The attachment  103  then bends and extends in parallel with the first side a 1 , leaving a space between the attachment  103  itself and the first side a 1 . The other one of the attachments  103  extends in the X-axis direction from an end of a third side a 3  (an end to a fourth side a 4 ) which faces the first side a 1 . The attachment  103  then bends and extends in parallel with a third side a 3 , leaving a space between the attachment  103  itself and the third side a 3 . The attachments  103  are formed of the first silicon layer b 1 . 
     The cylinder  104  is formed to cylindrically extend in the Z-axis direction, and is provided to a surface, of the mirror body  102 , opposite the dielectric multilayer film  121 . The cylinder  104  is formed of the oxide film layer b 2  and the second silicon layer b 3 . Specifically the cylinder  104  is integrally formed with the mirror body  102 . Such a feature improves the flatness of the mirror body  102 . 
     In the frame  500 , the two actuators  300  are arranged in the Y-axis direction with the first mirror  101  sandwiched therebetween. Each of the actuators  300  has a base end connected to the frame  500 , and a tip end to be a free end; that is, the actuator  300  is of a cantilever configuration. To the tip end (the free end), the first mirror  101  is connected. Each of the actuators  300  includes two beams connected together as if a single beam were folded into two in a principle surface of the SOI substrate B. The two beams include a first beam  301  curved toward one direction with respect to the principle surface, and a second beam  302  having no curve or curved less than the first beam  301 . The first beam  301  and the second beam  302  are arranged in parallel with each other. Note that, in  FIG. 2 , when the two actuators  300  are distinguished from each other, the actuator  300  above the first mirror  101  is referred to as a first actuator  300 A and the actuator  300  below the first mirror  101  is referred to as a second actuator  300 B. 
     Specifically, in the first actuator  300 A, the first beam  301  has a base end secured to the frame  500 . In  FIG. 2 , the first beam  301  extends from the frame  500  in the X-axis toward the observer&#39;s right. The first beam  301  has a tip end to which the second beam  302  is connected. The second beam  302  turns back from the first beam  301 , and extends in the X-axis direction toward the observer&#39;s left. The second beam  302  has a tip end bent toward the first mirror  101  in the Y-axis direction and extending. The tip end then enters the space between the mirror body  102  and the attachment  103  of the first mirror  101 , and extends in parallel with the attachment  103 . To the tip end of the second beam  302 , the first mirror  101  is connected. 
     Meanwhile, in the second actuator  300 B, the base end of the first beam  301  is secured to the frame  500 . The first beam  301  extends from the frame  500  in the X-axis direction toward the observer&#39;s left. The first beam  301  has a tip end to which the second beam  302  is connected. The second beam  302  turns back from the first beam  301 , and extends in the X-axis direction toward the observer&#39;s right. The second beam  302  has a tip end bent toward the first mirror  101  in the Y-axis direction and extending. The tip end then enters the space between the mirror body  102  and the attachment  103 , and extends in parallel with the attachment  103 . To the tip end of the second beam  302 , the first mirror  101  is connected. 
     Specifically, in the first actuator  300 A and the second actuator  300 B, the first beams  301  are connected to the frame  500 , and the second beams  302  are connected to the first mirror  101 . Note that the first actuator  300 A and the second actuator  300 B are opposite in direction in which the first beams  301  extend from the frame  500  and the second beams  302  extend from the first beams  301 . 
     Described next is a configuration of each beam. The first actuator  300 A and the second actuator  300 B are similar in configuration of each beam. For example, the first beams  301  of the first actuator  300 A and the first beams  301  of the second actuator  300 B are similar in configuration. 
     Each first beam  301  includes a beam body  313  and a piezoelectric element  314  stacked on a surface of the beam body  313 . 
     The beam body  313  is shaped into a bar whose cross-section is rectangular. The beam body  313  is formed of the first silicon layer b 1 . 
     The piezoelectric element  314  is provided to a surface of the beam body  313 . The SiO 2  film  318  is stacked on the surface of the beam body  313 , and the piezoelectric element  314  is stacked on the SiO 2  film  318 . The piezoelectric element  314  includes a lower electrode  315 , an upper electrode  317 , and a piezoelectric body layer  316  sandwiched between the lower electrode  315  and the upper electrode  317 . The lower electrode  315 , the piezoelectric body layer  316 , and the upper electrode  317  are stacked on top of another on the SiO 2  film  318  in the stated order. The piezoelectric element  314  and the SOI substrate B are formed of different materials. Specifically, the lower electrode  315  is formed of a Pt/Ti film or an Ir/Ti film. The piezoelectric body layer  316  is formed of lead zirconate titanate (PZT). The upper electrode  317  is formed of an Au/Ti film. 
     When a voltage is applied to the upper electrode  317  and the lower electrode  315  of the piezoelectric element  314 , the surface, of the beam body  313 , on which the piezoelectric element  314  is stacked expands and contracts. The beam body  313  then curves with the piezoelectric element  314  facing inward. 
     The second beam  302  includes the beam body  313  and a dummy film  319 . The beam body  313  has a surface on which the SiO 2  film  318  is deposited, and the dummy film  319  is stacked on the SiO 2  film  318 . The dummy film  319  includes the lower electrode  315 , the piezoelectric body layer  316 , and the upper electrode  317 . Specifically, the dummy film  319  and the piezoelectric element  314  are similar in configuration. However, no voltage is applied to the dummy film  319  and the dummy film  319  does not act as a piezoelectric element. Specifically, the lower electrode  315 , the piezoelectric body layer  316 , and the upper electrode  317  of the dummy film  319  are respectively insulated from the lower electrode  315 , the piezoelectric body layer  316 , and the upper electrode  317  of the piezoelectric element  314 . Even if a voltage is applied to the piezoelectric element  314 , such a configuration keeps the voltage from being applied to the dummy film  319 , and the dummy film  319  does not act as a piezoelectric element. 
     The dummy film  319  is provided to cancel a warp of beams in an initial stage and by temperature change. Specifically, the SiO 2  film  318 , the lower electrode  315 , the piezoelectric body layer  316 , and the upper electrode  317  are deposited by such a technique as sputtering on the surface of the beam body  313  included in the first beam  301  and formed of the first silicon layer b 1 . After the film, the electrodes, and the layer are deposited, the first beam  301  can warp due to, for example, a temperature change during the deposition. For example, a surface, of the beam body  313 , on which a thin film is deposited can contract, causing the first beam  301  to warp upward with the surface facing inward. However, for example, the first beam  301  is connected to the second beam  302  as if a single beam were folded into two. Hence, the dummy film  319  similar to the piezoelectric element  314  is also deposited on the beam body  313  of the second beam  302 . Specifically, the first beam  301  and the second beam  302  warp, while being arranged substantially in parallel with each other. As a result, the tip end of the first beam  301  and the base end of the second beam  302  rise; however, the tip end of the second beam  302  comes back to the same position, along the thickness of the SOI substrate B, as that of the base end of the first beam  301 . Hence, at the tip end of the second beam  302 , such a feature cancels the displacement of the SOI substrate B along the thickness due to the warp in the initial stage. Moreover, the first beam  301  includes such materials as silicon, SiO 2 , and Pt/Ti, each having a different coefficient of thermal expansion (CTE), stacked on top of another. Thus, a change in temperature causes the films to contract based on their respective CTEs. Hence, the first beam  301  can warp. However, the second beam  302  is similar in stack structure to the first beam  301 , causing the second beam  302  to warp as the first beam  301  does. As a result, the warp of the first beam  301  is reduced by the second beam  302 , similar to the warp in the initial stage. 
     Each of the second beams  302  is connected to a corresponding one of the attachments  103  of the first mirror  101  via two hinges  105 . 
       FIG. 3  is an enlarged plan view of the hinges  105  and a detection electrode  400 . Formed of a meandering line, each of the hinges  105  is elastic. Specifically, the hinge  105  includes straight lines and a turn connecting ends of neighboring straight lines. As a whole, the hinge  105  has a meandering form. Since the straight lines extend along the Y-axis, the hinge  105  tends to curve about an axis along the Y-axis. The hinge  105  has an end connected to the tip end of the second beam  302 , and another end connected to a portion, of the attachment  103 , facing the mirror body  102 . The hinge  105  is an example of a connector. 
     As illustrated in  FIG. 2 , the two hinges  105  are arranged to face each other across a straight line L 1  passing through the center C of the mirror body  102  and extending in the X-axis direction. The two hinges  105  are equally spaced from the straight line L 1  in the Y-axis direction. 
     The frame  500  is provided with drive terminals for applying a voltage to the first actuator  300 A and the second actuator  300 B. Specifically, the frame  500  has a surface provided with first feed terminals  511  and second feed terminals  512 . One of the first feed terminals  511  is wired to the upper electrode  317  of the first beam  301  in the first actuator  300 A. The other first feed terminal  511  is wired to the upper electrode  317  of the first beam  301  in the second actuator  300 B. Furthermore, one of the second feed terminals  512  is electrically connected to the lower electrode  315  of the first beam  301  in the first actuator  300 A. The other second feed terminal  512  is electrically connected to the lower electrode  315  of the first beam  301  in the second actuator  300 B. On a SiO 2  film  128  of the frame  500 , the lower electrode  315  and the piezoelectric body layer  316  are partially stacked. On the piezoelectric body layer  316 , the first feed terminals  511  and their wiring, and the second feed terminals  512  are provided. Note that in a portion, of the piezoelectric element  314 , to which a second feed terminal  512  is provided, an opening (illustrated by a broken line in  FIG. 2 ) is formed to reach a lower electrode  315 . Each of the second feed terminals  512  is provided to cover this opening, and electrically connected to a corresponding one of the lower electrodes  315 . Applying a voltage to a pair of a first feed terminal  511  and a second feed terminal  512  allows the voltage to be applied to the piezoelectric element  314  of the first actuator  300 A. Applying a voltage to another pair of a first feed terminal  511  and a second feed terminal  512  allows the voltage to be applied to the piezoelectric element  314  of the second actuator  300 B. 
     The detection electrode  400  includes a movable comb electrode  410  connected to the first mirror  101 , and a stationary comb electrode  420  provided to the frame  500 . 
     The movable comb electrode  410  includes a base  411  connected to the first mirror  101 , and movable combs  414  extending from the base  411 . The base  411  is connected to the attachment  103  and cantilevered. The base  411  includes a first base portion  412 , and second base portions  413 . The first base portion  412  extends on the straight line L 1  passing through the center C of the first mirror  101  and running along the X-axis. The second base portions  413  branch off, from portions of the first base portion  412 , in opposed directions relative to the Y-axis direction. The movable combs  414  branch off, and extend, from each of the second base portions  413 , in opposed directions relative to the X-axis direction. The movable combs  414  extend in parallel with one another. The movable comb electrode  410  is formed of the first silicon layer b 1 . 
     The stationary comb electrode  420  includes a base  421  connected to the frame  500 , and stationary combs  424  extending from the bases  421 . The base  421  is cantilevered and extends from the frame  500 . The base  421  includes two first base portions  422 , and multiple second base portions  423 . The two first base portions  422  extend in parallel with each other along the X-axis, so that the first base portion  412  of the movable comb  414  is sandwiched between the two first base portions  422 . The second base portions  423  branch off, from portions of each first base portion  422 , in the Y-axis direction toward the first base portion  412 . The second base portions  413  of the movable comb electrode  410  and the second base portions  423  are alternately arranged along the X-axis. The stationary combs  424  branch off, and extend, from each of the second base portions  423 , in opposed directions relative to the X-axis direction. The stationary comb electrode  420  is formed of the first silicon layer b 1 . Note that the stationary comb electrode  420  is insulated from the movable comb electrode  410 . Specifically, in the first silicon layer b 1 , the portion in which the stationary comb electrode  420  is formed is physically separated from its surrounding. 
     Hence, the movable combs  414  and the stationary combs  424  are interleaved each other. Specifically, the movable combs  414  and the stationary combs  424  are alternately arranged along the Y-axis. The movable combs  414  and the stationary combs  424  extend in parallel with each other in the X-axis direction, and face each other at spaced intervals along the Y-axis. 
     The surface of the first silicon layer b 1  in the frame  500  is provided with detection terminals for detecting capacitance between the movable comb electrode  410  and the stationary comb electrode  420 . Specifically, in the first silicon layer b 1 , a first detection terminal  521  is provided to a portion which is electrically conductive with the portion in which the movable comb electrode  410  is formed. Only one first detection terminal  521  is provided and shared with two movable comb electrodes  410 . Moreover, in the first silicon layer b 1 , second detection terminals  522  are provided to a portion which is electrically conductive with the portion in which the stationary comb electrode  420  is formed. Two second detection terminals  522  are provided so that each of the two terminals corresponds to one of two stationary comb electrodes  420 . 
     When the first mirror  101  is displaced, the movable comb electrode  410  is also displaced, followed by the displacement of the first mirror  101 . The details thereof will be described later. As a result, the capacitance between the movable comb electrode  410  and the stationary comb electrode  420  changes. This change in capacitance is detected via the first detection terminal  521  and the second detection terminals  522 . 
     Described next is a configuration of the second unit  200 . 
     The second unit  200  includes the second mirror  201 , and a frame  205  supporting the second mirror  201 . The second unit  200  is formed of a silicon substrate b 4 . 
     The frame  205  is shaped into a substantially rectangular frame in a planar view. In a planar view, the frame  205  is similar in shape to the frame  500  of the first unit  100 . 
     The second mirror  201  includes a mirror body  202  shaped into a substantial rectangle in a planar view. The mirror body  202  is formed of a silicon layer b 4  and a dielectric multilayer film  221  stacked on a surface of the silicon layer b 4 . The mirror body  202  is not provided with the cylinder  104  provided to the first mirror  101 ; however, the silicon layer b 4  of the mirror body  202  is thicker than the first silicon layer b 1  of the mirror body  102 . Such a feature ensures the flatness of the mirror body  202 . The dielectric multilayer film  221  is provided to a surface, of the silicon layer b 4  of the mirror body  202 , facing the first mirror  101 . The dielectric multilayer film  221  includes high refractive index layers and low refractive index layers alternately stacked one on top of another. 
     Moreover, protrusions  241  are provided to a surface, of the of the mirror body  202 , facing the first mirror  101 . The protrusions  241  are arranged at spaced intervals on a circumference of the first mirror  101  in the circumferential direction. These protrusions  241  face the first mirror  101  when the first unit  100  and the second unit  200  are laid on top of each other. Providing the protrusions  241  reduces a contact area between the first mirror  101  and the second mirror  201 , successfully keeping both of the mirrors from sticking together. 
     The second mirror  201  is connected to the frame  205  with the silicon layer b 4  extending into a flat-plate shape. 
     The first unit  100  and the second unit  200  in the above configuration are laid on top of each other, and the frame  500  and the frame  205  are bonded together via an adhesive. Here, the first unit  100  and the second unit  200  are laid on top of each other, with the dielectric multilayer film  221  of the second mirror  201  and the dielectric multilayer film  121  of the first mirror  101  facing each other. Such a feature allows the first mirror  101  and the second mirror  201  to be arranged in substantially parallel with each other at a spaced interval. Note that the frame  500  and the frame  205  may be bonded not with an adhesive but with another technique such as anodic boding. 
     The controller  900  includes a power source other than a processor and a memory, and controls the optical filter device  1000 . The controller  900  supplies the actuators  300  with the drive voltage to cause the actuators  300  to adjust the space between the first mirror  101  and the second mirror  201 . 
     Described next is how the optical filter device  1000  operates.  FIG. 4  is a perspective view of the detection electrode  400  in an initial state.  FIG. 5  is a schematic view illustrating how the movable combs  414  and the stationary combs  424  face each other in the initial state.  FIG. 6  is a perspective view of the detection electrode  400  when the first mirror  101  is displaced.  FIG. 7  is a schematic view illustrating how the movable combs  414  and the stationary combs  424  face each other when the first mirror  101  is displaced. 
     In the optical filter device  1000 , light enters the second mirror  201 . The light passing through the second mirror  201  enters between the second mirror  201  and the first mirror  101 . The light entering between the first mirror  101  and the second mirror  201  is reflected off the mirrors multiple times, and light having a wavelength corresponding to a space between the first mirror  101  and the second mirror  201  is output from the first mirror  101 . 
     Here, the first mirror  101  is displaced and the space between the first mirror  101  and the second mirror  201  is adjusted. Such adjustment allows for a change in the wavelength of the light exiting from the first mirror  101 . 
     Specifically, the controller  900  applies a drive voltage to the first feed terminals  511  and the second feed terminals  512 . This drive voltage is applied to the piezoelectric element  314  of the first actuator  300 A and the piezoelectric element  314  of the second actuator  300 B, such that the first beams  301  of the first actuator  300 A and the second actuator  300 B curve. Each of the first beam  301  warps upward with respect to the surface of the SOI substrate B (warps toward the piezoelectric element  314 ), with the piezoelectric element  314  facing inward. Meanwhile, the second beam  302  does not practically curve, and is left substantially straight. Specifically, the first beam  301  extend from the frame  500  to warp upward, and, at the tip end of the first beam  301 , the second beam  302  turns to extend substantially straight. Since the tip end of the first beam  301  slopes obliquely upward, the second beam  302  turning at the tip end of the first beam  301  also has the same slope as the tip end of the first beam  301  has. Specifically, the second beam  302  extends obliquely downward and subsequently straight. The tip end of the second beam  302  is positioned below the base end of the first beam  301 ; that is, below the surface of the SOI substrate B. As a result, the attachment  103  included in the first mirror  101  and to which the second beam  302  is connected also moves downward, opening the space between the first mirror  101  and the second mirror  201 . Note that, compared with the state before the application of the drive voltage, the tip end of the second beam  302  is slightly displaced inward along the X-axis (i.e., toward the center C of the first mirror  101 .) This displacement is absorbed by the hinge  105  extending along the X-axis. 
     Here, the controller  900  adjusts the drive voltage based on the result of detection by the detection electrode  400  to displace the first mirror  101  while keeping the first mirror  101  in substantially parallel with the second mirror  201 . 
     Specifically, the wavelength of the light exiting from the optical filter device  1000  (hereinafter referred to as an “output wavelength”) depends on the space between the first mirror  101  and the second mirror  201 . The space between the first mirror  101  and the second mirror  201  is determined based on a displacement of the first mirror  101 . The first mirror  101  has the movable comb electrode  410  integrally formed therewith. Hence, when the first mirror  101  is displaced, the movable comb electrode  410  is also displaced together with the first mirror  101 . The displacement in the movable comb electrode  410  changes overlapping areas S of the movable combs  414  and the stationary combs  424  corresponding to the respective movable combs  414  (hereinafter referred to as an “overlapping area”), changing the capacitance between the movable comb electrode  410  and the stationary comb electrode  420 . Specifically, the wavelength of the light exiting from the optical filter device may be changed through the adjustment of the space between the first mirror  101  and the second mirror  201 . The space between the first mirror  101  and the second mirror  201  may be detected based on the capacitance between the movable comb electrode  410  and the stationary comb electrode  420 . 
     Thus, the controller  900  previously stores in the memory (i) a drive voltage corresponding to an output wavelength and provided to the actuators  300 , and (ii) a capacitance of the detection electrode  400 . When the output wavelength is set, the controller  900  reads from the memory a drive voltage corresponding to the output wavelength, and applies the drive voltage to each of the first actuator  300 A and the second actuator  300 B. Then, based on the capacitance to be detected via the detection electrode  400 , the controller  900  performs feedback control on the drive voltage. 
     Specifically, one of the two movable comb electrodes  410  is provided to the attachment  103  included in the first mirror  101 , and to which the first actuator  300 A is attached. The other movable comb electrode  410  is provided to the attachment  103  included in the first mirror  101 , and to which the second actuator  300 B is attached. In other words, the one movable comb electrode  410  is displaced in response to the displacement of the first mirror  101  mainly by the first actuator  300 A. The other movable comb electrode  410  is displaced in response to the displacement of the first mirror  101  mainly by the second actuator  300 B. Hence, the controller  900  controls (i) a drive voltage applied to the first actuator  300 A based on the capacitance of one of the detection electrodes  400 , and (ii) a drive voltage applied to the second actuator  300 B based on the capacitance of the other detection electrode  400 . Specifically, the controller  900  adjusts the respective drive voltages for the first actuator  300 A and the second actuator  300 B so that the capacitance for each detection electrode  400  corresponds to a desired output wavelength. As a result, the first mirror  101  is in substantially parallel with the second mirror  201 , and the space between the first mirror  101  and the second mirror  201  is set to correspond to a desired output wavelength. 
     In this configuration, the movable combs  414  are displaced in parallel with the stationary combs  424 . Such a feature makes it possible to precisely detect the capacitance throughout a range of motion of the first mirror  101 . 
     Specifically, the movable comb electrode  410  and the stationary comb electrode  420  are formed of the same first silicon layer b 1 . In the initial state; that is, when the first mirror  101  is not displaced, the movable comb electrode  410  and the stationary comb electrode  420  are positioned on the same plane as illustrated in  FIGS. 4 and 5 . This plane is imaginary, and hereinafter referred to as “reference plane P.” The reference plane P is in parallel with the surface of the first silicon layer b 1 . Here, as illustrated in  FIG. 5 , an overlapping area S of each movable comb  414  and the corresponding stationary comb  424  is basically the largest. In other words, the capacitance is the highest. 
     Moreover, the mirror body  102  of the first mirror  101  is also formed of the first silicon layer b 1 . In the initial state, the first mirror  101  is also positioned on the reference plane P as the movable comb electrode  410  and the stationary comb electrode  420  are. 
     From this state, the first mirror  101  shifts substantially in parallel in the Z-axis direction as described before; that is, the first mirror  101  moves approximately in parallel with a reference plane P. Here, the movable comb electrode  410  is integrally connected to the first mirror  101 . Hence, as illustrated in  FIGS. 6 and 7 , the movable comb electrode  410  also moves approximately in parallel with the reference plane P. Specifically the movable comb  414  moves, staying in parallel with the stationary comb  424 . As a result, the overlapping area S of the movable comb  414  and the stationary comb  424  decreases as illustrated in  FIG. 7 . 
     Here, the overlapping area S reduces in proportion to a displacement of the first mirror  101 . The overlapping area of the movable comb  414  and the stationary comb  424  is shaped into a substantial rectangle. The overlapping area S is obtained by the product of a short side and a long side of the rectangle. When the movable comb  414  is displaced in the Z-axis direction, the long side of the overlapping area S does not change, and the short side becomes shorter in proportion to the displacement of the movable comb  414 . As a result, the overlapping area S also decreases in proportion to the displacement of the movable comb  414 . Since the movable comb  414  is displaced together with the first mirror  101 , the overlapping area S decreases in proportion to the displacement of the first mirror  101 . 
     As to a movable comb and a stationary comb, for example, the movable comb tilts with respect to the stationary comb. Here, an overlapping portion of the movable comb and the stationary comb is not always shaped into a rectangle. The shape of the overlapping portion changes depending on a tilted state of the movable comb. Accordingly, the overlapping area does not always change in proportion to the displacement of the movable comb. Furthermore, in the tilting, the displacement with respect to the tilt angle becomes greater as the tilted portion is farther distant from a center of the tilt. Hence, a tilted portion, of the movable comb, distant from the center of the tilt does not overlap the stationary comb when a displacement of a member to which the movable comb is connected becomes great. If the distance between the movable comb and the stationary comb is very short even though the movable comb does not overlap the stationary comb, a capacitance is created by the fringe effect; however, if the movable comb and the stationary comb are apart from each other at a certain distance, the distance keeps the capacitance from changing. Specifically, the configuration in which the movable comb tilts does not effectively utilize the overlapping area of the movable comb and the stationary comb for detecting the change of the capacitance. 
     Whereas, in the detection electrode  400 , the overlapping area S changes in proportion to a displacement of the first mirror  101 . Hence, the capacitance between the movable comb electrode  410  and the stationary comb electrode  420  also changes substantially in proportion to the displacement of the first mirror  101 . Hence, throughout a range of motion of the first mirror  101 , the capacitance uniformly changes as the first mirror  101  is displaced. As a result, no matter how much the first mirror  101  is displaced, the displacement of the first mirror  101  may be detected based on the capacitance with substantially the same precision as the capacitance is detected. Furthermore, the displacement of the movable combs  414  is substantially equal to that of the first mirror  101 . Such a feature makes it possible to effectively utilize the areas of the movable combs  414  and the stationary combs  424  so as to detect the change in the capacitance. 
     As described above, the optical filter device  1000  includes: the first mirror  101 ; the actuators  300  driving the first mirror  101 ; and the detection electrode  400  detecting the displacement of the first mirror  101 . The detection electrode  400  includes: the movable comb electrode  410  including movable combs  414  and connected to the first mirror  101 ; and the stationary comb electrode  420  including stationary combs  424  facing the movable combs  414  substantially in parallel with each other. The movable combs  414  are displaced in parallel with the stationary combs  424  when the movable comb electrode  410  is displaced together with the first mirror  101 . Note that the state where the movable combs  414  are displaced in parallel with the stationary combs  424  is that the movable combs  414  and the stationary combs  424  may be arranged so that the change in the capacitance between the movable comb electrode  410  and the stationary comb electrode  420  is substantially proportional to the displacement of the movable combs  414 . 
     Such features make it possible to detect the displacement of the first mirror  101  based on the change in the capacitance between the movable comb electrode  410  and the stationary comb electrode  420 . 
     In detecting the change in capacitance between two electrodes, another possible option is to arrange two plate electrodes in parallel with each other, and detect the capacitance created due to the change in the space between the two plate electrodes. However, the capacitance between the plate electrodes is inversely proportional to the space, and the wider the space is, the less precise the detection of the capacitance is. 
     In contrast, the use of comb electrodes solves the problem of the plate electrodes. In the comb electrodes, the movable combs  414  of the movable comb electrode  410  and the stationary combs  424  of the stationary comb electrode  420  face each other without contact. In this state, the movable comb electrode  410  is displaced such that the overlapping areas S of the movable combs  414  and the stationary combs  424  change, followed by the change in the capacitance between the movable combs  414  and the stationary combs  424 . The capacitance of the comb electrodes is proportional to the overlapping areas S. Such a feature makes it possible to precisely detect the change in the capacitance. 
     In addition, the movable combs  414 , which are displaced together with the first mirror  101 , are displaced in parallel with the stationary combs  424 . Thus, the overlapping areas S of the movable combs  414  and the stationary combs  424  change substantially in proportion to the displacement of the first mirror  101 . Such a feature makes it possible to detect the displacement of the first mirror  101  with uniform precision no matter how much the displacement is. As a result, precision may improve in detecting the displacement of the first mirror  101  throughout a displacement detectable area. Moreover, the relationship of a displacement of the first mirror  101  to a change in the capacitance is uniform throughout the displacement detectable area. Such a feature allows the displacement of the first mirror  101  to be more controllable. 
     Furthermore, the actuator  300  includes actuators  300 . Each of the actuators  300  is connected to a different portion of the first mirror  101 . The detection electrode  400  includes detection electrodes  400 . The movable comb electrode  410  includes movable comb electrodes  410 , and each of the movable comb electrodes  410  is connected to a different portion of the first mirror  101 . 
     In these features, the first mirror  101  is driven by the actuators  300 . Multiple actuators  300  are provided for multiple detection electrodes  400 . Hence, each of the detection electrodes  400  is provided to a corresponding one of the actuators  300 . Such a feature makes it possible to detect the displacement of the first mirror  101  caused by an actuator  300 , using a detection electrode  400  corresponding to the actuator  300 . 
     Moreover, the first mirror  101  is provided with the attachment  103  to which the actuator  300  is connected, and the movable comb electrode  410  is connected to the attachment  103 . 
     In this feature, the movable comb electrode  410  is connected to a portion, of the first mirror  101 , to which the actuator  300  is also connected. Specifically, the movable comb electrode  410  is displaced together with a portion, of the first mirror  101 , to be directly moved by the actuator  300 . Such a feature makes it possible to accurately detect, using the detection electrode  400 , the displacement of the first mirror  101  caused by the actuator  300 . 
     Furthermore, the first mirror  101  includes the mirror body  102 . The attachment  103  extends from the mirror body  102 . The actuator  300  is connected to the attachment  103  via the hinge  105  that is elastic and formed of a meandering line. The actuator  300  curves to drive the first mirror  101 . The hinge  105  stretches when the actuator  300  curves. The movable comb electrode  410  is connected to a portion, of the attachment  103 , across from a portion, of the attachment, to which the actuator  300  is attached. 
     In this feature, the actuator  300  curves when driving the first mirror  101 . The portion, of the actuator  300 , connected to the first mirror  101  is displaced in a direction (the Z-axis direction) to change the space between the first mirror  101  and the second mirror  201 . In addition, the portion is also slightly displaced in another direction (the X-axis direction.) Since the actuator  300  is connected to the attachment  103  via the elastic hinge  105 , the hinge  105  may absorb unnecessary displacement of the actuator  300 . Since the hinge  105  is placed to stretch when the actuator  300  curves, meandering lines do not interfere with one another, contributing to absorbing unnecessary displacement of the actuator  300 . Moreover, the attachment  103  extends from the mirror body  102  so that the actuator  300  and the hinge  105  may be arranged more flexibly. Consequently, the hinge  105  may be placed as described above. Then, the movable comb electrode  410  may be provided with the use of the attachment  103  disposed to flexibly arrange the actuator  300  and the hinge  105 . As described above, this attachment  103  is a part, of the first mirror  101 , to which the actuator  300  is attached. Such a feature makes it possible to accurately detect the displacement of the first mirror  101  caused by the actuator  300 . 
     In addition, the actuator  300  includes two actuators  300 , and the movable comb electrode  410  includes two movable comb electrodes  410 . The attachment  103  includes two attachments  103  provided on the straight line L 1  passing through the center C of the mirror body  102  and arranged to face each other across the center C. Each of the actuators  300  is connected to a corresponding one of the attachments  103  via the hinge  105  including hinges  105 . The hinges  105  include at least two hinges  105  arranged to face each other across the straight line L 1 . 
     In this feature, the attachments  103  are provided on the straight line L 1  passing through the center C of the mirror body  102 , and arranged to face each other across the center C. Such a feature allows the actuators  300 , as well as the movable comb electrodes  410 , to be provided on the straight line L 1  passing through the center C of the mirror body  102 , and arranged to face each other across the center C. Specifically, the first mirror  101  has two portions to be displaced by the actuators  300 . The two portions are (i) provided on the straight line L 1  passing through the center C of the mirror body  102 , and (ii) facing each other across the center C. In this feature, the first mirror  101  could rotate about the straight line L 1 . As a countermeasure, each actuator  300  is connected to an attachment  103  via the hinges  105 . The at least two hinges  105  are arranged to face each other across the straight line L 1 . Hence, for each actuator  300 , two hinges  105  may be arranged across the straight line L 1  to prevent the first mirror  101  from rotating about the straight line L 1 . As a result, the first mirror  101  may be displaced while being kept in parallel with the second mirror  201  as much as possible. 
     In addition, the optical filter device  1000  further includes the second mirror  201  spaced apart from the first mirror  101 . The actuators  300  drive the first mirror  101  to change the space between the first mirror  101  and the second mirror  201 . The first mirror  101  and the second mirror  201  transmit portion of the incident light, and let portion of the incident light having a wavelength in accordance with the space exit. 
     Such features make it possible to precisely detect the displacement of the first mirror  101  to precisely adjust the space between the first mirror  101  and the second mirror  201 . As a result, the features allow for precise control of the wavelength of the exiting light from the optical filter device  1000 . 
     &lt;&lt;Other Embodiments&gt;&gt; 
     As can be seen, the above embodiment is described as an example of the technique disclosed in the present application. However, the technique recited in the present disclosure shall not be limited to the one in the above embodiment. Instead, the technique may have any given modification, replacement of a feature with another feature, additional feature, and omission of a feature to be applied to other embodiments. The constituent elements described in the above embodiment may be combined to create a new embodiment. The constituent elements in the attached drawings and the detailed description may include not only those essential to solve the problems, but also those which might not be essential to solve the problems in order to show the technique as an example. Thus, those inessential constituent elements shall not be determined as essential ones simply because such elements are found in the attached drawings and the detailed description. 
     The above embodiment of the present invention may be configured as follows. 
     The optical element shall not be limited to the optical filter device  1000 . The detection by the above movable comb electrode and stationary comb electrode may be applied as long as the optical element causes an actuator to drive a mirror. Specifically, the detection technique is effective for an optical element causing the mirror to be displaced while maintaining the slope of the mirror as much as possible. 
     In the optical filter device  1000 , the first mirror  101  is displaced to move away from the second mirror  201 ; however, the displacement shall not be limited to this. The first mirror  101  may be displaced to come closer to the second mirror  201 . For example, the first unit  100  may be laid over the second unit  200 . 
     Two actuators  300  are provided; however, three or more actuators  300  may be provided. Two detection electrodes  400  are provided; however, three or more detection electrodes  400  may be provided. Note that the detection electrode  400  may beneficially be equal in number to the actuators  300 . 
     The movable comb electrode  410  may be secured to a portion, of the first mirror  101 , on which the actuator  300  is not secured. Specifically, the movable comb electrode  410  may be provided in any given place as long as the feedback control can be performed on a drive voltage of the actuators  300  based on capacitance of the detection electrodes  400 . 
     Each of the actuators  300  is connected to the first mirror  101  via two hinges  105 ; however, one hinge  105  or three or more hinges  105  may be connected. When three or more hinges  105  are connected, at least two of the hinges  105  are beneficially arranged across the straight line L 1 . 
     The actuator  300  is, but not limited to, a piezoelectric actuator which curves by a piezoelectric effect. For example, each actuator  300  may be a thermal actuator which comprises a beam including materials each having a different CTE. The thermal actuator curves due to a difference between the CTEs. 
     The actuator  300  includes two beams; namely, the first beam  301  and the second beam  302 . However, the actuator  300  may include one beam or three or more beams. 
     The second beam  302  is provided with the dummy film  319 ; however, the dummy film  319  may be omitted. 
     The first mirror  101  includes the cylinder  104 ; however, the cylinder  104  may be omitted. 
     Alternatively, the first mirror  101  may be replaced with a blade, so that a shutter device including the blade may precisely detect displacement of the blade.  FIG. 8  is a plan view of such a shutter device; namely a shutter device  2000 . The mirror  101  in  FIG. 2  is replaced with a blade  601 . Other constituent elements are directly adopted from the optical filter device  1000  to constitute the shutter device  2000 . 
     The blade  601  includes a blade body  602  and the two attachments  103 . The blade body  602  is shaped into a plate-like substantial square. The mirror  101  is connected to the tip end of the second beam  302  so that a surface of the mirror  101  is in parallel with the surfaces of the first beam  301  and the second beam  302  (see  FIG. 2 ); whereas, the blade  601  is connected to a tip end of the second beam  302  so that a surface of the blade  601  is vertical to surfaces of the first beam  301  and the second beam  302 . Specifically, in  FIG. 8 , the thickness (a side surface) of the blade body  602  is illustrated. A surface of the blade body  602  is in parallel with a plane defined by the Y-axis and the Z-axis. Then, when the actuators  300  drive the blade  601 , the blade  601  is displaced in the Z-axis direction. Such displacement may provide and close a not-shown light path in the X-axis direction. 
     In such a shutter device  2000 , the displacement of the blade  601  may be detected based on change in capacitance between the movable comb electrode  410  and the stationary comb electrode  420 . 
     Note that the surface of the blade body  602  does not have to be in parallel with the plane defined by the Y-axis and the Z-axis. The surface may be in parallel with a plane defined by, for example, the X-axis and Z-axis, depending on the not-shown light path to be blocked and provided by the blade  601 . 
     INDUSTRIAL APPLICABILITY 
     As can be seen, the technique disclosed here is useful for optical elements. 
     DESCRIPTION OF REFERENCE CHARACTERS 
       1000  Optical Filter Device (Optical Element) 
       101  First Mirror (Moving Unit) 
       103  Attachment 
       105  Hinge (Connector) 
       201  Second Mirror (Another Moving Unit) 
       300 A First Actuator 
       300 B Second Actuator 
       400  Detection Electrode 
       410  Movable Comb Electrode 
       414  Movable Comb 
       420  Stationary Comb Electrode 
       424  Stationary Comb 
       601  Blade (Moving Unit) 
       2000  Shutter Device (Optical Element)