Tunable interference filter, optical module, and method for driving a tunable interference filter

Abstract

This invention provides a tunable interference filter capable of improving the accuracy of wavelength tuning across a wide wavelength range. [Solution] The wavelength-tunable interference filter 1, as described above, comprises a first reflective film 4 and a second reflective film 5 facing each other, and an electrostatic actuator unit 6 that changes the gap amount G between the first reflective film 4 and the second reflective film 5. The electrostatic actuator unit 6 includes a bias electrode 61 to which a bias voltage corresponding to a target value of the gap amount G (i.e., gap target value Gt) is applied, a control electrode 62 to which a control voltage that is feedback-controlled according to the gap amount G is applied, and a variable electrode 63 to which the bias voltage and the control voltage are switchably applied.

Description

[Technical Field]

[0001] The present invention relates to a tunable interference filter, an optical module, and a method for driving a tunable interference filter. [Background technology]

[0002] Conventionally, there are tunable interference filters that include a pair of reflective films facing each other and an electrostatic actuator that changes the gap between the pair of reflective films (see, for example, Patent Document 1). In this tunable interference filter, light of a desired wavelength can be transmitted or reflected by adjusting the gap between the pair of reflective films.

[0003] In the tunable interference filter disclosed in Patent Document 1, the electrostatic actuator section has a plurality of drive electrodes that are voltage-controlled independently of each other. Specifically, the electrostatic actuator section has a bias electrode to which a bias voltage corresponding to a target gap value between a pair of reflective films is applied, and a control electrode to which a control voltage that is feedback-controlled based on the gap amount and the gap target value is applied. In this tunable interference filter, fine wavelength adjustment over a wide wavelength band can be achieved by using the bias electrode for coarse adjustment of the gap amount and the control electrode for fine adjustment of the gap amount. [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2018-128681 [Overview of the project] [Problems that the invention aims to solve]

[0005] However, the tunable interference filter disclosed in Patent Document 1 mentioned above does not resolve the problem of the voltage sensitivity of the electrostatic actuator section to the gap amount between a pair of reflective films exhibiting nonlinear behavior, resulting in insufficient accuracy in wavelength adjustment over a wide wavelength range. For example, when the gap amount is large, the sensitivity of the electrostatic actuator section to voltage decreases, making it susceptible to disturbances. On the other hand, when the gap amount is small, the sensitivity of the electrostatic actuator section to voltage increases, making fine adjustment of the gap amount difficult. [Means for solving the problem]

[0006] A tunable interference filter according to a first aspect of the present disclosure comprises a pair of reflective films facing each other, and an electrostatic actuator that changes the gap amount between the pair of reflective films, wherein the electrostatic actuator has a bias electrode to which a bias voltage corresponding to a target value of the gap amount is applied, a control electrode to which a control voltage that is feedback-controlled based on the gap amount and the target value is applied, and a variable electrode to which the bias voltage and the control voltage are switchably applied.

[0007] An optical module according to a second aspect of the present disclosure comprises any of the above-described tunable interference filters and a filter drive unit for driving the electrostatic actuator unit, wherein the filter drive unit includes a bias drive unit for applying the bias voltage to the bias electrode, a gap detection unit for detecting the gap amount between the pair of reflective films, a feedback control unit for applying the control voltage to the control electrode based on the gap amount detected by the gap detection unit, and a voltage switching unit for switchingly applying the bias voltage and the control voltage to the tunable electrode.

[0008] A method for driving a tunable interference filter according to a third aspect of the present disclosure includes the steps of: changing the gap amount by applying a bias voltage to a bias electrode that corresponds to a target value for the gap amount between a pair of reflective films; adjusting the gap amount by detecting the gap amount between the pair of reflective films and applying a control voltage to a control electrode that is feedback-controlled based on the detected gap amount and the target value; selecting one of the bias voltage and the control voltage as a select voltage based on the target value; and assisting in changing or adjusting the gap amount by applying the select voltage to a tunable electrode when a voltage is applied to the bias electrode or the control electrode. [Brief explanation of the drawing]

[0009] [Figure 1] A block diagram showing the schematic configuration of the spectroscopic measuring apparatus according to this embodiment. [Figure 2] A schematic cross-sectional view showing a tunable interference filter according to this embodiment. [Figure 3] This is a schematic plan view showing the electrode arrangement on the second substrate of the wavelength-tunable interference filter according to this embodiment. [Figure 4] A flowchart illustrating the driving method of the tunable interference filter according to this embodiment. [Figure 5] A graph illustrating the change in voltage sensitivity in the wavelength-tunable interference filter according to this embodiment. [Figure 6] A graph illustrating the change in the bias voltage value in the wavelength-tunable interference filter according to this embodiment. [Figure 7] A schematic cross-sectional view showing a tunable interference filter according to a modified example. [Figure 8] A schematic cross-sectional view showing a tunable interference filter relating to another modification. [Modes for carrying out the invention]

[0010] One embodiment of this disclosure will be described with reference to Figures 1 to 6. As shown in FIG. 1, the spectroscopic measurement device 100 of the present embodiment includes an optical module 101, a light receiving unit 102, a signal processing unit 103, and a spectroscopic control unit 104. The optical module 101 includes a wavelength-variable interference filter 1 and a filter driving unit 9. The spectroscopic measurement device 100 is a device that analyzes the light intensity of a predetermined wavelength in the light reflected by the measurement target by splitting the measurement light incident from the measurement target with the wavelength-variable interference filter 1 and receiving it with the light receiving unit 102, and measures the spectroscopic spectrum. This spectroscopic measurement device 100 can be incorporated and used in various devices such as a printer, a projector, or a drone.

[0011] [Configuration of the wavelength-variable interference filter 1] FIG. 2 is a cross-sectional view showing the wavelength-variable interference filter 1 of the present embodiment. The wavelength-variable interference filter 1 is a spectroscopic filter capable of changing the transmission wavelength according to a driving voltage input from the outside.

[0012] The wavelength-variable interference filter 1 includes a first substrate 2 and a second substrate 3 arranged to face each other, a first reflection film 4 provided on the first substrate 2, a second reflection film 5 provided on the second substrate 3, an electrostatic actuator unit 6 that changes the dimension of the gap (hereinafter, the gap amount G) between the first reflection film 4 and the second reflection film 5, and a capacitance detection unit 7 that detects the gap amount G.

[0013] In the following description, the direction in which the first substrate 2 and the second substrate 3 face each other (that is, the thickness direction of each of the first substrate 2 and the second substrate 3) is referred to as the thickness direction of the wavelength-variable interference filter 1. FIG. 2 corresponds to a cross-sectional view of the wavelength-variable interference filter 1 cut along its thickness direction.

[0014] The first substrate 2 and the second substrate 3 are each formed of a material that can transmit light, such as various glasses or crystals. Further, the first substrate 2 and the second substrate 3 are integrally configured as a structure that forms a cavity between them by being joined to each other.

[0015] The first substrate 2 has a first surface 21 facing the second substrate 3 and a second surface 22 on the opposite side of the first surface 21. When the first substrate 2 is viewed along the thickness direction, an annular groove 23 is formed on the second surface 22 of the first substrate 2. Thus, the first substrate 2 comprises a movable portion 24 surrounded by the annular groove 23, a diaphragm portion 25 thinned by the groove 23, and a base portion 26 that supports the movable portion 24 so as to be displaceable in the thickness direction via the diaphragm portion 25. The base portion 26 of the first substrate 2 is bonded to the second substrate 3 via an arbitrary bonding film.

[0016] The second substrate 3 has a third surface 31 facing the first substrate 2 and a fourth surface 32 opposite to the first surface 21. A recess 33 is formed in the center of the third surface 31 to form a cavity between the first substrate 2 and the second substrate 3. A reflective film mounting section 34, a detection electrode mounting section 35, and a drive electrode mounting section 36 are formed on the bottom surface of the recess 33 of the second substrate 3. The reflective film mounting section 34 is located in the center of the recess 33, and the detection electrode mounting section 35 and the drive electrode mounting section 36 are arranged in an annular shape surrounding the reflective film mounting section 34. Furthermore, each of the reflective film mounting section 34, the detection electrode mounting section 35, and the drive electrode mounting section 36 is formed according to the initial setting distance from the first substrate 2, and may have a base shape or a groove shape.

[0017] The first reflective film 4 is provided on the first surface 21 of the movable portion 24 of the first substrate 2, and the second reflective film 5 is provided on the reflective film installation portion 34 of the second substrate 3. The first reflective film 4 and the second reflective film 5 face each other with a gap between them, and the axis passing through the centers of the first reflective film 4 and the second reflective film 5 is the central axis C of the tunable interference filter 1. When the tunable interference filter 1 is viewed along its thickness, the region where the first reflective film 4 and the second reflective film 5 overlap forms the filter region of the tunable interference filter 1, and this filter region has a substantially circular shape centered on the central axis C. The gap amount G between the first reflective film 4 and the second reflective film 5 corresponds to the wavelength of light transmitted through the filter region of the tunable interference filter 1.

[0018] In this embodiment, although not particularly limited, the first reflective film 4 and the second reflective film 5 are dielectric multilayer films formed by alternately stacking Si (silicon layer) and SiO2 (silicon oxide layer).

[0019] The electrostatic actuator unit 6 of this embodiment changes the gap amount G between the first reflective film 4 and the second reflective film 5 by pulling the movable part 24 of the first substrate 2 toward the second substrate 3. This electrostatic actuator unit 6 has a bias electrode 61, a control electrode 62, and a variable electrode 63 as multiple drive electrodes to which a drive voltage is applied. The bias electrode 61, control electrode 62, and variable electrode 63 are provided on the drive electrode installation part 36 of the first substrate 2. Specifically, the bias electrode 61 is positioned to face the movable part 24, and the variable electrode 63 and control electrode 62 are positioned to face the diaphragm part 25. The electrostatic actuator unit 6 also has a ground electrode 64 provided on the first surface 21 of the first substrate 2 so as to face each of the bias electrode 61, control electrode 62, and variable electrode 63.

[0020] In this embodiment, the distances H1 to H3 between the bias electrode 61, control electrode 62, and variable electrode 63 and the ground electrode 64 are equal to each other, but may be different. Also, in this embodiment, the distances H1 to H3 between the bias electrode 61, control electrode 62, and variable electrode 63 and the ground electrode 64 are greater than the gap amount G between the first reflective film 4 and the second reflective film 5, but are not limited to this, and the relative sizes may be reversed.

[0021] Figure 3 is a plan view of the second substrate 3 along its thickness, schematically showing the arrangement of each electrode on the third surface 31 of the second substrate 3. As shown in Figure 3, when the second substrate 3 is viewed along its thickness, the bias electrode 61, control electrode 62, and variable electrode 63 are arranged around the second reflective film 5 and are arranged concentrically around the central axis C. In addition, the electrodes of the electrostatic actuator 6 on the second substrate 3 are arranged radially outward from the second reflective film 5 in the order of bias electrode 61, variable electrode 63, and control electrode 62.

[0022] In this embodiment, it is preferable that the area of ​​the bias electrode 61 is larger than the area of ​​the control electrode 62 and the variable electrode 63, and is larger than the combined total area of ​​the control electrode 62 and the variable electrode 63.

[0023] As will be described in more detail later, the bias electrode 61, together with the ground electrode 64, constitutes the first electrostatic actuator section 610, to which a bias voltage is applied. The control electrode 62, together with the ground electrode 64, constitutes the second electrostatic actuator section 620, to which a control voltage is applied. The variable electrode 63, together with the ground electrode 64, constitutes the third electrostatic actuator section 630, to which either the bias voltage or the control voltage is applied in a switchable manner. The first electrostatic actuator section 610 and the second electrostatic actuator section 620 are driven and controlled independently of each other, and the third electrostatic actuator section 630 can be switched between a bias drive mode, which is driven and controlled together with the first electrostatic actuator section 610, and a feedback control mode, which is driven and controlled together with the second electrostatic actuator section 620.

[0024] As shown in Figure 2, the capacitance detection unit 7 includes a first detection electrode 71 provided on the first surface 21 of the first substrate 2, and a second detection electrode 72 provided on the detection electrode installation section 35 of the second substrate 3 and facing the first detection electrode 71, and holds a charge between the first detection electrode 71 and the second detection electrode 72 corresponding to the gap amount G. When the tunable interference filter 1 is viewed along the thickness direction, the first detection electrode 71 and the second detection electrode 72 are arranged in an annular shape with a central axis C so as to surround the first reflective film 4 or the second reflective film 5. The first detection electrode 71 is positioned between the first reflective film 4 and the ground electrode 64, and the second detection electrode 72 is positioned between the second reflective film 5 and the bias electrode 61.

[0025] In this embodiment, although not particularly limited, the bias electrode 61, control electrode 62, variable electrode 63, and ground electrode 64 are each made of a metal oxide film such as ITO (Indium Tin Oxide) laminated on the dielectric multilayer film. Furthermore, the first detection electrode 71 and the second detection electrode 72 are each made of a metal film such as Au laminated on the dielectric multilayer film and the metal oxide film such as ITO (Indium Tin Oxide).

[0026] As shown in Figure 3, the bias electrode 61, control electrode 62, variable electrode 63, and second detection electrode 72 of the electrostatic actuator unit 6 are electrically connected to each of the multiple drive electrode terminals 38 located outside the cavity via lead wiring sections 37 formed on the second substrate 3. Although simplified in Figure 3, the lead wiring sections 37 and drive electrode terminals 38 are configured to allow independent voltage application to each of the bias electrode 61, control electrode 62, variable electrode 63, and second detection electrode 72. The drive electrode terminals 38 are also connected to an external filter drive unit 9 (see Figure 1) via an arbitrary wiring structure not shown. The ground electrode 64 and the first detection electrode 71 on the first substrate 2 are electrically connected to a common electrode terminal 39 located outside the cavity via wiring (such as through-wiring or lead-out wiring on the first substrate 2), which is not shown in the diagram. The common electrode terminal 39 is grounded.

[0027] [Filter drive unit 9] The filter drive unit 9 will be explained again with reference to Figure 1. The filter drive unit 9 is a circuit that drives and controls the wavelength-tunable interference filter 1 described above, and comprises a bias drive unit 91, a gap detection unit 92, a feedback control unit 93, a voltage switching unit 94, and a microcontroller 95.

[0028] The bias drive unit 91 applies a bias voltage to at least the first electrostatic actuator unit 610 (specifically, the bias electrode 61 in Figure 2). The bias voltage is a voltage corresponding to the target value of the gap amount G (hereinafter referred to as the gap target value Gt), and the bias drive unit 91 sets the voltage value of the bias voltage based on the bias signal input from the microcontroller 95. The bias drive unit 91 is composed of, for example, a D / A converter having a predetermined number of bits.

[0029] The gap detection unit 92 acquires a detection signal corresponding to the gap amount G from the capacitance detection unit 7 and outputs the acquired detection signal to the feedback control unit 93.

[0030] The feedback control unit 93 applies a control voltage to at least the second electrostatic actuator unit 620 (specifically, the control electrode 62 in Figure 2). Specifically, the feedback control unit 93 outputs a control voltage that is feedback-controlled so that each signal becomes equal in value, based on the detection signal input from the gap detection unit 92 (i.e., according to the gap amount G) and the target signal input from the microcontroller 95. The feedback control unit 93 is configured with an analog controller having a fixed gain, and the voltage variable range may be set to a predetermined width. For example, a PI controller or a PID controller can be used as this analog controller.

[0031] The voltage switching unit 94 applies either a bias voltage or a control voltage to the third electrostatic actuator unit 630 (specifically, the variable electrode 63 in Figure 2) in a switchable manner. In other words, the voltage switching unit 94 can switch the third electrostatic actuator unit 630 between a bias drive mode and a feedback control mode. For example, the voltage switching unit 94 in this embodiment is a switch that can switch the connection destination of the variable electrode 63 between the bias drive unit 91 and the feedback control unit 93 based on a switching signal input from the microcontroller 95.

[0032] The microcontroller 95 controls the bias drive unit 91, the feedback control unit 93, and the voltage switching unit 94, respectively, according to the target wavelength instructed by the spectral control unit 104.

[0033] [Spectrometer 100] The components of the spectroscopic measuring device 100, other than the optical module 101, will now be described. The light-receiving unit 102 is a sensor that receives light transmitted through the tunable interference filter 1. For example, an image sensor such as a CCD or CMOS can be used as the light-receiving unit 102. When the light-receiving unit 102 receives light transmitted through the tunable interference filter 1, it outputs a light-receiving signal corresponding to the amount of light received to the spectral control unit 104.

[0034] The signal processing unit 103 includes a sampling circuit for sampling the received light signal output from the light receiving unit 102, an amplification circuit for amplifying the received light signal, and an A / D conversion circuit for converting the received light signal into a digital signal. The signal processing unit 103 processes the received light signal using these circuits and inputs the processed received light signal to the spectral control unit 104.

[0035] The spectral control unit 104 is composed of components such as a CPU and memory, and controls the overall operation of the spectral measuring device 100. The spectral control unit 104 commands the filter drive unit 9 to start spectral measurement based on external input information, and performs spectral measurement on the object to be measured based on the light received signal input from the signal processing unit 103.

[0036] [Method for driving the tunable interference filter 1] Next, the driving method of the tunable interference filter 1 of this embodiment will be explained with further reference to the flowchart in Figure 4. In the following explanation, we will describe, as an example, the case in which a spectroscopic measurement is performed with an arbitrary wavelength as the target wavelength.

[0037] The spectral control unit 104 outputs a control signal to the filter drive unit 9 that indicates the target wavelength, in response to user operations, etc. When the microcontroller 95 receives the control signal from the spectral control unit 104, it calculates the target value of the gap amount G (target gap value Gt) required to extract light of the target wavelength from the tunable interference filter 1 (step S1).

[0038] The microcontroller 95 determines whether the gap target value Gt is smaller than a predetermined first threshold (step S2), and based on the comparison result, selects either the bias voltage or the control voltage as the selectable voltage to be applied to the variable electrode 63. For example, if the gap target value Gt is smaller than a predetermined first threshold (step S2; Yes), the microcontroller 95 selects the bias voltage as the selectable voltage to be applied to the variable electrode 63 (step S3). On the other hand, if the gap target value Gt is greater than or equal to a predetermined first threshold (step S2; No), the microcontroller 95 selects the control voltage as the selectable voltage to be applied to the variable electrode 63 (step S4). The microcontroller 95 then outputs a switching signal corresponding to the selected voltage to the voltage switching unit 94. This sets the switching connection destination of the voltage switching unit 94, and the third electrostatic actuator unit 630 is set to bias drive mode or feedback control mode (step S5).

[0039] Further, the microcomputer 95 calculates the voltage value of the bias voltage corresponding to the gap target value Gt (hereinafter, bias voltage value V , ,

[0041] )(step S6). At this time, the microcomputer 95 refers to an equation or a drive table stored in a memory (not shown) to calculate the bias voltage value V b corresponding to the gap target value Gt. Then, the microcomputer 95 outputs a bias signal indicating the bias voltage value V b to the bias drive unit 91.

[0040] Here, for the equation for calculating the bias voltage value V b , for example, the following equation (1) disclosed in Japanese Patent Laid-Open No. 2018-128681 can be referred to. <00​​​​​​​​​​​​​​​​​​​​​​​c This corresponds to the area of ​​the control electrode 62. On the other hand, when the third electrostatic actuator 630 is in feedback control mode, the area S of the electrode region to which the bias voltage is applied. c This corresponds to the area of ​​the bias electrode 61, and the area S of the electrode region to which the control voltage is applied. c This corresponds to the sum of the areas of the control electrode 62 and the variable electrode 63. Therefore, based on the setting of the voltage switching unit 94 set in step S5, the microcontroller 95 determines the area S of the electrode region to which the bias voltage is applied. c The area S of the electrode region to which the control voltage is applied. c The area is calculated, and the bias voltage value V is used with the calculated area. b It is preferable to calculate this.

[0042] Next, the bias drive unit 91 starts applying a bias voltage based on the bias signal input from the microcontroller 95 (step S7). Here, when the third electrostatic actuator unit 630 is in bias drive mode, the bias voltage output from the bias drive unit 91 is applied to the bias electrode 61 and the variable electrode 63, respectively. On the other hand, when the third electrostatic actuator unit 630 is in feedback control mode, the bias voltage output from the bias drive unit 91 is applied to the bias electrode 61. As a result, an electrostatic attraction force based on the bias voltage acts on the first electrostatic actuator section 610 (and the third electrostatic actuator section 630), causing the movable part 24 to be displaced toward the second substrate 3, thereby changing the gap amount G.

[0043] The feedback control unit 93 outputs a control voltage such that the detection signal input from the gap detection unit 92 and the target signal input from the microcontroller 95 are equal in value (step S8). Here, when the third electrostatic actuator unit 630 is in bias drive mode, the control voltage output from the feedback control unit 93 is applied to the control electrode 62. On the other hand, when the third electrostatic actuator unit 630 is in feedback control mode, the control voltage output from the bias drive unit 91 is applied to the control electrode 62 and the variable electrode 63, respectively. As a result, an electrostatic attraction force based on the control voltage acts on the second electrostatic actuator section 620 (and the third electrostatic actuator section 630), and the displacement of the movable section 24 (i.e., the gap amount G) is finely adjusted.

[0044] After step S8, the filter drive unit 9 applies a bias voltage and a control voltage until it receives a termination instruction, controlling the gap amount G so that the tunable interference filter 1 transmits light of the target wavelength (step S9).

[0045] While the gap amount G of the tunable interference filter 1 is controlled, the light receiving unit 102 detects the light transmitted through the tunable interference filter 1, and the spectral control unit 104 acquires the received light signal from the light receiving unit 102 via the signal processing unit 103. Then, the spectral control unit 104 calculates the optical characteristic value for the target wavelength of the object to be measured based on the acquired received light signal.

[0046] Furthermore, when measuring spectral spectra corresponding to each wavelength set at predetermined intervals within the measurement wavelength range, steps S1 to S9 described above can be repeated for each wavelength. For example, when changing the gap target value Gt at predetermined intervals, the third electrostatic actuator unit 630 can be set to bias drive mode when the gap target value Gt is smaller than the first threshold, and the third electrostatic actuator unit 630 can be set to feedback control mode when the gap target value Gt is greater than or equal to the first threshold.

[0047] [Operation of electrostatic actuator unit 6] The changes in voltage sensitivity and bias voltage in the electrostatic actuator section 6 will be explained with reference to Figures 5 and 6. The following explanation will utilize the Examples, Reference Example 1, Reference Example 2, and Comparative Examples.

[0048] In the embodiment, the voltage sensitivity and bias voltage were obtained when the gap target value Gt was changed at predetermined intervals using the wavelength tunable interference filter 1 of this embodiment. In the embodiment, when the gap target value Gt became greater than or equal to the first threshold, the third electrostatic actuator unit 630 was switched from bias drive mode to feedback control mode.

[0049] In Reference Example 1, the wavelength-tunable interference filter 1 of this embodiment was used, and the voltage sensitivity and bias voltage were obtained when the gap target value Gt was changed at predetermined intervals while the third electrostatic actuator unit 630 was fixed in bias drive mode. In Reference Example 2, the wavelength-tunable interference filter 1 of this embodiment was used, and the voltage sensitivity and bias voltage were obtained when the gap target value Gt was changed at predetermined intervals while the third electrostatic actuator unit 630 was fixed in feedback control mode.

[0050] The comparative example has a configuration in which the third electrostatic actuator section 630 and the voltage switching section 94 are omitted from the wavelength-tunable interference filter 1 of this embodiment. The area of ​​the bias electrode 61 in the comparative example and the area of ​​the bias electrode 61 in the embodiment are assumed to be approximately equal. Furthermore, the area of ​​the control electrode 62 in the comparative example is larger than the area of ​​each electrode of the control electrode 62 and variable electrode 63 in the embodiment, and is less than or equal to the combined area of ​​each electrode of the control electrode 62 and variable electrode 63 in the embodiment. In this comparative example, the voltage sensitivity and bias voltage were obtained when the gap target value Gt was changed at predetermined intervals.

[0051] Figures 5 and 6 show the changes in voltage sensitivity and bias voltage when the gap target value Gt is changed at predetermined intervals for each of the Examples, Reference Example 1, Reference Example 2, and Comparative Example. In the graphs of Figures 5 and 6, the horizontal axis represents the amount of gap displacement (gap displacement) when the gap amount G is changed according to the gap target value Gt, the vertical axis of Figure 5 represents the voltage sensitivity of the electrostatic actuator unit 6, and the vertical axis of Figure 6 represents the bias voltage value. Figures 5 and 6 also show the value of the gap displacement (threshold Th) corresponding to the first threshold of the gap target value Gt.

[0052] As shown in Figure 5, in each of the reference examples 1 and 2 and the comparative example, the voltage sensitivity increases as the gap displacement increases. On the other hand, in the embodiment, when the gap displacement is smaller than the threshold Th, the voltage sensitivity is ensured to be higher than in the comparative example, and when the gap displacement is greater than or equal to the threshold Th, the voltage sensitivity is kept lower than in the comparative example. Specifically, the voltage sensitivity when the gap displacement is greater than or equal to the threshold Th is kept to the same extent as the voltage sensitivity when the gap displacement is smaller than the threshold Th. Therefore, in this embodiment, it is clear that the voltage sensitivity of the electrostatic actuator 6 can be adjusted according to the gap displacement.

[0053] As shown in Figure 6, in each of the Reference Examples 1 and 2 and the Comparative Example, the larger the gap displacement, the greater the bias voltage value V required to achieve the target gap value Gt. b It gets bigger. On the other hand, in the embodiment, when the gap displacement is smaller than the threshold Th, similar to Reference Example 1, the larger the gap displacement, the greater the bias voltage value V required to achieve the target gap value Gt. b The value becomes larger. However, when the gap displacement is greater than or equal to the threshold Th, the bias voltage value V required to achieve the target gap value Gt is larger. b This can be kept smaller than in Reference Example 1 and the Comparative Example. Therefore, in the embodiment, it is clear that driving with a low voltage is possible when the gap displacement is large.

[0054] [Effects of this embodiment] (1) As described above, the wavelength-tunable interference filter 1 of this embodiment comprises a first reflective film 4 and a second reflective film 5 facing each other, and an electrostatic actuator unit 6 that changes the gap amount G between the first reflective film 4 and the second reflective film 5. The electrostatic actuator unit 6 includes a bias electrode 61 to which a bias voltage corresponding to a target value of the gap amount G (i.e., gap target value Gt) is applied, a control electrode 62 to which a control voltage that is feedback-controlled based on the gap amount G and the gap target value Gt is applied, and a variable electrode 63 to which the bias voltage and the control voltage are switchably applied.

[0055] In this configuration, the electrostatic actuator section 6 utilizes a bias electrode 61 to which a bias voltage is applied and a control electrode 62 to which a control voltage is applied, allowing for coarse and fine adjustment of the gap amount G. This enables precise wavelength adjustment over a wide wavelength range with a simple configuration. Furthermore, in the wavelength-tunable interference filter 1 of this embodiment, by utilizing a variable electrode 63 to which a bias voltage and a control voltage are switchedly applied as the electrostatic actuator unit 6, the ratio between the area of ​​the electrode region to which the bias voltage is applied and the area of ​​the electrode region to which the control voltage is applied can be changed in the electrostatic actuator unit 6. For example, when the gap target value Gt is large, by ensuring a large ratio of the area of ​​the electrode region to which the bias voltage is applied in the electrostatic actuator unit 6, it is possible to suppress a decrease in the voltage sensitivity of the electrostatic actuator unit 6, and as a result, the influence of disturbances can be suppressed. Also, when the gap target value Gt is small, by decreasing the ratio of the area of ​​the electrode region to which the bias voltage is applied in the electrostatic actuator unit 6, it is possible to suppress a high voltage sensitivity of the electrostatic actuator unit 6, and as a result, fine adjustment of the gap amount G becomes easier. Therefore, according to the tunable interference filter 1 of this embodiment, the voltage sensitivity of the electrostatic actuator 6 can be adjusted according to the gap target value Gt while coarse and fine adjustment of the gap amount G is performed by the electrostatic actuator 6, thereby improving the accuracy of wavelength adjustment over a wide wavelength range.

[0056] Furthermore, in the tunable interference filter 1 of this embodiment, even when the gap target value Gt is large, the maximum voltage value required for the bias voltage can be reduced by ensuring a large proportion of the area of ​​the electrode to which the bias voltage is applied in the electrostatic actuator section 6. In other words, with the tunable interference filter 1 of this embodiment, even when the gap target value Gt is large, driving with a low voltage becomes possible, and the accuracy of wavelength adjustment can be improved. Alternatively, with the tunable interference filter 1 of this embodiment, the range in which wavelength adjustment is possible can be made wider.

[0057] (2) When the tunable interference filter 1 of this embodiment is viewed along the thickness direction, the bias electrode 61, control electrode 62, and variable electrode 63 are arranged in concentric circles with the first reflective film 4 or the second reflective film 5 as the center. In this configuration, the bias electrode 61, control electrode 62, and variable electrode 63 improve the balance between the electrostatic forces they generate, allowing for uniform adjustment of the gap amount G between the first reflective film 4 and the second reflective film 5.

[0058] (3) When the electrostatic actuator section 6 of this embodiment is viewed along the thickness direction, the bias electrode 61, the variable electrode 63, and the control electrode 62 are arranged in that order radially outward from the first reflective film 4 or the second reflective film 5. With this configuration, when a bias voltage is applied to the variable electrode 63, the sensitivity of the electrostatic actuator unit 6 to the bias voltage can be suitably improved.

[0059] (4) In this embodiment, the area of ​​the bias electrode 61 is larger than the area of ​​the control electrode 62 and the variable electrode 63. This configuration allows for a good balance between coarse and fine adjustment of the gap amount G.

[0060] (5) The optical module 101 of this embodiment comprises the above-described wavelength tunable interference filter 1 and a filter drive unit 9 that drives the electrostatic actuator unit 6. The filter drive unit 9 includes a bias drive unit 91 that applies a bias voltage to the bias electrode 61, a gap detection unit 92 that detects the gap amount G between the first reflective film 4 and the second reflective film 5, a feedback control unit 93 that applies a control voltage to the control electrode 62 according to the gap amount G detected by the gap detection unit 92, and a voltage switching unit 94 that switchesly applies a bias voltage and a control voltage to the variable electrode 63. In this configuration, the effects of the tunable interference filter 1 described above can be effectively achieved.

[0061] (6) In this embodiment, the voltage switching unit 94 switches the voltage applied to the variable electrode 63 between the bias voltage and the control voltage based on the comparison result between the gap target value Gt, which is the target value of the gap amount G, and the first threshold. In this configuration, the voltage sensitivity of the electrostatic actuator unit 6 can be suitably adjusted.

[0062] (7) The driving method for the wavelength tunable interference filter 1 of this embodiment includes the steps of: changing the gap amount G by applying a bias voltage to the bias electrode 61, which is determined for each gap target value Gt, which is a target value of the gap amount G between the first reflective film 4 and the second reflective film 5 (step S7); adjusting the gap amount G by detecting the gap amount G between the first reflective film 4 and the second reflective film 5 and applying a control voltage to the control electrode 62 that is feedback-controlled based on the detected gap amount G and the gap target value Gt (step S8); selecting one of the bias voltage and the control voltage as a select voltage to be applied to the variable electrode 63 based on the target value of the gap amount G (steps S2 to S5); and assisting in the change or adjustment of the gap amount G by applying the select voltage to the variable electrode 63 when a voltage is applied to the bias electrode 61 or the control electrode 62. This method, similar to the effect of the tunable interference filter 1 described above, can improve the accuracy of wavelength tuning over a wide wavelength range.

[0063] [Differentiation] The present invention is not limited to the embodiments described above, and any modifications, improvements, etc., that can achieve the objectives of the present invention are included in the present invention.

[0064] [Example 1] As shown in Figure 7, the modified tunable interference filter 1A may include a variable electrode 63 configured in the same manner as in the first embodiment, and a variable auxiliary electrode 65 which is a different electrode from the variable electrode 63. The variable auxiliary electrode 65 can be arranged concentrically around the central axis C between the variable electrode 63 and the control electrode 62. Alternatively, the variable auxiliary electrode 65 is not limited to the region between the bias electrode 61 and the control electrode 62, but may also be in the region inside the bias electrode 61 or in the region outside the control electrode 62.

[0065] The variable auxiliary electrode 65, like the variable electrode 63, is to which either a bias voltage or a control voltage can be switched. This variable auxiliary electrode 65 may be controlled independently of the variable electrode 63. For example, the voltage switching unit 94 may be configured to switch the voltage applied to the variable auxiliary electrode 65 between a bias voltage and a control voltage based on a second threshold different from a first threshold corresponding to the variable electrode 63. That is, the voltage switching unit 94 may apply a bias voltage to the variable auxiliary electrode 65 when the gap target value Gt is less than the second threshold (e.g., second threshold > first threshold), and apply a control voltage to the variable auxiliary electrode 65 when the gap target value Gt is greater than or equal to the second threshold. In this modified example, the voltage sensitivity of the electrostatic actuator unit 6 can be adjusted more precisely.

[0066] [Differentiation 2] As shown in Figure 7, in the modified tunable interference filter 1A, the bias electrode 61, control electrode 62, and variable electrode 63 may be provided on the first substrate 2, and the ground electrode 64 may be provided on the second substrate 3. In this case, the bias electrode 61 may be provided on the movable part 24 of the first substrate 2, and the control electrode 62 may be provided on the diaphragm part 25 of the first substrate 2. The variable electrode 63 and variable auxiliary electrode 65 may be provided on either the movable part 24 or the diaphragm part 25 of the first substrate 2.

[0067] [Difference 3] As shown in Figure 7, in the modified tunable interference filter 1A, the first reflective film 4 and the second reflective film 5 may each be made of a metal film such as Ag or an alloy film such as an Ag alloy, instead of a dielectric multilayer film. In this case, the first detection electrode 71 may be provided on the first reflective film 4, and the second detection electrode 72 may be provided on the second reflective film 5.

[0068] [Differentiation Example 4] In the above embodiment, the variable electrode 63 may be divided into multiple electrodes. For example, as shown in Figure 7 with reference numerals, in the modified wavelength tunable interference filter 1B, the variable electrode 63 may have multiple divided electrodes 631. The multiple divided electrodes 631 are arranged concentrically around the central axis C, and a bias voltage and a control voltage are applied to them in a switchable manner. Alternatively, the multiple divided electrodes 631 are not limited to the region between the bias electrode 61 and the control electrode 62, but may be in the inner region of the bias electrode 61, the outer region of the control electrode 62, or in different regions from each other.

[0069] [Difference 5] In the above embodiment, when viewed along the thickness direction of the tunable interference filter 1, the electrostatic actuator section 6 is arranged radially outward from the first reflective film 4 or the second reflective film 5 in the order of bias electrode 61, variable electrode 63, and control electrode 62, but the order of arrangement of each electrode may be changed.

[0070] [Modification 6] In the above embodiment, a preferred relative size relationship is described for the areas of the bias electrode 61, the variable electrode 63, and the control electrode 62. However, the relative sizes of the areas may be changed depending on the structure for changing the gap amount G between the first substrate 2 and the second substrate 3.

[0071] [Difference 7] In each of the above embodiments, the voltage switching unit 94 is a switch that can switch the connection of the variable electrode 63 between the bias drive unit 91 and the feedback control unit 93 based on a switching signal input from the microcontroller 95, but the present invention is not limited thereto. For example, the voltage switching unit 94 may be configured to include a circuit structure capable of outputting a bias voltage and a control voltage separately from the bias drive unit 91 and the feedback control unit 93 described above, and a switch that applies either the bias voltage or the control voltage to the variable electrode 63 in a switchable manner based on a switching signal input from the microcontroller 95. In this case, the value of the bias voltage applied to the bias electrode 61 and the value of the bias voltage applied to the variable electrode 63 in bias drive mode may be different values ​​from each other.

[0072] [Differentiation 8] As shown in Figure 8, the modified tunable interference filter 1C may include a first substrate 2, a second substrate 3 facing the first surface 21 of the first substrate 2, and a third substrate 8 facing the second surface 22 of the first substrate 2. In this tunable interference filter 1C, the first reflective film 4 is provided on the second surface 22 of the first substrate 2, and the second reflective film 5 is provided on the side opposite to the second substrate 3 relative to the first substrate 2, that is, on the surface 81 of the third substrate 8 facing the first substrate 2. As a result, the first reflective film 4 and the second reflective film 5 are arranged to face each other.

[0073] In the modified wavelength tunable interference filter 1C, the electrostatic actuator 6 can adjust the gap amount G between the first reflective film 4 and the second reflective film 5 by pulling the first substrate 2 toward the second substrate 3 using electrostatic attraction. In the modified tunable interference filter 1C, the relationship between the magnitude of the voltage applied to each electrode of the electrostatic actuator 6 and the magnitude of the gap G between the first reflective film 4 and the second reflective film 5 is reversed compared to the above embodiment. Therefore, in the driving method for the tunable interference filter 1C, it is sufficient to reverse the relationship between the first threshold and the gap target value Gt in the driving method described in the first embodiment.

[0074] [Summary of this disclosure] A wavelength-tunable interference filter according to a first aspect of the present disclosure comprises a pair of reflective films facing each other, and an electrostatic actuator unit that changes the gap amount between the pair of reflective films, wherein the electrostatic actuator unit has a bias electrode to which a bias voltage corresponding to a target value of the gap amount is applied, a control electrode to which a control voltage that is feedback-controlled based on the gap amount and the target value is applied, and a variable electrode to which the bias voltage and the control voltage are switchably applied.

[0075] In the first embodiment described above, it is preferable that the bias electrode, the control electrode, and the variable electrode are arranged concentrically around the reflective film when viewed along the thickness direction of the reflective film.

[0076] In the first embodiment described above, it is preferable that the electrostatic actuator portion is arranged in the order of the bias electrode, the variable electrode, and the control electrode, radially outward from the pair of reflective films when viewed along the thickness direction.

[0077] In the first embodiment described above, it is preferable that the area of ​​the bias electrode is larger than the area of ​​the control electrode and the variable electrode.

[0078] An optical module according to a second aspect of the present disclosure comprises any of the above-described tunable interference filters and a filter drive unit for driving the electrostatic actuator unit, wherein the filter drive unit includes a bias drive unit for applying the bias voltage to the bias electrode, a gap detection unit for detecting the gap amount between the pair of reflective films, a feedback control unit for applying the control voltage to the control electrode based on the gap amount detected by the gap detection unit, and a voltage switching unit for switchingly applying the bias voltage and the control voltage to the tunable electrode.

[0079] In the second embodiment described above, it is preferable that the voltage switching unit switches the voltage applied to the variable electrode between the bias voltage and the control voltage based on the comparison result between the target value of the gap amount and the first threshold.

[0080] In the second embodiment described above, the electrostatic actuator further comprises a variable auxiliary electrode to which the bias voltage and the control voltage are switchedly applied, and the voltage switching unit preferably switches the voltage applied to the variable auxiliary electrode between the bias voltage and the control voltage based on the result of comparing the target value of the gap amount with a second threshold different from the first threshold.

[0081] A method for driving a tunable interference filter according to a third aspect of the present disclosure includes the steps of: changing the gap amount by applying a bias voltage to a bias electrode, which is determined for each target value of the gap amount between a pair of reflective films; adjusting the gap amount by detecting the gap amount between the pair of reflective films and applying a control voltage to a control electrode that is feedback-controlled based on the detected gap amount and the target value of the gap amount; selecting one of the bias voltage and the control voltage as a select voltage to be applied to the tunable electrode based on the target value of the gap amount; and assisting in changing or adjusting the gap amount by applying the select voltage to the tunable electrode when a voltage is applied to the bias electrode or the control electrode. [Explanation of symbols]

[0082] 1, 1A~1C...Wavelength tunable interference filter, 2...First substrate, 21...First surface, 22...Second surface, 23...Groove, 24...Movable part, 25...Diaphragm part, 26...Base part, 3...Second substrate, 31...Third surface, 32...Fourth surface, 33...Recess, 34...Reflective film installation part, 35...Detection electrode installation part, 36...Drive electrode installation part, 37...Outlet wiring part, 38...Drive electrode terminal, 39...Common electrode terminal, 4...First reflective film, 5...Second reflective film, 6...Electrostatic actuator part, 61...Bias electrode, 610...First electrostatic actuator part, 62...Control electrode, 620...Second electrostatic actuator Tuner section, 63... Variable electrode, 630... Third electrostatic actuator section, 631... Split electrode, 64... Ground electrode, 65... Variable spare electrode, 7... Capacitance detection section, 71... First detection electrode, 72... Second detection electrode, 8... Third substrate, 81... Opposing surface, 9... Filter drive section, 91... Bias drive section, 92... Gap detection section, 93... Feedback control section, 94... Voltage switching section, 95... Microcontroller, 100... Spectroscopic measuring device, 101... Optical module, 102... Light receiving section, 103... Signal processing section, 104... Spectroscopic control section, C... Central axis, G... Gap amount.

Claims

1. A pair of reflective films facing each other, The system includes an electrostatic actuator that changes the gap amount between the pair of reflective films, The electrostatic actuator section is, A bias electrode to which a bias voltage corresponding to the target value of the gap amount is applied, A control electrode to which a control voltage is applied, which is feedback-controlled based on the gap amount and the target value, A tunable interference filter having a variable electrode to which the bias voltage and the control voltage are switchedly applied.

2. The tunable interference filter according to claim 1, wherein, when viewed along the thickness direction of the reflective film, the bias electrode, the control electrode, and the variable electrode are arranged concentrically with respect to the reflective film.

3. The tunable interference filter according to claim 2, wherein the electrostatic actuator portion, when viewed along the thickness direction, is arranged in the order of the bias electrode, the variable electrode, and the control electrode from the pair of reflective films toward the radially outward direction.

4. The tunable interference filter according to claim 1, wherein the area of ​​the bias electrode is larger than the area of ​​the control electrode and the variable electrode.

5. A tunable interference filter according to claim 1, The filter drive unit drives the electrostatic actuator unit, The filter drive unit is A bias drive unit that applies the bias voltage to the bias electrode, A gap detection unit for detecting the amount of the gap between the pair of reflective films, A feedback control unit applies the control voltage to the control electrode based on the gap amount detected by the gap detection unit, An optical module having a voltage switching unit that switchesly applies the bias voltage and the control voltage to the variable electrode.

6. The optical module according to claim 5, wherein the voltage switching unit switches the voltage applied to the variable electrode between the bias voltage and the control voltage based on the result of comparing the target value of the gap amount with a first threshold.

7. The electrostatic actuator further comprises a variable pre-electrode to which the bias voltage and the control voltage are switchedly applied, The optical module according to claim 6, wherein the voltage switching unit switches the voltage applied to the variable auxiliary electrode between the bias voltage and the control voltage based on the result of comparing the target value of the gap amount with a second threshold different from the first threshold.

8. The steps include changing the gap amount by applying a bias voltage to a bias electrode that corresponds to a target value for the gap amount between a pair of reflective films, The steps include detecting the gap amount between the pair of reflective films and adjusting the gap amount by applying a control voltage to a control electrode, which is feedback-controlled based on the detected gap amount and the target value, A step of selecting one of the bias voltage and the control voltage as a selected voltage based on the target value, A method for driving a tunable interference filter, comprising the step of assisting in the change or adjustment of the gap amount by applying the selected voltage to a variable electrode when a voltage is applied to the bias electrode or the control electrode.

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