Light detection system and voltage determination method

Abstract

This invention provides a photodetection system and a voltage determination method that can accurately detect light of a desired wavelength. [Solution] The light detection system comprises a Fabry-Perot interference filter having a pair of mirrors whose distance from each other is variable, a light detection unit configured to detect light in the wavelength range of λ1 to λ2, and a control unit that applies a voltage to the Fabry-Perot interference filter so that the distance changes in the distance range of d1 to d2. When the distance is d1, the light transmission spectrum T1(λ) has a peak transmittance T1(λa) at λa corresponding to a single order, which appears within the wavelength range, and the transmittance T1(λ2) at λ2 is 1% or less. When the distance is d2, the light transmission spectrum T2(λ) has a peak transmittance T2(λb) at λb corresponding to a single order, which appears within the wavelength range, and the transmittance T2(λ1) at λ1 is 1% or less.

Description

Technical Field

[0001] The present invention relates to an optical detection system and a voltage determination method.

Background Art

[0002] An optical detection device including a Fabry-Perot interference filter having a first mirror and a second mirror with variable distance from each other, a photodetector for detecting light transmitted through the first mirror and the second mirror, and a band-pass filter disposed on the opposite side of the photodetector with respect to the Fabry-Perot interference filter is known (see, for example, Patent Document 1).

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] In the optical detection device as described above, when the distance between the first mirror and the second mirror changes within a predetermined distance range, light of wavelengths corresponding to a plurality of orders can pass through the first mirror and the second mirror at each distance within the predetermined distance range. Therefore, the band-pass filter is configured to mainly transmit light of a wavelength corresponding to a single order at each distance within the predetermined distance range. However, if the above distance range is not set appropriately (that is, if the voltage applied to the Fabry-Perot interference filter is not set appropriately), not only light of a wavelength corresponding to a single order but also the tail portion of light of a wavelength corresponding to an order before or after the single order may pass through the band-pass filter and the Fabry-Perot interference filter and be detected by the photodetector.

[0005] An object of the present invention is to provide an optical detection system and a voltage determination method capable of accurately detecting light of a desired wavelength. [Means for solving the problem]

[0006] The present invention provides a photodetection system comprising: a Fabry-Perot interference filter having a first mirror and a second mirror whose distance from each other is variable; a photodetector into which light transmitted through the first mirror and the second mirror is incident, and configured to detect light in the wavelength range of wavelength λ1 to wavelength λ2 (>λ1); and a control unit that applies a voltage to the Fabry-Perot interference filter so that the distance changes in the distance range of distance d1 to distance d2 (>d1). When the distance is distance d1, the transmission spectrum T1(λ) (λ is wavelength) of light transmitted through the first mirror and the second mirror has a peak transmittance T1(λa) at wavelength λa corresponding to a single order appearing within the wavelength range, and the transmittance T1(λ2) at wavelength λ2 is 1% or less. When the distance is distance d2, the transmission spectrum T2(λ) of light transmitted through the first mirror and the second mirror has a peak transmittance T2(λb) at wavelength λb corresponding to a single order appearing within the wavelength range, and the transmittance T2(λ1) at wavelength λ1 is 1% or less.

[0007] In this photodetection system, when the distance between the first and second mirrors is d1, the transmission spectrum T1(λ) of light transmitted through the first and second mirrors shows that the peak transmittance T1(λa) at wavelength λa, which corresponds to a single order, is within the wavelength range of the photodetector, and the transmittance T1(λ2) at wavelength λ2, which is the upper limit of that wavelength range, is 1% or less. As a result, when light at wavelength λa, which corresponds to a single order, is detected, not only light outside that wavelength range (such as light at wavelengths corresponding to orders before or after that single order) but also light at wavelength λ2 and its vicinity is less likely to be detected as noise light. Furthermore, when the distance between the first and second mirrors is d2, the transmission spectrum T2(λ) of light transmitted through the first and second mirrors shows that the peak transmittance T2(λb) at wavelength λb, which corresponds to a single order, is within the wavelength range of the photodetector, and the transmittance T2(λ1) at wavelength λ1, which is the lower limit of that wavelength range, is 1% or less. As a result, when light of wavelength λb corresponding to a single order is detected, not only light outside that wavelength range (such as light of wavelengths corresponding to orders before or after the single order) but also light of wavelength λ1 and its vicinity are less likely to be detected as noise. Therefore, this light detection system can accurately detect light of a desired wavelength.

[0008] In the photodetection system of the present invention, the difference between wavelength λ2 and wavelength λb may be greater than the difference between wavelength λ1 and wavelength λa. This makes it possible to more effectively suppress the detection of light of wavelength λ1 and nearby wavelengths as noise when light of wavelength λb corresponding to a single order is detected. This is because the peak spacing of multiple orders of light transmitted through the first and second mirrors when the distance between the first and second mirrors is distance d2 tends to be wider than the peak spacing of multiple orders of light transmitted through the first and second mirrors when the distance between the first and second mirrors is distance d1.

[0009] In the photodetection system of the present invention, the photodetector further includes a bandpass filter positioned on the opposite side of the Fabry-Perot interference filter from the photodetector, or between the Fabry-Perot interference filter and the photodetector. The bandpass filter is configured to transmit light in a specific wavelength range, and the transmission spectrum T3(λ) of the light transmitted through the bandpass filter may include a rising portion that rises in the wavelength range and a falling portion that falls below the wavelength range. This makes it easy and reliable to realize a configuration that detects light in the wavelength range of λ1 to λ2.

[0010] In the light detection system of the present invention, the transmittance at the intersection of the transmission spectrum T1(λ) and the falling edge is 1% or less, and the transmittance at the intersection of the transmission spectrum T2(λ) and the rising edge may also be 1% or less. This makes it possible to more effectively suppress the detection of light outside the "wavelength range of wavelength λ1 to wavelength λ2" (such as light with wavelengths corresponding to orders before or after the single order) as noise when light with wavelength λa corresponding to a single order is detected. Furthermore, when light with wavelength λb corresponding to a single order is detected, it is possible to more effectively suppress the detection of light outside the "wavelength range of wavelength λ1 to wavelength λ2" (such as light with wavelengths corresponding to orders before or after the single order) as noise when light with wavelength λb corresponding to a single order is detected.

[0011] In the photodetection system of the present invention, the wavelength at the intersection of the transmission spectrum T1(λ) and the rising edge may be greater than or equal to the wavelength at the bottom point of the transmission spectrum T1(λ) that appears on the shorter wavelength side of the peak transmittance T1(λa). This makes it possible to more effectively suppress the detection of light with wavelength λ1 and nearby wavelengths as noise when light with wavelength λa corresponding to a single order is detected.

[0012] In the photodetection system of the present invention, the wavelength at the intersection of the transmission spectrum T2(λ) and the falling edge may be less than or equal to the wavelength at the bottom point of the transmission spectrum T2(λ) that appears on the longer wavelength side of the peak transmittance T2(λb). This makes it possible to more effectively suppress the detection of light of wavelength λ2 and nearby wavelengths as noise when light of wavelength λb corresponding to a single order is detected.

[0013] In the photodetection system of the present invention, the wavelength at the bottom point of the transmission spectrum T2(λ) appearing on the longer wavelength side of the peak transmittance T2(λb) may be less than or equal to the long-wavelength cutoff wavelength λc of the photodetector. This makes it possible to more effectively suppress the detection of light at wavelength λ2 and nearby wavelengths as noise when light at wavelength λb corresponding to a single order is detected.

[0014] In the photodetection system of the present invention, the wavelength at the intersection of the transmission spectrum T1(λ) and the rising edge, and the wavelength at the intersection of the transmission spectrum T2(λ) and the rising edge, may be greater than or equal to the wavelength at the intersection of the transmission spectrum T1(λ) and the transmission spectrum T2(λ) that appear on the shorter wavelength side of the peak transmittance T1(λa). This makes it possible to more effectively suppress the detection of light with wavelength λ1 and nearby wavelengths as noise when light with wavelength λa corresponding to a single order and light with wavelength λb corresponding to a single order are detected.

[0015] In the photodetection system of the present invention, the wavelength at the intersection of the transmission spectrum T2(λ) and the falling edge, and the wavelength at the intersection of the transmission spectrum T1(λ) and the falling edge, may be less than or equal to the wavelength at the intersection of the transmission spectrum T1(λ) and the transmission spectrum T2(λ) that appear on the longer wavelength side of the peak transmittance T2(λb). This makes it possible to more effectively suppress the detection of light at wavelength λ2 and nearby wavelengths as noise when light at wavelength λa corresponding to a single order and light at wavelength λb corresponding to a single order are detected.

[0016] In the light detection system of the present invention, the transmittance at the intersection of the transmission spectrum T1(λ) and the rising edge is greater than or equal to the transmittance at the intersection of the transmission spectrum T2(λ) and the rising edge, and the transmittance at the intersection of the transmission spectrum T2(λ) and the falling edge may be greater than or equal to the transmittance at the intersection of the transmission spectrum T1(λ) and the falling edge. This makes it possible to more effectively suppress the detection of light with wavelength λ1 and nearby wavelengths as noise when light with wavelength λb corresponding to a single order is detected. Furthermore, when light with wavelength λa corresponding to a single order is detected, it is possible to more effectively suppress the detection of light with wavelength λ2 and nearby wavelengths as noise.

[0017] In the photodetection system of the present invention, the transmittance at the intersection of the transmission spectrum T1(λ) and the rising edge may be greater than or equal to the transmittance at the intersection of the transmission spectrum T1(λ) and the falling edge. This makes it possible to more effectively suppress the detection of light with wavelength λ2 and nearby wavelengths as noise when light with wavelength λa corresponding to a single order is detected.

[0018] In the light detection system of the present invention, the transmittance at the intersection of the transmission spectrum T2(λ) and the falling edge may be greater than or equal to the transmittance at the intersection of the transmission spectrum T2(λ) and the rising edge. This makes it possible to more effectively suppress the detection of light with wavelength λ1 and nearby wavelengths as noise when light with wavelength λb corresponding to a single order is detected.

[0019] In the photodetection system of the present invention, the wavelength λ2 may correspond to the long-wavelength cutoff wavelength λc of the photodetector. This makes it easy and reliable to realize a configuration that detects light in the wavelength range of λ1 to λ2.

[0020] In the photodetection system of the present invention, the transmittance T2(λ) of the transmission spectrum T2(λ) at the long-wavelength cutoff wavelength λc may be greater than or equal to the transmittance T1(λ) of the transmission spectrum T1(λ) at the long-wavelength cutoff wavelength λc. This makes it possible to more effectively suppress the detection of light at wavelength λ2 and nearby wavelengths as noise when light at wavelength λa corresponding to a single order is detected.

[0021] In the photodetection system of the present invention, the transmittance T2(λ1) may be less than or equal to the transmittance T2(λc) of the transmission spectrum T2(λ) at the long-wavelength cutoff wavelength λc. This makes it possible to more effectively suppress the detection of light at wavelength λ1 and nearby wavelengths as noise when light at wavelength λb corresponding to a single order is detected.

[0022] The present invention relates to a voltage determination method for a photodetector comprising a Fabry-Perot interference filter having a first mirror and a second mirror whose distance from each other is variable, and a photodetector into which light transmitted through the first mirror and the second mirror is incident, and configured to detect light in the wavelength range of wavelength λ1 to wavelength λ2 (>λ1), wherein the voltage is determined when a voltage is applied to the Fabry-Perot interference filter such that the distance changes in the distance range of distance d1 to distance d2 (>d1), comprising a first step of determining a voltage V1 at which the distance is distance d1, and a second step of determining a voltage V2 at which the distance is distance d2, wherein in the first step In the first step, with respect to the transmission spectrum T1(λ) (where λ is wavelength) of light transmitted through the first and second mirrors when the distance is d1, the voltage V1 is determined such that the peak transmittance T1(λa) at wavelength λa corresponding to a single order appears within the wavelength range and the transmittance T1(λ2) at wavelength λ2 is 1% or less. In the second step, with respect to the transmission spectrum T2(λ) of light transmitted through the first and second mirrors when the distance is d2, the voltage V2 is determined such that the peak transmittance T2(λb) at wavelength λb corresponding to a single order appears within the wavelength range and the transmittance T2(λ1) at wavelength λ1 is 1% or less.

[0023] In this voltage determination method, for the transmission spectrum T1(λ) of light transmitted through the first mirror and the second mirror when the distance between the first mirror and the second mirror is distance d1, the peak transmittance T1(λa) at the wavelength λa corresponding to a single order appears within the wavelength range from wavelength λ1 to wavelength λ2, and the transmittance T1(λ2) at wavelength λ2 is 1% or less, the voltage V1 is determined. Thereby, when light of wavelength λa corresponding to a single order is detected, not only light outside the wavelength range (such as light of wavelengths corresponding to orders before or after the single order) but also light of wavelength λ2 and wavelengths in its vicinity are less likely to be detected as noise light. Further, for the transmission spectrum T2(λ) of light transmitted through the first mirror and the second mirror when the distance between the first mirror and the second mirror is distance d2, the peak transmittance T2(λb) at the wavelength λb corresponding to a single order appears within the wavelength range from wavelength λ1 to wavelength λ2, and the transmittance T2(λ1) at wavelength λ1 is 1% or less, the voltage V2 is determined. Thereby, when light of wavelength λb corresponding to a single order is detected, not only light outside the wavelength range (such as light of wavelengths corresponding to orders before or after the single order) but also light of wavelength λ1 and wavelengths in its vicinity are less likely to be detected as noise light. According to the voltage determination method described above, light of a desired wavelength can be accurately detected.

Advantages of the Invention

[0024] According to the present invention, it is possible to provide an optical detection system and a voltage determination method capable of accurately detecting light of a desired wavelength.

Brief Description of the Drawings

[0025] [Figure 1] It is a block diagram of an optical detection system according to an embodiment. [Figure 2] It is a cross-sectional view of the optical detection device shown in FIG. 1. [Figure 3] It is a perspective view of the Fabry-Perot interference filter shown in FIG. 2. [Figure 4]Figure 3 is a cross-sectional view of a Fabry-Perot interference filter along the IV-IV line. [Figure 5] This is a flowchart of a voltage determination method according to one embodiment. [Figure 6] Figure 1 is a graph showing the transmission characteristics of the light detection device. [Figure 7] Figure 2 is a graph showing the relationship between the peak wavelength and the full width at half maximum in the Fabry-Perot interference filter. [Figure 8] This graph shows the transmission characteristics of a modified photodetector. [Figure 9] This is a diagram illustrating the configuration of a modified optical detection device. [Modes for carrying out the invention]

[0026] Embodiments of the present invention will be described in detail below with reference to the drawings. In each drawing, the same or corresponding parts are denoted by the same reference numerals, and redundant explanations are omitted. [Configuration of the photodetection system]

[0027] As shown in Figure 1, the photodetection system 100 comprises a photodetector 1 and a control unit 50. The control unit 50 is electrically connected to the photodetector 1. The control unit 50 performs input and output of electrical signals to the photodetector 1. As an example, the control unit 50 includes an integrated circuit such as an FPGA (field-programmable gate array), a non-volatile memory such as an EEPROM (Electrically Erasable Programmable Read-Only Memory), and a PC (Personal Computer). [Configuration of the photodetector]

[0028] As shown in Figure 2, the photodetector 1 comprises a package 2. Package 2 is a CAN package having a stem 3 and a cap 4. The cap 4 includes a cylindrical side wall 5 and a top wall 6. The side wall 5 and the top wall 6 are integrally formed from a metal material. The stem 3 is formed in a plate shape from a metal material. The stem 3 is hermetically bonded to the cylindrical side wall 5, facing the top wall 6.

[0029] A wiring board 7 is fixed to the inner surface 3a of the stem 3. The substrate material for the wiring board 7 can be, for example, silicon, ceramic, quartz, glass, or plastic. A photodetector 8 and temperature compensation elements such as thermistors (not shown) are mounted on the wiring board 7. The photodetector 8 is an infrared detector, such as a quantum sensor using InGaAs, a thermopile, or a bolometer. When detecting light in the ultraviolet, visible, and near-infrared wavelength ranges, a silicon photodiode can be used as the photodetector 8. The photodetector 8 may have a single light-receiving section, or multiple light-receiving sections may be arranged in an array. Furthermore, multiple photodetectors 8 may be mounted on the wiring board 7.

[0030] Multiple spacers 9 are fixed to the wiring board 7. Each spacer 9 can be made of, for example, silicon, ceramic, quartz, glass, or plastic. A Fabry-Perot interference filter 10 is fixed to the multiple spacers 9. The light-transmitting region 10a of the Fabry-Perot interference filter 10 faces the light-receiving portion of the photodetector 8. The spacers 9 may be integrally formed with the wiring board 7. Furthermore, the Fabry-Perot interference filter 10 may be supported by a single spacer 9 instead of multiple spacers 9.

[0031] Multiple lead pins 11 are fixed to the stem 3. Each lead pin 11 penetrates the stem 3 while maintaining electrical insulation and airtightness between it and the stem 3. Each lead pin 11 is electrically connected via wires 12 to electrode pads provided on the wiring board 7, terminals of the photodetector 8, terminals of the temperature compensation element, and terminals of the Fabry-Perot interference filter 10, respectively. The control unit 50 is electrically connected to each lead pin 11 via wiring (not shown). As a result, the control unit 50 can input and output electrical signals to the photodetector 8, the temperature compensation element, and the Fabry-Perot interference filter 10, respectively.

[0032] Package 2 has an opening 2a. More specifically, the opening 2a is formed in the top wall 6 of the cap 4 so as to face the light transmission region 10a of the Fabry-Perot interference filter 10. A light-transmitting member 13 is hermetically bonded to the inner surface 6a of the top wall 6 so as to close the opening 2a. The light-transmitting member 13 has a light incident surface 13a and a light emission surface 13b that face each other, and a side surface 13c. The light incident surface 13a of the light-transmitting member 13 is substantially flush with the outer surface of the top wall 6 at the opening 2a. The side surface 13c of the light-transmitting member 13 is in contact with the inner surface 5a of the side wall 5 of package 2. Such a light-transmitting member 13 is formed by placing glass pellets inside the cap 4 with the opening 2a facing downwards and melting the glass pellets.

[0033] A bandpass filter 14 is fixed to the light-emitting surface 13b of the light-transmitting member 13 by an adhesive member 15. The bandpass filter 14 is positioned on the opposite side of the Fabry-Perot interference filter 10 from the photodetector 8. The bandpass filter 14 has a light-incident surface 14a and a light-emitting surface 14b, and a side surface 14c, which face each other. As an example, the bandpass filter 14 is constructed by forming a dielectric multilayer film (for example, a multilayer film consisting of a combination of a high refractive index material such as TiO2 or Ta2O5 and a low refractive index material such as SiO2 or MgF2) on the surface of a light-transmitting member formed in a plate shape from a light-transmitting material (for example, silicon, glass, etc.).

[0034] In the photodetector 1, package 2 houses a wiring board 7, a photodetector 8, a temperature compensation element (not shown), a plurality of spacers 9, a Fabry-Perot interference filter 10, and a bandpass filter 14. In the photodetector 1, the aperture 2a, the light-transmitting member 13, and the bandpass filter 14 are arranged on one side relative to the Fabry-Perot interference filter 10, while the photodetector 8 is arranged on the other side relative to the Fabry-Perot interference filter 10.

[0035] In the light detection device 1 configured as described above, when the light to be measured is incident on the light transmission region 10a of the Fabry-Perot interference filter 10 via the aperture 2a, light transmission member 13, adhesive member 15, and bandpass filter 14, light of a predetermined wavelength from the light to be measured is transmitted through the first and second mirrors of the Fabry-Perot interference filter 10 (the configuration of the Fabry-Perot interference filter 10 will be described later). The light transmitted through the first and second mirrors of the Fabry-Perot interference filter 10 is incident on the photodetector 8 and detected by the photodetector 8. As an example, in the light detection device 1, the spectral spectrum of the light to be measured is obtained by detecting the light transmitted through the light transmission region 10a of the Fabry-Perot interference filter 10 by the photodetector 8 while the voltage applied to the Fabry-Perot interference filter 10 is changed (i.e., while the distance between the first and second mirrors in the Fabry-Perot interference filter 10 is changed). [Configuration of a Fabry-Perot interference filter]

[0036] As shown in Figure 3, the Fabry-Perot interference filter 10 has a light-transmitting region 10a. For example, the Fabry-Perot interference filter 10 has a rectangular plate shape, and the light-transmitting region 10a has a cylindrical shape. The light-transmitting region 10a selectively transmits light of a predetermined wavelength along its center line (the dashed line shown in Figure 3).

[0037] As shown in Figure 4, the Fabry-Perot interference filter 10 includes a substrate 21. On the light incident surface 21a of the substrate 21, an anti-reflective layer 31, a first laminate 32, an intermediate layer 33, and a second laminate 34 are laminated in this order. An air gap S is formed between the first laminate 32 and the second laminate 34 by the frame-shaped intermediate layer 33. The substrate 21 is made of, for example, silicon, quartz, glass, etc. If the substrate 21 is made of silicon, the anti-reflective layer 31 and the intermediate layer 33 are made of, for example, silicon oxide. The thickness of the intermediate layer 33 is, for example, an integer multiple of half the design center wavelength. The thickness of the intermediate layer 33 may be greater than an integer multiple of half the design center wavelength if necessary.

[0038] The portion of the first laminate 32 corresponding to the light-transmitting region 10a functions as the first mirror 35. The first mirror 35 is supported on the substrate 21 via an anti-reflective layer 31. As an example, the first laminate 32 is composed of multiple polysilicon layers and multiple silicon nitride layers stacked alternately one layer at a time. The optical thickness of each layer constituting the first mirror 35 is, for example, an integer multiple of 1 / 4 of the design center wavelength. Note that silicon oxide layers may be used instead of silicon nitride layers.

[0039] The portion of the second laminate 34 corresponding to the light-transmitting region 10a functions as a second mirror 36 facing the first mirror 35 via an air gap S. The second mirror 36 is supported on the substrate 21 via an anti-reflective layer 31, the first laminate 32, and an intermediate layer 33. As an example, the second laminate 34 is constructed by alternately stacking multiple polysilicon layers and multiple silicon nitride layers one layer at a time. The optical thickness of each layer constituting the second mirror 36 is, for example, an integer multiple of 1 / 4 of the design center wavelength. Note that silicon oxide layers may be used instead of silicon nitride layers.

[0040] In the second laminate 34, a plurality of through holes 24b are provided in the portion corresponding to the void S, extending from the surface 34a of the second laminate 34 to the void S. The plurality of through holes 24b are formed to such an extent that they do not substantially affect the function of the second mirror 36. The plurality of through holes 24b were used when forming the void S by removing a portion of the intermediate layer 33 by etching.

[0041] A first electrode 22 is formed on the first mirror 35 so as to surround the light-transmitting region 10a. A second electrode 23 is formed on the first mirror 35 so as to include the light-transmitting region 10a. Both the first electrode 22 and the second electrode 23 are formed by doping a portion of the polysilicon layer with impurities to reduce the resistance of that portion of the polysilicon layer. The size of the second electrode 23 is approximately the same as the size of the light-transmitting region 10a.

[0042] A third electrode 24 is formed on the second mirror 36. The third electrode 24 faces the first electrode 22 and the second electrode 23 through a gap S in a direction parallel to the center line of the light-transmitting region 10a (the dashed line shown in Figure 4). The third electrode 24 is formed by doping a portion of the polysilicon layer with impurities to reduce the resistance of that portion of the polysilicon layer.

[0043] In the Fabry-Perot interference filter 10, the second electrode 23 is located on the same plane as the first electrode 22 in a direction perpendicular to the center line of the light transmission region 10a. The distance between the second electrode 23 and the third electrode 24 is the same as the distance between the first electrode 22 and the third electrode 24. When viewed from a direction parallel to the center line of the light transmission region 10a, the second electrode 23 is surrounded by the first electrode 22.

[0044] The Fabry-Perot interference filter 10 is provided with a pair of terminals 25 flanking the light-transmitting region 10a. Each terminal 25 is located within a through-hole extending from the surface 34a of the second laminate 34 to the first laminate 32. Each terminal 25 is electrically connected to the first electrode 22 via wiring 22a.

[0045] The Fabry-Perot interference filter 10 is provided with a pair of terminals 26 flanking the light-transmitting region 10a. Each terminal 26 is located within a through-hole extending from the surface 34a of the second laminate 34 to just before the intermediate layer 33. Each terminal 26 is electrically connected to the second electrode 23 via wiring 23a and to the third electrode 24 via wiring 24a. The direction in which the pair of terminals 26 are aligned across the light-transmitting region 10a is perpendicular to the direction in which the pair of terminals 25 are aligned across the light-transmitting region 10a (see Figure 3).

[0046] The first laminate 32 has a plurality of trenches 27, 28 that open to the surface 32a. Each trench 27 extends in an annular shape to surround each portion of the wiring 23a that extends from each terminal 26 along a direction parallel to the center line of the light-transmitting region 10a. Each trench 27 electrically insulates the first electrode 22 from the wiring 23a. The trench 28 extends in an annular shape along the inner edge of the first electrode 22. The trench 28 electrically insulates the first electrode 22 from the second electrode 23. The areas within each trench 27, 28 may be made of insulating material or may be voids.

[0047] The second laminate 34 has a plurality of trenches 29 that open to the surface 34a. Each trench 29 extends in an annular shape so as to surround each terminal 25. Each trench 29 electrically insulates each terminal 25 from the third electrode 24. The area within each trench 29 may be made of insulating material or may be an air gap.

[0048] On the light-emitting surface 21b of the substrate 21, an anti-reflective layer 41, a third laminate 42, an intermediate layer 43, and a fourth laminate 44 are laminated in this order. The anti-reflective layer 41 and the intermediate layer 43 have the same configuration as the anti-reflective layer 31 and the intermediate layer 33, respectively. The third laminate 42 and the fourth laminate 44 have a laminated structure that is symmetrical to the first laminate 32 and the second laminate 34 with respect to the substrate 21, respectively. The anti-reflective layer 41, the third laminate 42, the intermediate layer 43, and the fourth laminate 44 have the function of suppressing warping of the substrate 21.

[0049] The anti-reflective layer 41, the third laminate 42, the intermediate layer 43, and the fourth laminate 44 are provided with an opening 40a that includes a light-transmitting region 10a. When viewed from a direction parallel to the center line of the light-transmitting region 10a, the size of the opening 40a is approximately the same as the size of the light-transmitting region 10a. The opening 40a opens on the light-emitting side, and the bottom surface of the opening 40a extends to the anti-reflective layer 41. A light-shielding layer 45 is formed on the light-emitting surface of the fourth laminate 44. The light-shielding layer 45 is made of, for example, aluminum. A protective layer 46 is formed on the surface of the light-shielding layer 45 and on the inner surface of the opening 40a. The protective layer 46 is made of, for example, aluminum oxide. By setting the thickness of the protective layer 46 to 1 to 100 nm (preferably about 30 nm), the optical influence of the protective layer 46 can be ignored.

[0050] In the Fabry-Perot interference filter 10 configured as described above, when a voltage is applied to the first electrode 22 and the third electrode 24 via multiple terminals 25 and 26, a potential difference is generated between the first electrode 22 and the third electrode 24, and an electrostatic force corresponding to this potential difference is generated between the first electrode 22 and the third electrode 24. The electrostatic force generated between the first electrode 22 and the third electrode 24 attracts the second mirror 36 to the first mirror 35, and the distance between the first mirror 35 and the second mirror 36 is adjusted. At this time, the second electrode 23, which is at the same potential as the third electrode 24, functions as a compensating electrode, and the second mirror 36 is kept flat in the light transmission region 10a. Thus, in the Fabry-Perot interference filter 10, the distance between the first mirror 35 and the second mirror 36 is variable.

[0051] The wavelength of light transmitted through the Fabry-Perot interference filter 10 depends on the distance between the first mirror 35 and the second mirror 36 in the light transmission region 10a. Therefore, by adjusting the voltage applied to the first electrode 22 and the third electrode 24 (the potential difference generated between the first electrode 22 and the third electrode 24), the wavelength of light transmitted through the Fabry-Perot interference filter 10 can be selected.

[0052] However, in the Fabry-Perot interference filter 10, light with a peak wavelength λp that satisfies λp = 2nd / a (n: refractive index, d: distance between the first mirror 35 and the second mirror 36, a: integer) is transmitted through the first mirror 35 and the second mirror 36. In other words, even if the distance d between the first mirror 35 and the second mirror 36 is constant, light with wavelengths corresponding to each of the multiple orders (a) is transmitted through the first mirror 35 and the second mirror 36. Therefore, in the photodetector 1, the bandpass filter 14 and the photodetector 8 are configured such that, for a predetermined distance d, light with a wavelength corresponding to a single order (e.g., secondary light) is detected, and light with wavelengths corresponding to orders other than that single order (e.g., primary light, and higher-order light of the third order or higher) is not detected. [Transmission characteristics of light detection devices]

[0053] As a premise, the photodetector, which consists of a bandpass filter 14 and a photodetector 8, is configured to detect light in the wavelength range of wavelength λ1 to wavelength λ2 (>λ1). The configuration of the photodetector with the photodetector 8 to detect light in the wavelength range of wavelength λ1 to wavelength λ2 means that the photodetector 8 has sufficient light-receiving sensitivity for light in that wavelength range, and that 50% or more of the light incident on the photodetector reaches the photodetector 8. In this embodiment, the bandpass filter 14 has the function of transmitting 50% or more of the light in the wavelength range of wavelength λ1 to wavelength λ2, and the photodetector 8 has sufficient light-receiving sensitivity for light in the wavelength range of wavelength λ1 to wavelength λ2. In other words, assuming that the Fabry-Perot interference filter 10 does not exist, the bandpass filter 14 and the photodetector 8 work together to detect light in the wavelength range of wavelength λ1 to wavelength λ2 by the photodetector 8. Furthermore, for the photodetector 8 to have sufficient light detection sensitivity for light in the wavelength range of λ1 to λ2, it means that the photodetector 8 has a sensitivity of 5% or more of its maximum sensitivity for light in the wavelength range of λ1 to λ2.

[0054] Under the above premise, the control unit 50 applies a voltage to the Fabry-Perot interference filter 10 such that the distance between the first mirror 35 and the second mirror 36 changes within a distance range of d1 or greater and d2 (>d1) or less. Here, the voltage determination method for determining the voltage applied to the Fabry-Perot interference filter 10 will be described. Note that either the first step or the second step described below may be performed first, or both may be performed simultaneously.

[0055] First, as shown in Figure 5, in the first step S01, a voltage V1 is determined such that the distance between the first mirror 35 and the second mirror 36 is distance d1. More specifically, as shown in Figure 6, when the distance between the first mirror 35 and the second mirror 36 is distance d1, the voltage V1 is determined such that the peak transmittance T1(λa) at wavelength λa corresponding to a single order appears within the wavelength range of wavelength λ1 to λ2, and the peak transmittance T1 at wavelengths other than the single order does not appear within that wavelength range. Furthermore, the voltage V1 is determined such that the transmittance T1(λ2) at wavelength λ2 is 1% or less. Note that T1(λ) means that the transmittance of light transmitted through the first mirror 35 and the second mirror 36 when the distance between them is distance d1 is a function of wavelength λ.

[0056] Next, as shown in Figure 5, in the second step S02, a voltage V2 is determined such that the distance between the first mirror 35 and the second mirror 36 is distance d2. More specifically, as shown in Figure 6, when the distance between the first mirror 35 and the second mirror 36 is distance d2, the voltage V2 is determined such that the peak transmittance T2(λb) at wavelength λb corresponding to a single order appears within the wavelength range of wavelength λ1 to wavelength λ2, and the peak transmittance T2 at wavelengths corresponding to orders other than that single order does not appear within that wavelength range. Furthermore, the voltage V2 is determined such that the transmittance T2(λ1) at wavelength λ1 is 1% or less. Note that T2(λ) means that the transmittance of light transmitted through the first mirror 35 and the second mirror 36 when the distance between them is distance d2 is a function of wavelength λ.

[0057] The transmission characteristics of the photodetector 1 will be described in more detail with reference to Figure 6. As described above, for the transmission spectrum T1(λ), ​​the peak transmittance T1(λa) at wavelength λa corresponding to a single order appears within the wavelength range from wavelength λ1 to wavelength λ2, and the peak transmittance T1 at wavelengths corresponding to orders other than the single order does not appear within that wavelength range. Furthermore, for the transmission spectrum T1(λ), ​​the transmittance T1(λ2) at wavelength λ2 is 1% or less (0.1% or less in this embodiment). For the transmission spectrum T2(λ), the peak transmittance T2(λb) at wavelength λb corresponding to a single order appears within the wavelength range from wavelength λ1 to wavelength λ2, and the peak transmittance T2 at wavelengths corresponding to orders other than the single order does not appear within that wavelength range. Furthermore, for the transmission spectrum T2(λ), the transmittance T2(λ1) at wavelength λ1 is 1% or less (0.1% or less in this embodiment). The difference between wavelength λ2 and wavelength λb is greater than the difference between wavelength λ1 and wavelength λa.

[0058] The bandpass filter 14 is configured to transmit light in the wavelength range of λ1 to λ2. In other words, the bandpass filter 14 has the function of transmitting 50% or more of light in the wavelength range of λ1 to λ2. The transmission spectrum T3(λ) of light transmitted through the bandpass filter 14 includes a rising portion RP that rises in the wavelength range of λ1 to λ2, and a falling portion FP that falls from the wavelength range of λ1 to λ2. Note that T3(λ) means that the transmittance of light transmitted through the bandpass filter 14 is a function of wavelength λ.

[0059] The transmittance at the intersection D of the transmission spectrum T1(λ) and the falling edge FP is 1% or less. The transmittance at the intersection B of the transmission spectrum T2(λ) and the rising edge RP is 1% or less. The wavelength at the intersection A of the transmission spectrum T1(λ) and the rising edge RP is greater than or equal to the wavelength at the bottom point G of the transmission spectrum T1(λ). The bottom point G of the transmission spectrum T1(λ) is the first downward-convex point (i.e., convex towards the lower transmittance) that appears on the shorter wavelength side of the peak transmittance T1(λa). The wavelength at the intersection C of the transmission spectrum T2(λ) and the falling edge FP is less than or equal to the wavelength at the bottom point H of the transmission spectrum T2(λ). The bottom point H of the transmission spectrum T2(λ) is the first downward-convex point (i.e., convex towards the lower transmittance) that appears on the longer wavelength side of the peak transmittance T2(λb).

[0060] The wavelength at the bottom point H of the transmission spectrum T2(λ) is less than or equal to the long-wavelength cutoff wavelength λc of the photodetector 8. The long-wavelength cutoff wavelength λc of the photodetector 8 is a value that indicates the limit of the spectral sensitivity characteristics on the long-wavelength side, for example, the wavelength at which the maximum sensitivity is 10%.

[0061] The wavelength at intersection A of transmission spectrum T1(λ) and the rising edge RP, and the wavelength at intersection B of transmission spectrum T2(λ) and the rising edge RP, are greater than or equal to the wavelength at intersection E of transmission spectrum T1(λ) and transmission spectrum T2(λ). Intersection E of transmission spectrum T1(λ) and transmission spectrum T2(λ) is the first intersection point that appears on the short wavelength side of peak transmittance T1(λa). The wavelength at intersection C of transmission spectrum T2(λ) and the falling edge FP, and the wavelength at intersection D of transmission spectrum T1(λ) and the falling edge FP, are less than or equal to the wavelength at intersection F of transmission spectrum T1(λ) and transmission spectrum T2(λ). Intersection F of transmission spectrum T1(λ) and transmission spectrum T2(λ) is the first point that appears on the long wavelength side of peak transmittance T2(λb).

[0062] The transmittance at the intersection A of the transmission spectrum T1(λ) and the rising portion RP is greater than or equal to the transmittance at the intersection B of the transmission spectrum T2(λ) and the rising portion RP. The transmittance at the intersection C of the transmission spectrum T2(λ) and the falling portion FP is greater than or equal to the transmittance at the intersection D of the transmission spectrum T1(λ) and the falling portion FP. In this embodiment, the transmittance at each intersection A, B, C, and D is 1% or less.

[0063] The transmittance at the intersection A of the transmission spectrum T1(λ) and the rising edge RP is greater than or equal to the transmittance at the intersection D of the transmission spectrum T1(λ) and the falling edge FP. The transmittance at the intersection C of the transmission spectrum T2(λ) and the falling edge FP is greater than or equal to the transmittance at the intersection B of the transmission spectrum T2(λ) and the rising edge RP.

[0064] Figure 7 is a graph showing the relationship between peak wavelength and full width at half maximum in the Fabry-Perot interference filter 10. As shown in Figure 7, in the Fabry-Perot interference filter 10, the transmission center wavelength λt is shifted to the shorter wavelength side relative to the design center wavelength λd. The design center wavelength λd is the peak wavelength used as a reference when determining the thickness of the intermediate layer 33, the optical thickness of each layer constituting the first mirror 35, and the optical thickness of each layer constituting the second mirror 36. The transmission center wavelength λt is the peak wavelength at the center between "the peak wavelength λs of light transmitted through the first mirror 35 and the second mirror 36 when the distance between the first mirror 35 and the second mirror 36 is distance d1" and "the peak wavelength λl of light transmitted through the first mirror 35 and the second mirror 36 when the distance between the first mirror 35 and the second mirror 36 is distance d2". If the full width at half maximum for each peak wavelength tends to be smaller on the shorter wavelength side compared to the longer wavelength side, then a shift of the transmission center wavelength λt to the shorter wavelength side relative to the design center wavelength λd is effective in improving the uniformity of wavelength separation. The design center wavelength λd can be determined by measuring the thickness of the intermediate layer 33, the optical thickness of each layer constituting the first mirror 35, and the optical thickness of each layer constituting the second mirror 36. [Mechanism of Action and Effects]

[0065] In the photodetection system 100, when the distance between the first mirror 35 and the second mirror 36 is d1, the transmission spectrum T1(λ) of light transmitted through the first mirror 35 and the second mirror 36 shows that the peak transmittance T1(λa) at wavelength λa corresponding to a single order appears within the wavelength range of wavelength λ1 to wavelength λ2, and the transmittance T1(λ2) at wavelength λ2, which is the upper limit of this wavelength range, is 1% or less. As a result, when light at wavelength λa corresponding to a single order is detected, not only light outside that wavelength range (such as light at wavelengths corresponding to orders before or after the single order) but also light at wavelength λ2 and its vicinity are less likely to be detected as noise light. Furthermore, when the distance between the first mirror 35 and the second mirror 36 is d2, the transmission spectrum T2(λ) of light transmitted through the first mirror 35 and the second mirror 36 shows that the peak transmittance T2(λb) at wavelength λb corresponding to a single order appears within the wavelength range of wavelength λ1 to wavelength λ2, and the transmittance T2(λ1) at wavelength λ1, which is the lower limit of this wavelength range, is 1% or less. As a result, when light at wavelength λb corresponding to a single order is detected, not only light outside that wavelength range (such as light at wavelengths corresponding to orders before or after the single order) but also light at wavelength λ1 and its vicinity is less likely to be detected as noise light. Therefore, the photodetection system 100 can accurately detect light of a desired wavelength.

[0066] In the photodetection system 100, the difference between wavelength λ2 and wavelength λb is greater than the difference between wavelength λ1 and wavelength λa. This makes it possible to more effectively suppress the detection of light of wavelength λ1 and nearby wavelengths as noise when light of wavelength λb corresponding to a single order is detected. This is because the peak spacing of multiple orders of light transmitted through the first mirror 35 and the second mirror 36 when the distance between the first mirror 35 and the second mirror 36 is distance d2 tends to be wider than the peak spacing of multiple orders of light transmitted through the first mirror 35 and the second mirror 36 when the distance between the first mirror 35 and the second mirror 36 is distance d1.

[0067] In the photodetection system 100, the bandpass filter 14 is configured to transmit light in the wavelength range of λ1 to λ2, and the transmission spectrum T3(λ) of the light transmitted through the bandpass filter 14 includes a rising portion RP that rises in the wavelength range of λ1 to λ2, and a falling portion FP that falls from the wavelength range of λ1 to λ2. This makes it easy and reliable to realize a configuration that detects light in the wavelength range of λ1 to λ2.

[0068] In the photodetection system 100, the transmittance at the intersection D of the transmission spectrum T1(λ) and the falling edge FP is 1% or less, and the transmittance at the intersection B of the transmission spectrum T2(λ) and the rising edge RP is 1% or less. This makes it possible to more effectively suppress the detection of light outside the "wavelength range of wavelength λ1 to λ2" (such as light with wavelengths corresponding to orders before or after the single order) as noise when light with wavelength λa corresponding to a single order is detected. Furthermore, when light with wavelength λb corresponding to a single order is detected, it is possible to more effectively suppress the detection of light outside the "wavelength range of wavelength λ1 to λ2" (such as light with wavelengths corresponding to orders before or after the single order) as noise when light with wavelength λb corresponding to a single order is detected.

[0069] In the photodetection system 100, the wavelength at the intersection point A of the transmission spectrum T1(λ) and the rising edge RP is greater than or equal to the wavelength at the bottom point G of the transmission spectrum T1(λ) that appears on the shorter wavelength side of the peak transmittance T1(λa). This makes it possible to more effectively suppress the detection of light with wavelength λa, corresponding to a single order, as noise light, when light with wavelength λ1 and nearby wavelengths is detected.

[0070] In the photodetection system 100, the wavelength at the intersection point C of the transmission spectrum T2(λ) and the falling edge FP is less than or equal to the wavelength at the bottom point H of the transmission spectrum T2(λ) that appears on the longer wavelength side of the peak transmittance T2(λb). This makes it possible to more effectively suppress the detection of light of wavelength λ2 and nearby wavelengths as noise when light of wavelength λb corresponding to a single order is detected.

[0071] In the photodetection system 100, the wavelength at the bottom point H of the transmission spectrum T2(λ) is less than or equal to the long-wavelength cutoff wavelength λc of the photodetector 8. This makes it possible to more effectively suppress the detection of light with wavelength λ2 and nearby wavelengths as noise when light with wavelength λb corresponding to a single order is detected.

[0072] In the photodetection system 100, the wavelength at the intersection A of the transmission spectrum T1(λ) and the rising edge RP, and the wavelength at the intersection B of the transmission spectrum T2(λ) and the rising edge RP, are greater than or equal to the wavelength at the intersection E of the transmission spectrum T1(λ) and the transmission spectrum T2(λ), which appear on the shorter wavelength side of the peak transmittance T1(λa). This makes it possible to more effectively suppress the detection of light with wavelengths λ1 and nearby wavelengths as noise when light with wavelength λa corresponding to a single order and light with wavelength λb corresponding to a single order are detected.

[0073] In the photodetection system 100, the wavelength at the intersection C of the transmission spectrum T2(λ) and the falling edge FP, and the wavelength at the intersection D of the transmission spectrum T1(λ) and the falling edge FP, are less than or equal to the wavelength at the intersection F of the transmission spectrum T1(λ) and the transmission spectrum T2(λ), which appear on the longer wavelength side of the peak transmittance T2(λb). This makes it possible to more effectively suppress the detection of light with wavelengths λ2 and nearby wavelengths as noise when light with wavelength λa corresponding to a single order and light with wavelength λb corresponding to a single order are detected.

[0074] In the photodetection system 100, the transmittance at the intersection A of the transmission spectrum T1(λ) and the rising edge RP is greater than or equal to the transmittance at the intersection B of the transmission spectrum T2(λ) and the rising edge RP, and the transmittance at the intersection C of the transmission spectrum T2(λ) and the falling edge FP is greater than or equal to the transmittance at the intersection D of the transmission spectrum T1(λ) and the falling edge FP. This makes it possible to more effectively suppress the detection of light with wavelength λ1 and nearby wavelengths as noise when light with wavelength λb corresponding to a single order is detected. Furthermore, when light with wavelength λa corresponding to a single order is detected, it is possible to more effectively suppress the detection of light with wavelength λ2 and nearby wavelengths as noise.

[0075] In the photodetection system 100, the transmittance at the intersection A of the transmission spectrum T1(λ) and the rising edge RP is greater than or equal to the transmittance at the intersection D of the transmission spectrum T1(λ) and the falling edge FP. This makes it possible to more effectively suppress the detection of light with wavelength λ2 and nearby wavelengths as noise when light with wavelength λa corresponding to a single order is detected.

[0076] In the photodetection system 100, the transmittance at the intersection C of the transmission spectrum T2(λ) and the falling edge FP is greater than or equal to the transmittance at the intersection B of the transmission spectrum T2(λ) and the rising edge RP. This makes it possible to more effectively suppress the detection of light with wavelength λ1 and nearby wavelengths as noise when light with wavelength λb corresponding to a single order is detected.

[0077] In the voltage determination method described above, when the distance between the first mirror 35 and the second mirror 36 is d1, the voltage V1 is determined such that the peak transmittance T1(λa) at wavelength λa corresponding to a single order appears within the wavelength range of λ1 to λ2, and the transmittance T1(λ2) at wavelength λ2 is 1% or less. This makes it less likely that light at wavelength λ2 and its vicinity will be detected as noise when light at wavelength λa corresponding to a single order is detected. Furthermore, when the distance between the first mirror 35 and the second mirror 36 is d2, the voltage V2 is determined such that the peak transmittance T2(λb) at wavelength λb corresponding to a single order appears within the wavelength range of λ1 to λ2, and the transmittance T2(λ1) at wavelength λ1 is 1% or less. This makes it less likely that light of wavelength λb, corresponding to a single order, will be detected as noise when light of wavelength λ1 and its vicinity is detected. According to the voltage determination method described above, light of a desired wavelength can be detected with high accuracy. [Differentiation]

[0078] The present invention is not limited to the embodiments described above. For example, as shown in Figure 8, wavelength λ2 may correspond to the long-wavelength cutoff wavelength λc of the photodetector 8. In the example shown in Figure 8, the bandpass filter 14 has the function of transmitting 50% or more of light in the wavelength range of wavelength λ1 to "wavelength greater than wavelength λ2", and the photodetector 8 has sufficient light detection sensitivity for light in the wavelength range of "wavelength less than wavelength λ1" to wavelength λ2. Even in the example shown in Figure 8, assuming that the Fabry-Perot interference filter 10 does not exist, the bandpass filter 14 and the photodetector 8 work together to detect light in the wavelength range of wavelength λ1 to wavelength λ2 by the photodetector 8. Thus, even in the example shown in Figure 8, a configuration for detecting light in the wavelength range of wavelength λ1 to wavelength λ2 can be easily and reliably realized.

[0079] In the example shown in Figure 8, the transmittance T2(λ) of the transmission spectrum T2(λ) at the long-wavelength cutoff wavelength λc is greater than or equal to the transmittance T1(λ) of the transmission spectrum T1(λ) at the long-wavelength cutoff wavelength λc. This makes it possible to more effectively suppress the detection of light at wavelength λ2 and nearby wavelengths as noise when light at wavelength λa corresponding to a single order is detected.

[0080] In the example shown in Figure 8, the transmittance T2(λ1) at wavelength λ1 is less than or equal to the transmittance T2(λ) of the transmission spectrum T2(λ) at the longer wavelength cutoff wavelength λc. This makes it possible to more effectively suppress the detection of light at wavelength λ1 and nearby wavelengths as noise when light at wavelength λb corresponding to a single order is detected.

[0081] As shown in Figure 9, the bandpass filter 14 may be placed between the Fabry-Perot interference filter 10 and the photodetector 8. Alternatively, components of the Fabry-Perot interference filter 10 other than those constituting the first mirror 35 and the second mirror 36 (e.g., the substrate 21) may function as a bandpass filter together with the bandpass filter 14. Or, components of the Fabry-Perot interference filter 10 other than those constituting the first mirror 35 and the second mirror 36 (e.g., the substrate 21) may function as a bandpass filter instead of the bandpass filter 14. Furthermore, the photodetector 1 does not have to include the bandpass filter 14. As an example of this, the photodetector 8 may have a light receiving unit and a bandpass filter placed in front of the light receiving unit. As described above, the photodetector system 100 is configured as a photodetector unit having a photodetector 8 into which light transmitted through the first mirror 35 and the second mirror 36 is incident, and which detects light in the wavelength range of wavelength λ1 to wavelength λ2.

[0082] In the Fabry-Perot interference filter 10, the transmission center wavelength λt does not have to be shifted to the shorter wavelength side relative to the design center wavelength λd. In the Fabry-Perot interference filter 10, the transmission center wavelength λt may coincide with the design center wavelength λd, or the transmission center wavelength λt may be shifted to the longer wavelength side relative to the design center wavelength λd. [Explanation of symbols]

[0083] 1...Photodetector, 8...Photodetector (photodetector unit), 10...Fabry-Perot interference filter, 14...Bandpass filter (photodetector unit), 35...First mirror, 36...Second mirror, 50...Control unit, 100...Photodetector system.

Claims

1. A Fabry-Perot interference filter having a first mirror and a second mirror whose distance from each other is variable, A photodetector having a photodetector into which light transmitted through the first mirror and the second mirror is incident, and configured to detect light in the wavelength range of wavelength λ1 or greater and wavelength λ2 (>λ1) or less, The system includes a control unit that applies a voltage to the Fabry-Perot interference filter such that the distance changes within a distance range of d1 or greater and d2 or less (>d1), When the distance is the distance d1, the transmission spectrum T1(λ) (where λ is wavelength) of light passing through the first mirror and the second mirror is such that the peak transmittance T1(λa) at wavelength λa corresponding to a single order appears within the wavelength range, and the transmittance T1(λ2) at wavelength λ2 is 1% or less. A photodetection system in which, when the distance is the distance d2, the transmission spectrum T2(λ) of light transmitted through the first mirror and the second mirror has a peak transmittance T2(λb) at wavelength λb corresponding to a single order appearing within the wavelength range, and the transmittance T2(λ1) at wavelength λ1 is 1% or less.

2. A Fabry-Perot interference filter having a first mirror and a second mirror whose distance from each other is variable, A photodetector comprising: a photodetector having a photodetector into which light transmitted through the first mirror and the second mirror is incident, and a photodetector configured to detect light in the wavelength range of wavelength λ1 or greater and wavelength λ2 (>λ1) or less, A voltage determination method for determining the voltage when applying a voltage to the Fabry-Perot interference filter such that the distance changes within a distance range of d1 or greater and d2 or less (>d1), The first step is to determine the voltage V1 such that the distance is the distance d1, The second step is to determine a voltage V2 such that the distance is the distance d2, In the first step, the voltage V1 is determined such that, when the distance is the distance d1, the transmission spectrum T1(λ) (where λ is wavelength) of light transmitted through the first mirror and the second mirror is such that the peak transmittance T1(λa) at wavelength λa corresponding to a single order appears within the wavelength range and the transmittance T1(λ2) at wavelength λ2 is 1% or less. A voltage determination method in which, in the second step, the voltage V2 is determined such that, when the distance is the distance d2, the transmission spectrum T2(λ) of light transmitted through the first mirror and the second mirror is such that the peak transmittance T2(λb) at wavelength λb corresponding to a single order appears within the wavelength range and the transmittance T2(λ1) at wavelength λ1 is 1% or less.

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