Calibration method, calibration device, and optical element

Abstract

This calibration method comprises: a step for preparing an optical element having a transmittance or reflectance peak at each of a plurality of wavelengths including those from a first wavelength to an n-th wavelength, a light source, a Fabry-Perot interference filter, and a detector; a first detection step for controlling a voltage applied to the Fabry-Perot interference filter so as to cause light rays of the first to n-th wavelengths to successively pass through the Fabry-Perot interference filter, and detecting a photocurrent by using the detector; and a first calibration step for calibrating a relational expression between the applied voltage and the transmission wavelength on the basis of a plurality of voltage values at a plurality of peaks of the photocurrent. At each of the first to n-th wavelengths, the full width at half maximum of the peak of the optical element is less than the full width at half maximum of the transmittance peak of the Fabry-Perot interference filter.

Description

Calibration method, calibration device, and optical element

[0001] The present disclosure relates to a calibration method, a calibration device, and an optical element.

[0002] A Fabry-Perot interference filter is known that has a pair of mirrors whose distance from each other is variable. In such a Fabry-Perot interference filter, the wavelength of light transmitted through the Fabry-Perot interference filter can be controlled by adjusting the distance between the pair of mirrors by controlling the voltage applied to the Fabry-Perot interference filter.

[0003] Even among Fabry-Perot interference filters with the same specifications (design), the relationship between the applied voltage and the transmission wavelength may vary between individual filters due to manufacturing errors, the usage environment, etc. Patent Document 1 discloses a method for performing calibration when such variations occur. In the method described in Patent Document 1, calibration is performed using a peak in the absorption wavelength spectrum of a known component (e.g., hemoglobin under the skin of a living body).

[0004] JP 2016-70776 A JP 2018-197780 A

[0005] However, when calibration is performed using the method described in Patent Document 1, it may be difficult to identify the peak position of the photocurrent corresponding to the detected light depending on the peak shape of the absorption wavelength spectrum of the known component, which may result in a decrease in calibration accuracy. Furthermore, it is desirable to perform such calibration using a simple method.

[0006] An object of the present disclosure is to provide a calibration method, a calibration device, and an optical element that can perform calibration easily with high accuracy.

[0007] A calibration method according to one aspect of the present disclosure includes the steps of: [1] "preparing an optical element having a transmittance or reflectance peak at each of a plurality of wavelengths including a first wavelength to an n-th wavelength (n is an integer of 2 or more); a light source that emits light including the plurality of wavelengths; a Fabry-Perot interference filter having a pair of mirror portions whose distance from each other is variable; and a detector that detects light that has passed through the Fabry-Perot interference filter; arranging the optical element on an optical path from the light source to the Fabry-Perot interference filter; and controlling a voltage applied to the Fabry-Perot interference filter to change the distance, thereby detecting the light that has passed through the Fabry-Perot interference filter. a first detection step of sequentially transmitting light of the first wavelength to the nth wavelength through the Fabry-Perot interference filter and detecting the transmitted light of the first wavelength to the nth wavelength with the detector; and a first calibration step of calibrating a relational expression between a voltage applied to the Fabry-Perot interference filter and the transmission wavelength of the Fabry-Perot interference filter based on a plurality of voltage values ​​at a plurality of peaks of a photocurrent corresponding to the detected light, wherein the half width of the peak of the optical element is smaller than the half width of the transmittance peak of the Fabry-Perot interference filter for each of the first wavelength to the nth wavelength.

[0008] In the calibration method, the half-width of the transmittance or reflectance peak of the optical element is smaller than the half-width of the transmittance peak of the Fabry-Perot interference filter at each of the first to nth wavelengths. In other words, because the optical element has high spectral resolution, the detection results from the detector clearly show undulations in the photocurrent. Therefore, the positions of the peaks in the photocurrent corresponding to the first to nth wavelengths of light can be easily and accurately identified, improving the accuracy of the calibration. Furthermore, in the calibration method, because the optical element has multiple transmittance or reflectance peaks, it is possible to obtain multiple correspondence relationships between the applied voltage and the transmission wavelength of the Fabry-Perot interference filter, for example, without switching the wavelength of the light emitted from the light source. In other words, the relationship between the applied voltage and the transmission wavelength can be calibrated using a simple method. Therefore, the calibration method allows for easy and accurate calibration.

[0009] A calibration method according to one aspect of the present disclosure may be [2] "the calibration method according to the above [1], wherein n is an integer equal to or greater than 4." In this case, four or more peaks appear in the photocurrent corresponding to the detected light, and therefore, based on the four or more voltage values ​​at these peaks, it is possible to calibrate with high precision the relationship between the applied voltage and the transmitted wavelength, which is expressed as a cubic function, for example.

[0010] A calibration method according to one aspect of the present disclosure may be [3] "the calibration method according to the above [2], wherein the variable range of the transmission wavelength of the Fabry-Perot interference filter is divided into four equal parts, and each of the four equal parts includes at least one of the first wavelength to the nth wavelength." In this case, the relationship between the applied voltage and the transmission wavelength can be calibrated over the entire variable range of the transmission wavelength of the Fabry-Perot interference filter.

[0011] A calibration method according to one aspect of the present disclosure may be [4] "the calibration method according to any one of the above [1] to [3], wherein n is an integer equal to or greater than 10." In this case, 10 or more peaks appear in the photocurrent corresponding to the detected light, and therefore, the relationship between the applied voltage and the transmitted wavelength can be calibrated with higher accuracy based on the 10 or more voltage values ​​at these peaks.

[0012] A calibration method according to one aspect of the present disclosure may be [5] "the calibration method according to the above [4], wherein the variable range of the transmission wavelength of the Fabry-Perot interference filter is divided into 10 equal parts, and each of the 10 equal parts includes at least one of the first wavelength to the nth wavelength." In this case, the relationship between the applied voltage and the transmission wavelength can be calibrated over the entire variable range of the transmission wavelength of the Fabry-Perot interference filter.

[0013] The calibration method according to one aspect of the present disclosure may be [6] "the calibration method according to any one of [1] to [5] above, wherein in the first detection step, a voltage applied to the Fabry-Perot interference filter is controlled so as to sweep the transmission wavelength of the Fabry-Perot interference filter in a wavelength range including the first wavelength to the nth wavelength." In this case, a single wavelength sweep can obtain multiple correspondence relationships between the voltage applied to the Fabry-Perot interference filter and the transmission wavelength of the Fabry-Perot interference filter, thereby reducing the time required for the calibration process.

[0014] The calibration method according to one aspect of the present disclosure may be [7] "the calibration method according to any one of [1] to [6] above, wherein the optical element is a bandpass filter having a dielectric multilayer film." In this case, the optical element can be configured so that the characteristics (shape, position, etc.) of the transmittance or reflectance peak are desired.

[0015] A calibration method according to one aspect of the present disclosure may be [8] "the calibration method according to any one of [1] to [7] above, wherein the optical element is a bandpass filter, and in the arranging step, a collimating lens is further arranged on the optical path between the light source and the optical element." Performance such as the optical transmittance and transmission wavelength of a bandpass filter changes depending on the angle of incidence of light (the performance is dependent on the angle of incidence). However, in this calibration method, a collimating lens is arranged between the light source and the optical element, thereby eliminating problems caused by such dependency on the angle of incidence (for example, light of a desired wavelength can be transmitted at a desired transmittance).

[0016] A calibration method according to one aspect of the present disclosure may be [9] "the calibration method according to any one of [1] to [8] above, wherein in the first detection step, the voltage applied to the Fabry-Perot interference filter is controlled within a range from 0 V to a first voltage that is the maximum operating voltage of the Fabry-Perot interference filter." In this case, even if there is individual variation in the Fabry-Perot interference filters, the applied voltage is controlled within a voltage range that does not damage the Fabry-Perot interference filter, and therefore, the relationship between the applied voltage and the transmission wavelength can be appropriately calibrated.

[0017] A calibration method according to one aspect of the present disclosure may be

[10] "the calibration method according to any one of [1] to [9] above, including: a step of calculating a second voltage corresponding to the shortest transmission wavelength of the Fabry-Perot interference filter using the relational expression calibrated in the first calibration step; a second detection step of changing the distance by controlling the voltage applied to the Fabry-Perot interference filter within a voltage range including a third voltage higher than the second voltage, and detecting light transmitted through the Fabry-Perot interference filter with the detector; and a second calibration step of further calibrating the relational expression based on a voltage value at a peak of a photocurrent corresponding to the light newly detected in the second detection step." In this case, the relational expression between the applied voltage and the transmission wavelength can be calibrated with higher accuracy based on the voltage value at the peak newly detected in the second detection step (not detected in the first detection step).

[0018] a voltage control unit that controls a voltage applied to the Fabry-Perot interference filter to change the distance, thereby sequentially transmitting light of the first wavelength to the nth wavelength out of the light emitted from the light source through the Fabry-Perot interference filter; a detector that detects the transmitted light of the first wavelength to the nth wavelength; and a calibration unit that calibrates a relational expression between the voltage applied to the Fabry-Perot interference filter and the transmission wavelength of the Fabry-Perot interference filter based on a plurality of voltage values ​​at a plurality of peaks of a photocurrent corresponding to the detected light, wherein a half width of the peak of the peak of the optical element is smaller than a half width of the transmittance peak of the Fabry-Perot interference filter at each of the first wavelength to the nth wavelength.

[0019] In the calibration device, the half-width of the transmittance or reflectance peak of the optical element is smaller than the half-width of the transmittance peak of the Fabry-Perot interference filter at each of the first to nth wavelengths. In other words, because the optical element has high spectral resolution, the photocurrent undulations appear clearly in the detection results from the detector. Therefore, the positions of the peaks in the photocurrent corresponding to the first to nth wavelengths of light can be easily and accurately identified, improving the accuracy of calibration. Furthermore, because the optical element has multiple transmittance or reflectance peaks, it is possible to obtain multiple correspondence relationships between the applied voltage and the transmission wavelength of the Fabry-Perot interference filter, for example, without switching the wavelength of light emitted from the light source. In other words, the relationship between the applied voltage and the transmission wavelength can be calibrated using a simple method. Therefore, the calibration method allows for easy and accurate calibration.

[0020] The calibration device according to one aspect of the present disclosure may be

[12] "the calibration device according to the above

[11] , wherein n is an integer equal to or greater than 4." In this case, four or more peaks appear in the photocurrent corresponding to the detected light, and therefore, based on the four or more voltage values ​​at these peaks, it is possible to calibrate with high precision the relationship between the applied voltage and the transmitted wavelength, which is expressed as a cubic function, for example.

[0021] The calibration device according to one aspect of the present disclosure may be

[13] "the calibration device according to the above

[12] , wherein the variable range of the transmission wavelength of the Fabry-Perot interference filter is divided into four equal parts, and each of the four equal parts includes at least one of the first wavelength to the nth wavelength." In this case, the relationship between the applied voltage and the transmission wavelength can be calibrated over the entire variable range of the transmission wavelength of the Fabry-Perot interference filter.

[0022] The calibration device according to one aspect of the present disclosure may be

[14] "the calibration device according to any one of

[11] to

[13] above, wherein n is an integer equal to or greater than 10." In this case, 10 or more peaks appear in the photocurrent corresponding to the detected light, and therefore, the relationship between the applied voltage and the transmitted wavelength can be calibrated with higher accuracy based on the 10 or more voltage values ​​at these peaks.

[0023] The calibration device according to one aspect of the present disclosure may be

[15] "the calibration device according to the above

[14] , wherein the variable range of the transmission wavelength of the Fabry-Perot interference filter is divided into 10 equal parts, and each of the 10 equal parts includes at least one of the first wavelength to the nth wavelength." In this case, the relationship between the applied voltage and the transmission wavelength can be calibrated over the entire variable range of the transmission wavelength of the Fabry-Perot interference filter.

[0024] The calibration device according to one aspect of the present disclosure may be

[16] "the calibration device according to any one of

[11] to

[15] above, wherein the voltage control unit controls the voltage applied to the Fabry-Perot interference filter so as to sweep the transmission wavelength of the Fabry-Perot interference filter in a wavelength range including the first wavelength to the nth wavelength." In this case, a plurality of correspondence relationships between the voltage applied to the Fabry-Perot interference filter and the transmission wavelength of the Fabry-Perot interference filter can be obtained by a single wavelength sweep, thereby reducing the time required for the calibration process.

[0025] The calibration device according to one aspect of the present disclosure may be

[17] "the calibration device according to any one of

[11] to

[16] above, wherein the optical element is a bandpass filter having a dielectric multilayer film." In this case, the optical element can be configured so that the characteristics (shape, position, etc.) of the transmittance or reflectance peak are as desired.

[0026] The calibration device according to one aspect of the present disclosure may be

[18] "the calibration device according to any one of the above

[11] to

[17] , further comprising a collimating lens disposed on an optical path between the light source and the optical element, the optical element being a band-pass filter." In this case, since the collimating lens is disposed between the light source and the optical element, it is possible to solve the problem caused by the dependency of the performance of the band-pass filter on the angle of incidence.

[0027] An optical element according to one aspect of the present disclosure is

[19] "an optical element that can be used to calibrate the relationship between the voltage applied to a Fabry-Perot interference filter and the transmission wavelength of the Fabry-Perot interference filter, the optical element having a transmittance or reflectance peak at each of a plurality of wavelengths including a first wavelength to an nth wavelength (n is an integer of 2 or more) that are located within a variable range of the transmission wavelength of the Fabry-Perot interference filter, and the half-width of the peak of the optical element at each of the first wavelength to the nth wavelength is smaller than the half-width of the transmittance peak of the Fabry-Perot interference filter."

[0028] In the above optical element, the half-width of the transmittance or reflectance peak of the optical element is smaller than the half-width of the transmittance peak of the Fabry-Perot interference filter at each of the first to nth wavelengths. In other words, because the optical element has high spectral resolution, when used for calibration using the above-described calibration method and calibration device, the detection results of the detector clearly show undulations in the photocurrent. Therefore, the positions of the peaks in the photocurrent corresponding to light of the first to nth wavelengths can be easily and accurately identified, improving the accuracy of the calibration. Furthermore, because the optical element has multiple transmittance or reflectance peaks, it is possible to obtain multiple correspondence relationships between the applied voltage and the transmission wavelength of the Fabry-Perot interference filter, for example, without switching the wavelength of light emitted from the light source. In other words, the relationship between the applied voltage and the transmission wavelength can be calibrated using a simple method. Therefore, the above optical element allows for easy calibration with high accuracy.

[0029] According to the present disclosure, it is possible to provide a calibration method, a calibration device, and an optical element that can perform calibration easily with high accuracy.

[0030] FIG. 1 is a diagram showing the overall configuration of a calibration device according to one embodiment. FIG. 2 is a perspective view of the Fabry-Perot interference filter shown in FIG. 1. FIG. 3 is a cross-sectional view of the Fabry-Perot interference filter taken along line III-III in FIG. 2. FIG. 4 is a diagram schematically showing the light transmission characteristics of an optical element. FIG. 5 is a diagram schematically showing the light transmission characteristics of a bandpass filter. FIG. 6 is a diagram schematically showing the transmittance of a bandpass filter for light transmitted through an optical element. FIG. 7 shows the relationship between the applied voltage and the transmission wavelength of a Fabry-Perot interference filter. FIG. 8 is a diagram showing the light transmission characteristics of a Fabry-Perot interference filter. FIG. 9 is a flowchart showing a calibration method according to one embodiment. FIG. 10 is a diagram showing the time change in the voltage applied to the Fabry-Perot interference filter. FIG. 11 is a graph showing the relationship between the voltage applied to the Fabry-Perot interference filter and the photocurrent corresponding to light detected by a detector. FIG. 12 is a diagram showing the relationship between the transmission wavelength when a second voltage is applied and the transmission wavelength when a third voltage is applied. FIG. 13 is a graph showing the relationship between the transmission wavelength of the Fabry-Perot interference filter and the photocurrent corresponding to the light detected by the detector.

[0031] Hereinafter, embodiments of the present disclosure will be described in detail with reference to the drawings. In each drawing, the same or corresponding parts are denoted by the same reference numerals, and duplicated explanations will be omitted.

[0032] A calibration device 1 according to this embodiment will be described. The calibration device 1 shown in FIG. 1 is a device for calibrating a Fabry-Perot interference filter 10. The Fabry-Perot interference filter 10 is an optical filter that transmits light of a wavelength corresponding to an applied voltage. The calibration device 1 calibrates the relationship between the voltage applied to the Fabry-Perot interference filter 10 and the transmission wavelength of the Fabry-Perot interference filter 10 (hereinafter also referred to as the "relationship between applied voltage and transmission wavelength"). The Fabry-Perot interference filter 10 may be mounted on an optical device such as a hyperspectral camera. A hyperspectral camera is a camera that can disperse light into tens to hundreds of bands according to wavelength and acquire images for each band. In a hyperspectral camera, light passes through the Fabry-Perot interference filter 10 along the optical axis, and the light transmitted through the Fabry-Perot interference filter 10 is captured by an imaging element. First, the Fabry-Perot interference filter 10 will be described below with reference to FIGS. 2 and 3. [Fabry-Perot Interference Filter]

[0033] 2 and 3, the Fabry-Perot interference filter 10 has a light-transmitting region 10a. The Fabry-Perot interference filter 10 is, for example, a rectangular plate-shaped element. The thickness direction of the Fabry-Perot interference filter 10 is arranged so as to be parallel to the optical axis direction D (a direction parallel to the optical axis A of the light incident on the Fabry-Perot interference filter 10). The light-transmitting region 10a is, for example, a cylindrical region having a center line parallel to the optical axis direction D. When viewed from the optical axis direction D, the center of the light-transmitting region 10a coincides with the center of the Fabry-Perot interference filter 10.

[0034] The Fabry-Perot interference filter 10 includes a substrate 11 whose thickness direction is the optical axis direction D. The substrate 11 is made of a material such as silicon, quartz, or glass. The substrate 11 has a first surface 11a and a second surface 11b opposite to the first surface 11a. The first surface 11a and the second surface 11b are, for example, flat surfaces perpendicular to the optical axis direction D. A first laminated structure 12 is laminated on the first surface 11a, and a second laminated structure 13 is laminated on the second surface 11b.

[0035] The first stacked structure 12 includes an antireflection layer 121, a first stacked body 122, an intermediate layer 123, and a second stacked body 124. The antireflection layer 121, the first stacked body 122, the intermediate layer 123, and the second stacked body 124 are stacked in this order on the first surface 11a of the substrate 11. That is, the first stacked body 122 is disposed on the first surface 11a via the antireflection layer 121, and the second stacked body 124 is disposed on the opposite side of the first stacked body 122 from the substrate 11 (upper side in FIG. 3 ). An air gap S is formed between the first stacked body 122 and the second stacked body 124 by the frame-shaped intermediate layer 123. When the substrate 11 is made of silicon, the antireflection layer 121 and the intermediate layer 123 are each made of, for example, silicon oxide. The thickness of the intermediate layer 123 is, for example, an integer multiple of ½ of the design center wavelength. The thickness of the intermediate layer 123 may be larger than an integral multiple of 1 / 2 of the design central wavelength, if necessary.

[0036] The portion of the first stack 122 corresponding to the light-transmitting region 10a functions as the mirror section 14. That is, the first stack 122 has the mirror section 14. The mirror section 14 is supported on the substrate 11 via an anti-reflection layer 121. As an example, the first stack 122 is configured by alternately stacking a plurality of polysilicon layers and a plurality of silicon nitride layers. The optical thickness of each layer that configures the mirror section 14 is, for example, an integral multiple of ¼ of the design central wavelength. Note that a silicon oxide layer may be used instead of the silicon nitride layer.

[0037] The portion of the second stack 124 corresponding to the light-transmitting region 10a functions as the mirror portion 15. That is, the second stack 124 includes the mirror portion 15. The mirror portion 15 is supported on the substrate 11 via the anti-reflection layer 121, the first stack 122, and the intermediate layer 123, and faces the mirror portion 14 across the gap S in the optical axis direction D. As an example, the second stack 124 is configured by alternately stacking multiple polysilicon layers and multiple silicon nitride layers. The optical thickness of each layer constituting the mirror portion 15 is, for example, an integral multiple of ¼ of the design center wavelength. Note that a silicon oxide layer may be used instead of the silicon nitride layer. A plurality of through-holes are formed in the portion of the second stack 124 corresponding to the gap S to an extent that does not substantially affect the function of the mirror portion 15. The multiple through-holes are used when forming the gap S by removing a portion of the intermediate layer 123 by etching.

[0038] A first electrode 125 and a second electrode 126 are formed on the mirror portion 14. The first electrode 125 surrounds the light-transmitting region 10a when viewed from the optical axis direction D. The second electrode 126 overlaps the light-transmitting region 10a when viewed from the optical axis direction D. The shape of the second electrode 126 when viewed from the optical axis direction D is substantially the same as the shape of the light-transmitting region 10a when viewed from the optical axis direction D. Each of the first electrode 125 and the second electrode 126 is formed by doping impurities into a portion of a polysilicon layer to reduce the resistance of that portion.

[0039] A third electrode 127 is formed on the mirror section 15. The third electrode 127 faces the first electrode 125 and the second electrode 126 across a gap S. The third electrode 127 is formed by doping an impurity into a portion of the polysilicon layer to reduce the resistance of that portion.

[0040] A pair of terminals 16 are provided on the first laminate structure 12, sandwiching the light-transmitting region 10a (see FIG. 2). Each terminal 16 is disposed in a through-hole formed in the second laminate 124 and the intermediate layer 123, opening on the opposite side from the substrate 11 and reaching the first laminate 122. Each terminal 16 is electrically connected to the first electrode 125 via a wiring 125a.

[0041] The first laminate structure 12 is provided with a pair of terminals 17 on either side of the light-transmitting region 10a (see FIG. 2). Each terminal 17 is disposed in a through-hole formed in the second laminate 124 and the intermediate layer 123, opening on the side opposite the substrate 11 and reaching the intermediate layer 123. Each terminal 17 is electrically connected to the second electrode 126 via wiring 126a and to the third electrode 127 via wiring 127a. The direction in which the pair of terminals 17 sandwich the light-transmitting region 10a is perpendicular to the direction in which the pair of terminals 16 sandwich the light-transmitting region 10a (see FIG. 2).

[0042] A pair of trenches 122a are formed in the first stack 122. Each trench 122a extends in an annular shape so as to surround a portion of the wiring 126a extending from each terminal 17 in the optical axis direction D. Each trench 122a electrically insulates the first electrode 125 from the wiring 126a. A trench 122b is formed in the first stack 122. The trench 122b extends in an annular shape along the inner edge of the first electrode 125. The trench 122b electrically insulates the first electrode 125 from the second electrode 126. The region within each trench 122a, 122b may be filled with an insulating material or may be an empty space.

[0043] A pair of trenches 124a are formed in the second stack 124. Each trench 124a extends in an annular shape so as to surround each terminal 16. Each trench 124a electrically insulates each terminal 16 from the third electrode 127. The area within each trench 124a may be filled with an insulating material or may be an empty space.

[0044] The second stacked structure 13 includes an antireflection layer 131, a third stacked body 132, an intermediate layer 133, and a fourth stacked body 134. The antireflection layer 131, the third stacked body 132, the intermediate layer 133, and the fourth stacked body 134 are stacked in this order on the second surface 11b of the substrate 11. The antireflection layer 131 and the intermediate layer 133 have the same configurations as the antireflection layer 121 and the intermediate layer 123, respectively. The third stacked body 132 and the fourth stacked body 134 have stacked structures symmetrical to the first stacked body 122 and the second stacked body 124, respectively, with respect to the substrate 11. The antireflection layer 131, the third stacked body 132, the intermediate layer 133, and the fourth stacked body 134 have the function of suppressing warpage of the substrate 11.

[0045] A recess 18 is formed on the surface 13a of the second laminate structure 13 opposite the substrate 11. The recess 18 is open on the side opposite the substrate 11. The recess 18 overlaps with the light-transmitting region 10a when viewed from the optical axis direction D. The shape of the recess 18 when viewed from the optical axis direction D is substantially the same as the shape of the light-transmitting region 10a when viewed from the optical axis direction D, and is circular in this example. The center line of the recess 18 coincides with the center line of the light-transmitting region 10a. The recess 18 is formed in the third laminate 132, the intermediate layer 133, and the fourth laminate 134, and extends to the anti-reflection layer 131.

[0046] A light-shielding layer 135 is formed on the surface 13a of the second laminate structure 13. The light-shielding layer 135 is formed, for example, over the entire surface 13a. The material of the light-shielding layer 135 is, for example, aluminum. The light-shielding layer 135 blocks light incident on the Fabry-Perot interference filter 10. In this example, the light-shielding layer 135 is a reflective layer that blocks light incident on the Fabry-Perot interference filter 10 by reflecting it. On the other hand, in areas where the light-shielding layer 135 is not formed (areas where the recesses 18 are formed in this example), light incident on the Fabry-Perot interference filter 10 passes through. In other words, the light-transmitting region 10a corresponds to the area where the light-shielding layer 135 is not formed. In this way, in the Fabry-Perot interference filter 10, the light-transmitting region 10a (aperture, light entrance opening) is defined by the light-shielding layer 135. That is, the light-transmitting region 10a (aperture, light entrance opening) is formed by providing a light-shielding layer 135 (reflective layer) in the region other than the light-transmitting region 10a in the Fabry-Perot interference filter 10 when viewed from the optical axis direction D, while not providing a light-shielding layer 135 in the light-transmitting region 10a.

[0047] A protective layer 136 is formed on the light-shielding layer 135 and on the inner surface of the recess 18. The material of the protective layer 136 is, for example, aluminum oxide. Note that by setting the thickness of the protective layer 136 to 100 nm or less (preferably about 30 nm), the optical effect of the protective layer 136 can be ignored.

[0048] In the Fabry-Perot interference filter 10 configured as described above, when a voltage is applied to the first electrode 125 and the third electrode 127 via the multiple terminals 16, 17, causing a potential difference between the first electrode 125 and the third electrode 127, an electrostatic force corresponding to the potential difference is generated between the first electrode 125 and the third electrode 127. The generation of the electrostatic force between the first electrode 125 and the third electrode 127 attracts the mirror portion 15 to the mirror portion 14, thereby adjusting the distance between the mirror portion 14 and the mirror portion 15. At this time, the second electrode 126, which has the same potential as the third electrode 127, functions as a compensation electrode, and the mirror portion 15 is kept flat in the light transmission region 10a.

[0049] In this way, in the Fabry-Perot interference filter 10, the pair of mirror portions 14, 15 facing each other in the optical axis direction D function as a pair of mirror portions with a variable distance between them. Here, the wavelength of light transmitted through the Fabry-Perot interference filter 10 depends on the distance between the mirror portion 14 and the mirror portion 15. Therefore, by adjusting the voltage (potential difference generated between the first electrode 125 and the third electrode 127) applied to the Fabry-Perot interference filter 10 (the first electrode 125 and the third electrode 127), it is possible to select the wavelength of light transmitted through the Fabry-Perot interference filter 10. In this way, the Fabry-Perot interference filter 10 transmits light of a wavelength corresponding to the distance between the mirror portions 14, 15 out of the incident light. [Calibration Device]

[0050] Next, a description will be given of the calibration device 1. As shown in Fig. 1, the calibration device 1 includes an optical element 2, a light source 3, a collimating lens 4, a bandpass filter 5, a condensing lens 6, a detector 7, a control device 8, and a Fabry-Perot interference filter 10.

[0051] The optical element 2 is an optical element that can be used to calibrate the relationship between applied voltage and transmission wavelength. The optical element 2 is a bandpass filter that selectively transmits light of a specific wavelength, such as an emission line filter. The optical element 2 has a substrate 21 and a dielectric multilayer film 22. The substrate 21 is a plate-shaped member made of a light-transmitting material such as glass or silicon. The dielectric multilayer film 22 is formed on one main surface 21a of the substrate 21. The dielectric multilayer film 22 is a multilayer film made of, for example, a combination of a high-refractive index material and a low-refractive index material.

[0052] The optical element 2 has a transmittance (light transmittance) peak at each of a plurality of wavelengths (hereinafter also referred to as peak wavelengths) including a first wavelength λ1 to an nth wavelength λn (n is an integer of 2 or greater). In this example, n is an integer of 10 or greater. FIG. 4 is a diagram schematically illustrating the light transmission characteristics of the optical element 2. As shown in FIG. 4, the optical element 2 has a transmittance peak at each of a plurality of peak wavelengths. The optical element 2 is designed to transmit light of wavelengths corresponding to the peak wavelengths while reducing or blocking light of other wavelengths. In this example, the transmittance at each peak is approximately 100%. The transmittance at each peak may be, for example, 50% or greater, or 80% or greater. Each peak has a shape that is line-symmetric (bilaterally symmetric in FIG. 4) with respect to an axis of symmetry passing through the center position (vertex) of the peak. The multiple peaks are arranged with a constant interference period. The spacing between the peaks may be approximately several tens of nanometers (e.g., approximately 50 nm). The spacing between the peaks increases slightly toward the longer wavelength side. The first wavelength λ1 to the n-th wavelength λn become shorter from the first wavelength λ1 side toward the n-th wavelength λn side (in the order of the first wavelength λ1, the second wavelength λ2, . . . , the n-th wavelength λn).

[0053] The light source 3 emits light L including a plurality of peak wavelengths. The light source 3 is, for example, a light source that emits light included in the near-infrared wavelength range. In this example, the emitted light L includes not only light from the first wavelength λ1 to the nth wavelength λn, but also light with a peak wavelength longer than the first wavelength λ1 (for example, the 0th wavelength λ0 shown in FIG. 4) and light with a peak wavelength shorter than the nth wavelength λn (for example, the n+1th wavelength λn+1 to the n+3rd wavelength λn+3 shown in FIG. 4).

[0054] Light L emitted from the light source 3 passes through an optical fiber (not shown), then passes through the collimating lens 4, the optical element 2, the bandpass filter 5, and the condenser lens 6 in this order, and is incident on the Fabry-Perot interference filter 10. As described above, the optical element 2 is designed to transmit light of a wavelength corresponding to the peak wavelength while reducing or blocking light of other wavelengths. Therefore, in this example, of the light L emitted from the light source 3, light having wavelengths other than the peak wavelength is reduced or blocked by the optical element 2, and light of the peak wavelength reaches the bandpass filter 5 without being reduced or blocked.

[0055] The collimating lens 4 is disposed on the optical path between the light source 3 and the optical element 2, and collimates the light L emitted from the light source 3. The collimating lens 4 collimates the light L so that the light L travels in a direction perpendicular to the extending direction of the dielectric multilayer film 22 of the optical element 2 (a direction perpendicular to the main surface 21 a). In this example, the collimating lens 4 is composed of a single lens. The collimating lens 4 may also be composed of multiple lens elements.

[0056] The bandpass filter 5 is an optical filter that transmits light in a specific wavelength band. As an example, the bandpass filter 5 is configured by forming a dielectric multilayer film (e.g., a multilayer film made of a combination of a high refractive index material and a low refractive index material) on the surface of a light-transmitting member formed in a plate shape from a light-transmitting material (e.g., silicon, glass, etc.).

[0057] The bandpass filter 5 is designed to transmit light of wavelengths within a specific wavelength band (transmission wavelength band) while reducing or blocking light of wavelengths outside the transmission wavelength band. FIG. 5 is a diagram schematically illustrating the light transmission characteristics of the bandpass filter 5. As shown in FIG. 5, the bandpass filter 5 transmits light of a transmission wavelength band of several hundred nanometers (e.g., 400 nm). In this example, the transmittance of the bandpass filter 5 in the transmission wavelength band is approximately 100%. The transmittance in the transmission wavelength band may be, for example, 50% or more, or 80% or more. While the wavelength band of light transmitted through the optical element 2 is extremely narrow, centered on the peak wavelength, the transmission wavelength band of the bandpass filter 5 is designed to be wider. In this example, the transmission wavelength band of the bandpass filter 5 ranges from wavelength λA to wavelength λB (λA<λB). The transmission wavelength band of the bandpass filter 5 includes the first wavelength λ1 to the n+2th wavelength λn+2 of the peak wavelengths of the optical element 2, but does not include light with a peak wavelength longer than the first wavelength λ1 (for example, the 0th wavelength λ0 shown in FIG. 4 ) or light with a peak wavelength shorter than the n+2th wavelength λn+2 (for example, the n+3rd wavelength λn+3 shown in FIG. 4 ). Therefore, in this example, as shown in FIG. 6 , of the light with multiple peak wavelengths incident on the bandpass filter 5, only light with the first wavelength λ1 to the n+2th wavelength λn+2 passes through the bandpass filter 5, and light with a peak wavelength longer than the first wavelength λ1 and light with a peak wavelength shorter than the n+2th wavelength λn+2 is blocked by the bandpass filter 5.

[0058] The condenser lens 6 is disposed on the optical path between the optical element 2 and the Fabry-Perot interference filter 10, and condenses the light L transmitted through the bandpass filter 5 onto the light transmission region 10a of the Fabry-Perot interference filter 10. In this example, the condenser lens 6 is composed of a single lens. The condenser lens 6 may also be composed of multiple lens elements.

[0059] The detector 7 detects the light L transmitted through the Fabry-Perot interference filter 10. Specifically, the detector 7 receives the light L transmitted through the Fabry-Perot interference filter 10 and acquires a photocurrent corresponding to the light L. The detector 7 may be, for example, an infrared detector such as a quantum sensor using InGaAs or the like, a thermal sensor using a thermopile or a bolometer, or a photodiode. The detector 7 outputs an electrical signal corresponding to the photocurrent to the control device 8.

[0060] The control device 8 may be configured by a computer including an arithmetic circuit such as a CPU for performing arithmetic processing, a recording medium including memories such as RAM and ROM, and an input / output device. The control device 8 may be configured by a computer such as a desktop personal computer, a laptop personal computer, or a smart device (e.g., a smartphone or a tablet terminal). As shown in FIG. 1 , the control device 8 has a memory unit 81, a voltage control unit 82, and a calibration unit 83.

[0061] The storage unit 81 is a recording medium constituted by, for example, a memory. The storage unit 81 stores various programs and data for controlling the operations of the Fabry-Perot interference filter 10 and the detector 7. The calibration method for the Fabry-Perot interference filter 10 executed in the calibration device 1 can be executed based on the programs stored in the storage unit 81. The storage unit 81 stores a relational expression between the applied voltage and the transmission wavelength. The storage unit 81 also stores the peak wavelength (first wavelength λ1, etc.) of the optical element 2.

[0062] The voltage control unit 82 and the calibration unit 83 are configured by an arithmetic circuit such as a CPU. Specifically, the arithmetic circuit functions as the voltage control unit 82 and the calibration unit 83 by reading and executing various programs stored in the storage unit 81.

[0063] The voltage control unit 82 generates a control signal for controlling a power supply device (not shown) connected to the Fabry-Perot interference filter 10 and outputs the control signal to the power supply device. The power supply device applies a voltage to the Fabry-Perot interference filter 10 (the first electrode 125 and the third electrode 127) based on the control signal received from the voltage control unit 82. As described above, the wavelength of light transmitted through the Fabry-Perot interference filter 10 depends on the distance between the mirror unit 14 and the mirror unit 15, and this distance is determined by the voltage applied to the Fabry-Perot interference filter 10. The voltage control unit 82 controls the voltage applied to the Fabry-Perot interference filter 10 using the control signal to change the distance between the mirror unit 14 and the mirror unit 15, thereby transmitting light of a specific wavelength through the Fabry-Perot interference filter 10. For example, the voltage control unit 82 controls the applied voltage to sequentially transmit light of wavelengths λ1 to λn transmitted through the bandpass filter 5 through the Fabry-Perot interference filter 10. The voltage control unit 82 may sequentially transmit light of wavelengths from the nth wavelength λn to the first wavelength λ1 through the Fabry-Perot interference filter 10. That is, the voltage control unit 82 may change the transmission wavelength of the Fabry-Perot interference filter 10 from the long wavelength side to the short wavelength side, or from the short wavelength side to the long wavelength side.

[0064] The calibration unit 83 calibrates the relational expression between the applied voltage and the transmitted wavelength stored in the storage unit 81. Specifically, the calibration unit 83 calibrates the relational expression between the applied voltage and the transmitted wavelength stored in the storage unit 81 based on a plurality of voltage values ​​at a plurality of peaks of the photocurrent corresponding to the light detected by the detector 7. The relational expression between the applied voltage and the transmitted wavelength is expressed by, for example, a cubic function (cubic equation). Details of the calibration process performed by the calibration unit 83 will be described later.

[0065] Light L emitted from the light source 3 is incident on the Fabry-Perot interference filter 10 via the optical element 2. As described above, the Fabry-Perot interference filter 10 transmits light of a wavelength corresponding to the applied voltage. Here, the transmission wavelength of the Fabry-Perot interference filter 10 will be described. The variable range of the transmission wavelength of the Fabry-Perot interference filter 10 is defined as variable range R. The longest transmission wavelength within variable range R is defined as longest transmission wavelength λmax, and the shortest transmission wavelength is defined as shortest transmission wavelength λmin. By controlling the voltage applied to the Fabry-Perot interference filter 10 by a voltage control unit 82, the transmission wavelength of the Fabry-Perot interference filter 10 can be appropriately changed within the range from the longest transmission wavelength λmax to the shortest transmission wavelength λmin. Furthermore, as shown in FIG. 6 , the first wavelength λ1 to the (n+2)th wavelength λn+2 (the first wavelength λ1 to the nth wavelength λn) are located within the variable range R. When the variable range R is divided into four equal parts, each range includes at least one of the first wavelength λ1 to the nth wavelength λn, which are the peak wavelengths of the optical element 2. The variable range R is a range determined by the specifications (design) of the Fabry-Perot interference filter 10, and an actual Fabry-Perot interference filter 10 can transmit light with wavelengths shorter than the shortest transmission wavelength λmin by controlling the applied voltage.

[0066] FIG. 7 shows the relationship between applied voltage and transmission wavelength for each of multiple Fabry-Perot interference filters 10 having similar specifications (designs). The relationship between applied voltage and transmission wavelength (the graph shown in FIG. 7) is represented by a cubic function. FIG. 7 shows three graphs corresponding to three Fabry-Perot interference filters 10. As shown in FIG. 7, the transmission wavelength of the Fabry-Perot interference filter 10 changes depending on the voltage applied to the Fabry-Perot interference filter 10. Specifically, as the applied voltage increases, the transmission wavelength of the Fabry-Perot interference filter 10 decreases. The rate of change in the transmission wavelength increases as the applied voltage increases. In other words, the amount of change in the transmission wavelength when the voltage is increased by a unit amount is greater the shorter the transmission wavelength (the closer to the short wavelength side).

[0067] As shown in FIG. 7 , even among Fabry-Perot interference filters 10 with similar specifications, variations (individual differences) can occur in the relationship between the applied voltage and the transmission wavelength. Such variations are due to, for example, manufacturing errors related to the distance between the mirror portions 14 and 15, the stress (tension) or thickness of the mirror portion 15 (membrane), or environmental errors related to the temperature of the environment in which the Fabry-Perot interference filter 10 is used or the fixing method of the Fabry-Perot interference filter 10. Therefore, even among Fabry-Perot interference filters 10 with the same specifications, the applied voltage required to reach the shortest transmission wavelength λ min varies. For actual Fabry-Perot interference filters 10, a range of applied voltages (operating voltages) that will not damage the Fabry-Perot interference filter 10 is predetermined, taking into account individual differences (even if individual differences exist). The operating voltage is determined, for example, by the specifications (design) of the Fabry-Perot interference filter 10. Hereinafter, the maximum value of the operating voltage is also referred to as the first voltage V1. As described above, due to individual differences in actual Fabry-Perot interference, variations occur in the applied voltage required to reach the shortest transmission wavelength λ min. The minimum voltage in the range (variation range) of the voltage that can be applied to reach the shortest transmission wavelength λmin corresponds to the first voltage V1.

[0068] Next, the relationship between the half-width of the transmittance peak of the Fabry-Perot interference filter 10 and the half-width of the transmittance peak of the optical element 2 will be described. FIG. 8 is a diagram showing the light transmission characteristics of the Fabry-Perot interference filter 10. FIG. 8 shows the transmittance of light transmitted through the Fabry-Perot interference filter 10 when different voltages are applied sequentially. In the example shown in FIG. 8, the transmittance of light transmitted through the Fabry-Perot interference filter 10 when voltages corresponding to transmission wavelengths of 1550 nm, 1650 nm, 1700 nm, 1750 nm, and 1850 nm are applied is shown. As shown in FIG. 8, the half-width W1 of the transmittance peak of the Fabry-Perot interference filter 10 is approximately 10 nm to 20 nm. In contrast, the half-width W2 of the transmittance peak of the optical element 2 (see FIG. 4) is approximately 1 nm, which is smaller than the half-width W1. 8 (for example, a voltage for transmitting light corresponding to the peak wavelength of the optical element 2) is applied to the Fabry-Perot interference filter 10, the half-width W1 is approximately 10 nm to 20 nm. That is, at each of a plurality of peak wavelengths (wavelengths transmitted through the optical element 2) including the first wavelength λ1 to the nth wavelength λn, the half-width W2 is smaller than the half-width W1. The half-width W2 may be one-half or less, or one-fourth or less, of the half-width W1. [Calibration Method]

[0069] A description will now be given of a method for calibrating (calibration processing) the Fabry-Perot interference filter 10 in the calibration device 1. Fig. 9 is a flowchart showing the calibration method of this embodiment.

[0070] First, a calibration device 1 is prepared (step S11). Specifically, an optical element 2, a light source 3, a collimating lens 4, a bandpass filter 5, a condensing lens 6, a detector 7, a control device 8, and a Fabry-Perot interference filter 10 are prepared. The peak wavelength (first wavelength λ1, etc.) of the optical element 2 is measured in advance using a calibrated device and stored in a storage unit 81.

[0071] Next, the collimator lens 4, the optical element 2, the band-pass filter 5, and the condenser lens 6 are arranged in this order on the optical path from the light source 3 to the Fabry-Perot interference filter 10 (step S12). That is, on the optical path, the collimator lens 4 is arranged between the light source 3 and the optical element 2, and the band-pass filter 5 and the condenser lens 6 are arranged between the optical element 2 and the Fabry-Perot interference filter 10.

[0072] Next, the light source 3 emits light L (step S13). The light L includes light of multiple wavelengths corresponding to the multiple peak wavelengths of the optical element 2. That is, the light L includes not only light of the first wavelength λ1 to the n-th wavelength λn, but also light of peak wavelengths longer than the first wavelength λ1 (e.g., corresponding to the 0th wavelength λ0 shown in FIG. 4 ) and light of peak wavelengths shorter than the n-th wavelength λn (e.g., corresponding to the n+1th wavelength λn+1 to the n+3rd wavelength λn+3 shown in FIG. 4 ).

[0073] The emitted light L is collimated by the collimating lens 4 and enters the optical element 2. As described above, the optical element 2 is designed to transmit light of a wavelength corresponding to its peak wavelength while reducing or blocking light of other wavelengths. In this example, of the light L emitted from the light source 3, light of wavelengths other than the peak wavelength is blocked by the optical element 2, while the light of the peak wavelength reaches the bandpass filter 5 without being blocked.

[0074] Light transmitted through the optical element 2 is incident on the bandpass filter 5. In this example, the transmission wavelength band of the bandpass filter 5 includes the first wavelength λ1 to the n+2th wavelength λn+2 of the peak wavelengths of the optical element 2, but does not include light with a peak wavelength longer than the first wavelength λ1 (e.g., the 0th wavelength λ0 shown in FIG. 4 ) or light with a peak wavelength shorter than the n+2th wavelength λn+2 (e.g., the n+3rd wavelength λn+3 shown in FIG. 4 ). Therefore, of the light with multiple peak wavelengths incident on the bandpass filter 5, only light with the first wavelength λ1 to the n+2th wavelength λn+2 passes through the bandpass filter 5, while light with peak wavelengths longer than the first wavelength λ1 and light with peak wavelengths shorter than the n+2th wavelength λn+2 are blocked by the bandpass filter 5 (see FIG. 6 ). The light L transmitted through the bandpass filter 5 (light with the first wavelength λ1 to the n+2th wavelength λn+2) is collected by the collecting lens 6 and enters the light transmission region 10 a of the Fabry-Perot interference filter 10.

[0075] Next, the calibration device 1 executes a first detection step (step S14). In the first detection step, the voltage control unit 82 controls the voltage applied to the Fabry-Perot interference filter 10 to change the distance between the pair of mirror units 14, 15, thereby sequentially transmitting light of the first wavelength λ1 to the nth wavelength λn through the Fabry-Perot interference filter 10. The detector 7 then acquires photocurrents corresponding to the transmitted light of the first wavelength λ1 to the nth wavelength λn.

[0076] Specifically, the voltage control unit 82 controls the voltage applied to the Fabry-Perot interference filter 10 so as to sweep the transmission wavelength of the Fabry-Perot interference filter 10 in a wavelength range including the first wavelength λ1 to the nth wavelength λn. In this example, the voltage control unit 82 controls (sweeps in this example) the applied voltage in the range from 0 V to the first voltage V1. "Sweeping" refers to continuously changing a target value. In this example, when the applied voltage is the first voltage V1, the transmission wavelength of the Fabry-Perot interference filter 10 is located between the nth wavelength λn and the (n+1)th wavelength λn+1. Therefore, sweeping the applied voltage in the range from 0 V to the first voltage V1 corresponds to sweeping the transmission wavelength of the Fabry-Perot interference filter 10 in a wavelength range including the first wavelength λ1 to the nth wavelength λn. That is, when the applied voltage is swept in the range from 0 V to the first voltage V1, light of the first wavelength λ1 to the nth wavelength λn passes through the Fabry-Perot interference filter 10 sequentially, and light of the n+1st wavelength λn+1 and the n+2nd wavelength λn+2 does not pass through the Fabry-Perot interference filter 10.

[0077] The applied voltage (voltage sweep interval) may be determined based on a polynomial, a sine function, or the like so that the rate of increase of the applied voltage decreases over time. As described above, the relationship between the applied voltage and the transmission wavelength is expressed, for example, by a cubic function, and the rate of change of the transmission wavelength increases as the applied voltage increases (see FIG. 7 ). Therefore, by determining the applied voltage so that the rate of increase of the applied voltage decreases over time, highly accurate detection can be performed even on the short wavelength side. Specifically, FIG. 10 shows an example of the change over time of the applied voltage to the Fabry-Perot interference filter 10. In this example, the applied voltage changes according to the portion indicated by the solid line in the graph of the applied voltage (sine function) shown in FIG. 10 . That is, as time passes, the rate of increase of the applied voltage increases and then gradually decreases. As a result, in the relationship between the transmission wavelength and applied voltage of the Fabry-Perot interference filter 10, the sweep voltage interval can be increased to reduce the number of measurement points on the long wavelength side where the rate of variation of the transmission wavelength (the slope of the graph in Figure 7) is small, and the sweep voltage interval can be decreased to increase the number of measurement points on the short wavelength side where the rate of variation of the transmission wavelength is large, making it possible to achieve (improve) both measurement accuracy and measurement time.

[0078] Next, the calibration unit 83 executes a first calibration step (step S15). In the first calibration step, the relationship equation between the applied voltage and the transmission wavelength stored in the memory unit 81 is calibrated based on multiple voltage values ​​at multiple peaks of the photocurrent corresponding to the detected light. FIG. 11 is a graph showing the relationship between the voltage applied to the Fabry-Perot interference filter 10 and the photocurrent corresponding to the light detected by the detector 7 in the first detection step. In the first detection step, light having the first wavelength λ1 to the nth wavelength λn is sequentially transmitted through the Fabry-Perot interference filter 10. Therefore, multiple peaks corresponding to the first wavelength λ1 to the nth wavelength λn appear in the photocurrent. Furthermore, the Fabry-Perot interference filter 10 transmits light having the first wavelength λ1 to the nth wavelength λn in order as the applied voltage increases. Therefore, peak P1 (the first peak) shown in FIG. 11 corresponds to the peak when light having the first wavelength λ1 is detected, and peak P2 corresponds to the peak when light having the second wavelength λ2 is detected. The same applies to peaks P3 to Pn.

[0079] As described above, the memory unit 81 stores a plurality of peak wavelengths (e.g., the first wavelength λ1 to the nth wavelength λn) possessed by the optical element 2. Therefore, the calibration unit 83 can identify the correspondence between the peak wavelengths (the wavelengths of light actually transmitted through the Fabry-Perot interference filter 10) and the applied voltage by associating the peak wavelengths stored in the memory unit 81 with peaks in the photocurrent. The calibration unit 83 calibrates the relational expression between the applied voltage and the transmitted wavelength stored in the memory unit 81 (calculates a correction coefficient) based on the identified correspondence between the peak wavelengths and the applied voltages.

[0080] Next, the voltage control unit 82 calculates (step S16) a second voltage V2 corresponding to the shortest transmission wavelength λmin of the Fabry-Perot interference filter 10, using the relational expression between the applied voltage and the transmission wavelength calibrated in the first calibration step (step S15). For example, the voltage control unit 82 substitutes the shortest transmission wavelength λmin into the relational expression between the applied voltage and the transmission wavelength, and calculates the applied voltage corresponding to the shortest transmission wavelength λmin as the second voltage V2.

[0081] Next, the voltage control unit 82 calculates the third voltage V3 (step S17). The actual Fabry-Perot interference filter 10 is configured so as not to be damaged even if a third voltage V3 slightly greater than the applied voltage (second voltage V2) corresponding to the shortest transmission wavelength λmin is applied. For example, the third voltage V3 may be a voltage obtained by adding a predetermined voltage (e.g., 0.5 V) to the second voltage V2. Because the third voltage V3 is greater than the second voltage V2, as shown in FIG. 12 , the transmission wavelength when the third voltage V3 is applied to the Fabry-Perot interference filter 10 is shorter than the transmission wavelength when the second voltage V2 is applied. As an example, when the difference between the second voltage V2 and the third voltage V3 is 0.5 V, the transmission wavelength when the third voltage V3 is applied is approximately 25 nm shorter than the transmission wavelength when the second voltage V2 is applied. The third voltage V3 (the predetermined voltage added to the second voltage V2) may be determined based on the position of the peak wavelength of the optical element 2. Specifically, the third voltage V3 (a predetermined voltage to be added to the second voltage V2) may be determined so that the third voltage V3 is located within a range of applied voltages that will not damage the Fabry-Perot interference filter 10, and so that one or more peak wavelengths of the optical element 2 are included between the transmission wavelength corresponding to the second voltage V2 and the transmission wavelength corresponding to the third voltage V3.

[0082] Next, the calibration device 1 executes a second detection step (step S18). In the second detection step, the voltage control unit 82 controls the voltage applied to the Fabry-Perot interference filter 10 within a voltage range including the third voltage V3, thereby changing the distance between the pair of mirror units 14, 15. Then, the detector 7 acquires a photocurrent corresponding to the light L transmitted through the Fabry-Perot interference filter 10.

[0083] In this example, the voltage control unit 82 controls (sweeps, in this example) the applied voltage in the range from 0 V to the third voltage V3. The transmission wavelength of the Fabry-Perot interference filter 10 when the applied voltage is the third voltage V3 is located between the n+2th wavelength λn+2 and the n+3rd wavelength λn+3. Therefore, sweeping the applied voltage in the range from 0 V to the third voltage V3 corresponds to sweeping the transmission wavelength of the Fabry-Perot interference filter 10 in a wavelength range including the first wavelength λ1 to the n+2th wavelength λn+2. In other words, when the applied voltage is swept in the range from 0 V to the third voltage V3, light of the first wavelength λ1 to the n+2nd wavelength λn+2 is sequentially transmitted through the Fabry-Perot interference filter 10. In the first detection step (step S14), the light of the n+1th wavelength λn+1 and the n+2nd wavelength λn+2 did not pass through the Fabry-Perot interference filter 10, but in the second detection step, the n+1th wavelength λn+1 and the n+2nd wavelength λn+2 pass through the Fabry-Perot interference filter 10. Therefore, the number of photocurrent peaks acquired in the second detection step is n+2, which is more than the number of photocurrent peaks (n) acquired in the first detection step.

[0084] Subsequently, the calibration unit 83 executes a second calibration step (step S19). In the second calibration step, the relationship between the applied voltage and the transmission wavelength calibrated in the first calibration step (step S15) is further calibrated based on multiple voltage values ​​at multiple peaks of the photocurrent corresponding to the light newly detected in the second detection step. As described above, the memory unit 81 stores multiple peak wavelengths (e.g., the first wavelength λ1 to the n+2nd wavelength λn+2) possessed by the optical element 2. As in the processing in the first calibration step, the calibration unit 83 can identify the correspondence between the peak wavelength (the wavelength of the light actually transmitted through the Fabry-Perot interference filter 10) and the applied voltage by associating the peak wavelengths stored in the memory unit 81 with the peaks in the photocurrent. The calibration unit 83 calibrates the relationship between the applied voltage and the transmission wavelength based on the identified correspondence between the peak wavelength and the applied voltage.

[0085] Next, the calibration unit 83 determines whether the number of peaks detected in the photocurrent is the expected number (step S20). Specifically, it determines whether the number of peaks included in the photocurrent matches the number of peaks expected based on the relational expression between the applied voltage and the transmission wavelength (the number of peak wavelengths of the optical element 2 included in the transmission wavelength range corresponding to 0 V to the third voltage). If it is determined that they do not match, the output device of the control device 8 may output information indicating that they do not match.

[0086] Next, the calibration unit 83 inspects (calculates) the wavelength resolution (FWHM) of the Fabry-Perot interference filter 10 (step S21). Specifically, the calibration unit 83 first creates a graph (corresponding to FIG. 11 ) showing the relationship between the voltage applied to the Fabry-Perot interference filter 10 and the photocurrent corresponding to the light detected by the detector 7 in the second detection step. Next, the calibration unit 83 performs interpolation or approximation on the created graph to create a graph showing the relationship between the transmission wavelength of the Fabry-Perot interference filter 10 and the photocurrent, as shown in FIG. 13 . Next, the calibration unit 83 calculates the wavelength resolution of the Fabry-Perot interference filter 10 based on the height (photocurrent value) of the target peak and the height (photocurrent value) of the lower of the bottoms located on the left and right of the peak (the peak is sandwiched between two bottoms). This completes the processing of the calibration device 1. [Operation and Effects]

[0087] In the calibration method, calibration device 1, and optical element 2 according to this embodiment, the half-width W2 of the transmittance peak of the optical element 2 is smaller than the half-width W1 of the transmittance peak of the Fabry-Perot interference filter 10 at each of the first wavelength λ1 to the nth wavelength λn. That is, because the optical element 2 has high spectral resolution, the photocurrent undulations appear clearly in the detection results from the detector 7. Therefore, the positions of the photocurrent peaks (peaks P1 to Pn) corresponding to light of the first wavelength λ1 to the nth wavelength λn can be easily and accurately identified, improving the accuracy of calibration. Furthermore, because the optical element 2 has multiple transmittance peaks, it is possible to obtain multiple correspondence relationships between the applied voltage and the transmission wavelength in the Fabry-Perot interference filter 10, for example, without switching the wavelength of light emitted from the light source 3. That is, the relationship between the applied voltage and the transmission wavelength can be calibrated using a simple method. Therefore, the calibration method, calibration device 1, and optical element 2 enable easy calibration with high accuracy.

[0088] The calibration method described in Patent Document 1 performs calibration using the peak wavelengths (intrinsic wavelengths) of known components such as hemoglobin, making it impossible to freely set the number of peaks and the interval between the peaks in the variable transmission wavelength range of the Fabry-Perot interference filter. In contrast, the calibration device 1 allows the optical element 2 to be designed to have any number and interval of transmittance peaks. Furthermore, the half-widths of the peaks of the known components described in Patent Document 1 are large and the peak shapes are asymmetric. Therefore, the method described in Patent Document 1 makes it difficult to identify the peak positions in the acquired photocurrent, which may reduce the accuracy of the calibration. In contrast, the optical element 2 allows the transmittance peak shape to be set so that the peak positions in the photocurrent are easily identified. Specifically, the half-width W2 of the transmittance peak of the optical element 2 is smaller than the half-width W1 of the transmittance peak of the Fabry-Perot interference filter 10, and the transmittance peak of the optical element 2 has a shape that is axisymmetric with respect to the axis of symmetry passing through the central position (vertex). This allows the peak positions in the detected photocurrent to be easily and accurately identified, improving the accuracy of the calibration.

[0089] A conventional calibration method involves applying a predetermined voltage to a Fabry-Perot interference filter, transmitting light of a single peak wavelength while sweeping the wavelength using a monochromator, and measuring the wavelength at which the amount of light transmitted (output) through the Fabry-Perot interference filter peaks. This process is performed at multiple (e.g., 10 or more) applied voltages, thereby calibrating the relationship between the applied voltage and the transmission wavelength. However, monochromators are generally expensive and may be difficult to install. Furthermore, the method of sequentially changing the applied voltage to multiple voltage values ​​and sweeping the wavelength of light of a single peak wavelength at each voltage value (switching wavelengths) is complex and requires a lot of time and effort to obtain the data used for calibration. In contrast, the calibration method and calibration device 1 according to this embodiment use an optical element 2 designed to have multiple transmittance peaks, and a photocurrent with multiple peaks is obtained by sweeping the applied voltage to the Fabry-Perot interference filter 10. That is, in this embodiment, the relationship between the applied voltage and the transmission wavelength can be calibrated with a single wavelength sweep. That is, the relationship between the applied voltage and the transmission wavelength can be calibrated using a simpler method.

[0090] Furthermore, as described above, even for Fabry-Perot interference filters 10 with the same specifications (design), the relationship between the applied voltage and the transmission wavelength in the Fabry-Perot interference filter 10 may vary between individual filters due to manufacturing errors, the usage environment, or the like. Therefore, it is preferable to calibrate the relationship between the applied voltage and the transmission wavelength in the optical system and environment in which the Fabry-Perot interference filter 10 is actually used. According to the calibration method and calibration device 1 of this embodiment, calibration can be performed without using special equipment, etc., and therefore calibration can be performed in the optical system and environment in which the Fabry-Perot interference filter 10 is actually used, thereby improving the accuracy of the calibration.

[0091] n is an integer equal to or greater than 4. As a result, four or more peaks appear in the photocurrent corresponding to the detected light, and therefore, based on the four or more voltage values ​​at these peaks, it is possible to calibrate with high precision the relationship between the applied voltage and the transmitted wavelength, which is expressed as, for example, a cubic function.

[0092] The variable range R of the transmission wavelength of the Fabry-Perot interference filter 10 is divided into four equal parts, and each of the four equal parts includes at least one of the first wavelength λ1 to the nth wavelength λn. This makes it possible to calibrate the relationship between the applied voltage and the transmission wavelength over the entire variable range R of the transmission wavelength of the Fabry-Perot interference filter 10.

[0093] n is an integer equal to or greater than 10. As a result, 10 or more peaks appear in the photocurrent corresponding to the detected light, and the relationship between the applied voltage and the transmitted wavelength can be calibrated with higher accuracy based on the 10 or more voltage values ​​at these peaks.

[0094] In the first detection step, the voltage control unit 82 controls the voltage applied to the Fabry-Perot interference filter 10 so as to sweep the transmission wavelength of the Fabry-Perot interference filter 10 in a wavelength range including the first wavelength λ1 to the nth wavelength λn. This makes it possible to obtain multiple correspondence relationships between the voltage applied to the Fabry-Perot interference filter 10 and the transmission wavelength of the Fabry-Perot interference filter 10 by a single wavelength sweep, thereby shortening the time required for the calibration process.

[0095] The optical element 2 is a bandpass filter having a dielectric multilayer film 22. This allows the optical element 2 to be configured so that the transmittance peak characteristics (shape, position, etc.) are as desired.

[0096] The optical element 2 is a bandpass filter, and the collimating lens 4 is disposed on the optical path between the light source 3 and the optical element 2. This makes it possible to solve the problem caused by the dependency of the performance of the bandpass filter on the angle of incidence.

[0097] In the first detection step, the voltage control unit 82 controls the voltage applied to the Fabry-Perot interference filter 10 within a range from 0 V to a first voltage V1, which is the maximum operating voltage of the Fabry-Perot interference filter 10. As a result, even if there are individual differences in the Fabry-Perot interference filters 10, the applied voltage is controlled within a voltage range in which the Fabry-Perot interference filter 10 will not be damaged, and therefore the relationship between the applied voltage and the transmission wavelength can be appropriately calibrated.

[0098] The calibration method of this embodiment includes the steps of: calculating a second voltage V2 corresponding to the shortest transmission wavelength of the Fabry-Perot interference filter 10 using the relational expression calibrated in the first calibration step (step S15) (step S16); changing the distance between the mirror portions 14 and 15 by controlling the voltage applied to the Fabry-Perot interference filter 10 within a voltage range including a third voltage V3 greater than the second voltage V2; and detecting light transmitted through the Fabry-Perot interference filter 10 with the detector 7 (step S18); and further calibrating the relational expression based on the voltage value at the peak of the photocurrent corresponding to the light newly detected in the second detection step (step S19). This allows the relational expression between the applied voltage and the transmission wavelength to be calibrated with higher accuracy based on the voltage value at the peak newly detected in the second detection step (not detected in the first detection step).

[0099] If the calibration device 1 does not include a bandpass filter 5 and the maximum transmission wavelength λmax of the Fabry-Perot interference filter 10 varies (e.g., by about 10 nm to 20 nm) due to individual differences, it may be difficult to determine the correspondence between the peak in the photocurrent and the wavelength. Specifically, if the calibration device 1 does not include a bandpass filter 5, light with a peak wavelength near the maximum transmission wavelength λmax that is longer than the first wavelength λ1 also reaches the Fabry-Perot interference filter 10. Therefore, if the maximum transmission wavelength λmax varies, it is difficult to determine which peak wavelength the peak P1 in the photocurrent corresponds to. In contrast, in this embodiment, the calibration device 1 includes a bandpass filter 5, and the bandpass filter 5 blocks light with a peak wavelength near the maximum transmission wavelength λmax that is longer than the first wavelength λ1 (e.g., within 20 nm). This makes it easy to determine that the peak P1 that first appears in the photocurrent corresponds to the first wavelength λ1. That is, the transmission wavelength band of the bandpass filter 5 is narrower than the variable range R of the transmission wavelength of the Fabry-Perot interference filter 10 (the maximum wavelength of the transmission wavelength band of the bandpass filter 5 is shorter than the longest transmission wavelength λmax by a predetermined wavelength or more), so that the peaks can be easily associated.

[0100] The present disclosure is not limited to the above-described embodiments. For example, the optical element 2 may have a peak of reflectance (light reflectance) at each of a plurality of peak wavelengths including the first wavelength λ1 to the nth wavelength λn. In this case, the description of the peak of transmittance of the optical element 2 in the embodiments and modified examples can be replaced with a description of the peak of reflectance of the optical element 2. Light L emitted from the light source 3 is reflected by the optical element 2 and enters the Fabry-Perot interference filter 10. Furthermore, the description "an object transmits (or reflects) light of a specific wavelength" in the embodiments and modified examples includes the object absorbing light of a specific wavelength and transmitting (or reflecting) light other than the specific wavelength.

[0101] When the variable range R of the transmission wavelength of the Fabry-Perot interference filter 10 is divided into 10 equal parts, each range may include at least one of the first wavelength λ1 to the nth wavelength λn. In this case, the relationship between the applied voltage and the transmission wavelength can be calibrated over the entire variable range R of the transmission wavelength of the Fabry-Perot interference filter 10.

[0102] The Fabry-Perot interference filter 10 and the detector 7 may constitute an integrated (e.g., packaged) sensor. The band-pass filter 5 may be provided as a part of the sensor (may be built into the sensor), or may be provided as a separate component from the sensor as in the above embodiment. The band-pass filter 5 may be disposed between the condenser lens 6 and the Fabry-Perot interference filter 10 on the optical path.

[0103] In the first detection step (step S14) and the second detection step (step S18), the voltage control unit 82 may change the transmission wavelength of the Fabry-Perot interference filter 10 from a wavelength on the short wavelength side to a wavelength on the long wavelength side (changing the applied voltage from the high voltage side to the low voltage side).

[0104] In the first detection step (step S14) and the second detection step (step S18), the voltage control unit 82 may intermittently (spot-wise) sweep the applied voltage within a voltage range having a predetermined width, rather than continuously sweeping the applied voltage from 0 V to the maximum voltage to be applied. For example, the voltage control unit 82 may apply (sweep) a voltage within a range including a voltage corresponding to a specific peak wavelength among the multiple peak wavelengths possessed by the optical element 2 (voltages before and after the specific peak wavelength), and then apply (sweep) a voltage within a range including a voltage corresponding to another peak wavelength, thereby detecting multiple transmittance peaks (peaks of photocurrent detected by the detector 7) of the Fabry-Perot interference filter 10. As another example, in the second detection step, the voltage control unit 82 may sweep the applied voltage within a predetermined voltage range including the third voltage V3 (a range around the third voltage V3 that does not include 0 V).

[0105] In the first calibration step (step S15) and the second calibration step (step S19), the calibration unit 83 identifies at least two peaks, but is not limited to this number. However, the number of peaks to be identified in the photocurrent varies depending on the type of the relational expression between the applied voltage and the transmitted wavelength. For example, when the relational expression between the applied voltage and the transmitted wavelength is expressed as a cubic function, it is preferable that four or more peaks be identified in the photocurrent. Therefore, when the relational expression is expressed as a cubic function, the calibration unit 83 may terminate the calibration process if four or more peaks cannot be identified in the photocurrent (if no peaks exist) in the first calibration step (step S15) and the second calibration step (step S19). In other words, the calibration unit 83 may terminate the calibration process if the calibration unit 83 cannot identify a predetermined number of peaks determined according to the type of the relational expression in the first calibration step (step S15) and the second calibration step (step S19).

[0106] In the first calibration step (step S15) and the second calibration step (step S19), the calibration unit 83 may identify peaks in the photocurrent after performing interpolation or peak fitting on the acquired photocurrent data. The interpolation may be, for example, linear interpolation, spline interpolation, Lagrange interpolation, or Hermino interpolation. The peak fitting may be least-squares fitting, polynomial fitting, or Gaussian fitting.

[0107] The method of associating the peak in the photocurrent with the peak wavelength, which is performed by the calibration unit 83 in the first calibration step (step S15) and the second calibration step (step S19), is not limited. For example, the calibration unit 83 determines a predetermined wavelength range that includes the peak wavelength of the optical element 2. Then, the calibration unit 83 may associate a peak that appears in the photocurrent (the transmission peak of the Fabry-Perot interference filter 10) with the peak wavelength when a voltage is applied to the Fabry-Perot interference filter 10 so as to transmit light of wavelengths in that range. Note that if no peak appears in the photocurrent (if there is no transmission peak within the predetermined wavelength range), the calibration unit 83 may determine that the Fabry-Perot interference filter 10 is defective and terminate the calibration process.

[0108] The interval between the peaks of the transmittance or reflectance of the optical element 2 may be larger than the width (e.g., 20 nm) of the variation in the transmission wavelength of the Fabry-Perot interference filter 10. In this case, it is easy to associate the peaks in the detected photocurrent with the peak wavelengths of the optical element 2.

[0109] The relational expression between the applied voltage and the transmission wavelength is not limited to a cubic function (cubic expression), but may be a linear function (linear expression), a quadratic function (quadratic expression), or a quartic function (quartic expression) or higher order relational expression, or may be expressed by, for example, a relational expression between the elastic coefficient and the deflection amount of the mirror portion as disclosed in Japanese Patent Application Laid-Open No. 2018-197780. The elastic coefficient is the elastic coefficient related to a virtual spring provided on the mirror portion when the deflection resistance of the mirror portion is expressed as the virtual spring.

[0110] In the above calibration method, the step of calculating the third voltage V3 (step S17) from the first detection step (step S14) may be omitted. For example, if the value of the third voltage V3 has already been calculated, steps S14 to S17 may be omitted, and the second detection step (step S18) and the second calibration step (step S19) may be performed using the calculated third voltage V3. In this case, by considering that the minimum value of the peak wavelength detectable by applying up to the third voltage V3 (the (n+2)th wavelength λn+2 in the above embodiment) corresponds to the nth wavelength λn in the first detection step, it can also be said that the second detection step corresponds to the first detection step, and the second calibration step corresponds to the first calibration step.

[0111] In the above-described calibration steps (the first calibration step of step S15 and the second calibration step of step S18), the calibration unit 83 may determine the correspondence between the peaks in the photocurrent and the peak wavelengths based on the voltage intervals between the peaks in the photocurrent. Specifically, since the change (slope) in the transmission wavelength with respect to the fluctuation of the voltage applied to the Fabry-Perot interference filter 10 is smaller on the long wavelength side than on the short wavelength side, the voltage intervals between the peaks are different from each other, as shown in FIG. 11 . For example, the voltage interval between peak P1 corresponding to the first wavelength λ1 and peak P2 corresponding to the second wavelength λ2 is larger than the voltage interval between peak P3 corresponding to the third wavelength λ3 and peak P4 corresponding to the fourth wavelength λ4, which are located on the shorter wavelength side. The calibration unit 83 may determine the correspondence between the peaks in the photocurrent and the peak wavelengths based on the amount of change in this voltage interval.

[0112] The above-described calibration method, calibration device 1, and optical element 2 can also be applied to the calibration of other spectroscopic modules (e.g., spectrometers, FTIR devices (Fourier transform infrared spectroscopic analyzers), etc.) other than Fabry-Perot interference filters.

[0113] 1...calibration device, 2...optical element, 3...light source, 4...collimating lens, 5...bandpass filter, 7...detector, 10...Fabry-Perot interference filter, 14, 15...mirror section, 22...dielectric multilayer film, 82...voltage control section, 83...calibration section, W1, W2...half width

Claims

1. The method includes the steps of preparing an optical element having a transmittance or reflectance peak at each of a plurality of wavelengths including a first wavelength to an nth wavelength (n is an integer of 2 or more), a light source that emits light including the plurality of wavelengths, a Fabry-Perot interference filter having a pair of mirror portions whose distance from each other is variable, and a detector that detects light that has passed through the Fabry-Perot interference filter; arranging the optical element on an optical path from the light source to the Fabry-Perot interference filter; a first detection step of controlling a voltage applied to the Fabry-Perot interference filter to change the distance, thereby causing light of the first wavelength to the nth wavelength out of the light emitted from the light source to pass through the Fabry-Perot interference filter in sequence, and detecting the transmitted light of the first wavelength to the nth wavelength with the detector; and a first calibration step of calibrating a relational expression between the voltage applied to the Fabry-Perot interference filter and the transmission wavelength of the Fabry-Perot interference filter based on a plurality of voltage values ​​at a plurality of peaks of a photocurrent corresponding to the detected light, A calibration method, wherein a half width of the peak of the optical element is smaller than a half width of a transmittance peak of the Fabry-Perot interference filter at each of the first wavelength to the nth wavelength.

2. The calibration method according to claim 1, wherein n is an integer of 4 or greater.

3. The calibration method according to claim 2, wherein the variable range of the transmission wavelength of the Fabry-Perot interference filter is divided into four equal parts, and each of the four equal parts includes at least one of the first wavelength to the nth wavelength.

4. A calibration method according to any one of claims 1 to 3, wherein n is an integer of 10 or greater.

5. The calibration method according to claim 4, wherein the variable range of the transmission wavelength of the Fabry-Perot interference filter is divided into ten equal parts, and each of the ten ranges includes at least one of the first wavelength to the nth wavelength.

6. A calibration method according to any one of claims 1 to 5, wherein in the first detection step, the voltage applied to the Fabry-Perot interference filter is controlled so as to sweep the transmission wavelength of the Fabry-Perot interference filter in a wavelength range including the first wavelength to the nth wavelength.

7. A calibration method according to any one of claims 1 to 6, wherein the optical element is a bandpass filter having a dielectric multilayer film.

8. A calibration method according to any one of claims 1 to 7, wherein the optical element is a band-pass filter, and the arranging step further comprises arranging a collimating lens on the optical path between the light source and the optical element.

9. A calibration method according to any one of claims 1 to 8, wherein in the first detection step, the voltage applied to the Fabry-Perot interference filter is controlled in a range from 0 V to a first voltage that is the maximum operating voltage of the Fabry-Perot interference filter.

10. A calibration method according to any one of claims 1 to 9, comprising: a step of calculating a second voltage corresponding to the shortest transmission wavelength of the Fabry-Perot interference filter using the relational expression calibrated in the first calibration step; a second detection step of changing the distance by controlling the voltage applied to the Fabry-Perot interference filter within a voltage range including a third voltage greater than the second voltage, and detecting light transmitted through the Fabry-Perot interference filter with the detector; and a second calibration step of further calibrating the relational expression based on the voltage value at the peak of the photocurrent corresponding to the light newly detected in the second detection step.

11. A calibration device comprising: an optical element having a transmittance or reflectance peak at each of a plurality of wavelengths including a first wavelength to an nth wavelength (n is an integer of 2 or more); a Fabry-Perot interference filter having a pair of mirror sections whose distance from each other is variable, and into which light including the plurality of wavelengths emitted from a light source is incident via the optical element; a voltage control section that controls the voltage applied to the Fabry-Perot interference filter to change the distance, thereby causing light of the first wavelength to the nth wavelength of the light emitted from the light source to be transmitted sequentially through the Fabry-Perot interference filter; a detector that detects the transmitted light of the first wavelength to the nth wavelength; and a calibration section that calibrates the relationship between the voltage applied to the Fabry-Perot interference filter and the transmission wavelength of the Fabry-Perot interference filter based on a plurality of voltage values ​​at a plurality of peaks of a photocurrent corresponding to the detected light, wherein the half-width of the peak of the optical element is smaller than the half-width of the transmittance peak of the Fabry-Perot interference filter at each of the first wavelength to the nth wavelength.

12. The calibration device according to claim 11, wherein n is an integer of 4 or greater.

13. The calibration device according to claim 12, wherein when the variable range of the transmission wavelength of the Fabry-Perot interference filter is divided into four equal parts, each range includes at least one of the first wavelength to the nth wavelength.

14. The calibration device according to any one of claims 11 to 13, wherein n is an integer of 10 or greater.

15. The calibration device according to claim 14, wherein when the variable range of the transmission wavelength of the Fabry-Perot interference filter is divided into ten equal parts, each range includes at least one of the first wavelength to the nth wavelength.

16. A calibration device according to any one of claims 11 to 15, wherein the voltage control unit controls the voltage applied to the Fabry-Perot interference filter so as to sweep the transmission wavelength of the Fabry-Perot interference filter in a wavelength range including the first wavelength to the nth wavelength.

17. The calibration device according to any one of claims 11 to 16, wherein the optical element is a bandpass filter having a dielectric multilayer film.

18. The calibration device according to any one of claims 11 to 17, further comprising a collimating lens disposed on the optical path between the light source and the optical element, wherein the optical element is a band-pass filter.

19. An optical element that can be used to calibrate the relationship between the voltage applied to a Fabry-Perot interference filter and the transmission wavelength of the Fabry-Perot interference filter, the optical element having a transmittance or reflectance peak at each of a plurality of wavelengths including a first wavelength to an nth wavelength (n is an integer of 2 or more) located within the variable range of the transmission wavelength of the Fabry-Perot interference filter, and the half-width of the peak of the optical element at each of the first wavelength to the nth wavelength is smaller than the half-width of the transmittance peak of the Fabry-Perot interference filter.

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