Potential difference measurement device

Abstract

A potential difference measurement device (101) comprises: a light source (1); a semiconductor optical element (3) in which free carriers generated by the presence of charged particles absorb incident light incident from the light source (1), thereby reducing the intensity of emitted light that is emitted, relative to the intensity of the incident light; a detector (5) that detects the intensity of the emitted light; and a calculation unit (6) that calculates the intensity difference of the emitted light between before and after the reduction. The intensity difference varies according to the potential difference between outer space and a charged body (10) charged by the charged particles. The calculation unit (6) measures the potential difference by calculating the intensity difference in a state in which the semiconductor optical element (3) is disposed on the charged body (10) and the surface potential of the semiconductor optical element (3) is set to the same potential as the potential of the charged body (10) or to a potential having a predetermined potential difference with the potential of the charged body (10).

Description

Potential difference measuring device

[0001] The present invention relates to a potentiometric device.

[0002] Spacecraft used outside the atmosphere, such as artificial satellites and planetary probes, are exposed to charged particles such as plasma in outer space and high-energy cosmic rays. As a result, the surface of the spacecraft's structure, insulators such as the cover glass of solar panels, and internal components of the structure become charged. As charging of spacecraft can cause malfunctions and accidents, measures to prevent charging are required. To take such measures, it is necessary to know the degree of charge on the spacecraft.

[0003] For example, Patent Document 1 discloses that the potential difference between outer space and the surface of a spacecraft is determined based on a detection signal from a potential detection sensor such as a Langmuir probe.

[0004] Japanese Patent Publication No. 8-130097

[0005] However, to reduce the influence of the sheath, the Langmuir probe must be placed at a distance of several tens of centimeters or more from the spacecraft. Moreover, since the Langmuir probe is relatively large, it is difficult to install many of them at various locations on the spacecraft. Therefore, it is practically difficult to use the Langmuir probe to measure the potential difference between the spacecraft and outer space.

[0006] Furthermore, not only in space but also in plasma devices that perform various processes in a plasma environment, if the workpiece to be processed becomes charged, problems may occur during the process. However, even in plasma devices, using a Langmuir probe to measure the potential difference between the workpiece and the plasma space is practically difficult from the viewpoint of avoiding an increase in size. Moreover, there is the disadvantage that a relatively large Langmuir probe disturbs the plasma space.

[0007] An object of one aspect of the present invention is to miniaturize an apparatus for measuring the potential difference between a charged body and outer space or plasma space.

[0008] In order to solve the above problem, one aspect of the present invention provides a potential difference measuring device comprising: a light source; a semiconductor optical element that reduces the intensity of outgoing light relative to the intensity of incident light by absorbing incident light from the light source with free carriers generated by the presence of charged particles; a detector that detects the intensity of the outgoing light; and a calculation unit that calculates the difference in intensity of the outgoing light before and after the reduction in the intensity of the outgoing light, wherein the intensity difference changes depending on the potential difference between a charged body charged by the charged particles and outer space, and the calculation unit measures the potential difference by placing the semiconductor optical element on the charged body and setting the surface potential of the semiconductor optical element to the same potential as the charged body or a potential that is a predetermined potential difference from the potential of the charged body. The magnitude of the change in the potential difference depends on the change in the amount of free carriers generated by the presence of charged particles depending on the potential difference.

[0009] In order to solve the above problem, one aspect of the present invention provides a potential difference measuring device comprising: a light source; a semiconductor optical element that reduces the intensity of outgoing light relative to the intensity of incident light by absorbing incident light from the light source with free carriers generated by the presence of charged particles; a detector that detects the intensity of the outgoing light; and a calculation unit that calculates the difference in intensity of the outgoing light before and after the reduction in the intensity of the outgoing light, wherein the intensity difference changes depending on the potential difference between a charged body charged by the charged particles and a plasma space, and the calculation unit measures the potential difference by calculating the intensity difference while the semiconductor optical element is placed on the charged body and the surface potential of the semiconductor optical element is set to the same potential as the charged body or to a potential that is a predetermined potential difference from the potential of the charged body. The magnitude of the change in the potential difference depends on the amount of free carriers generated by the presence of charged particles changing depending on the potential difference.

[0010] According to one aspect of the present invention, it is possible to reduce the size of an apparatus for measuring the potential difference between a charged body and outer space or plasma space.

[0011] 8 is a block diagram showing the configuration of a potential difference measuring device according to a first embodiment of the present invention. FIG. 9 is a perspective view showing the external configuration of a satellite on which the potential difference measuring device is mounted. FIG. 10 is a plan view showing the configuration of a semiconductor optical element in the potential difference measuring device. FIG. 11 is a plan view showing the configuration of another semiconductor optical element in the potential difference measuring device. FIG. 12 is a view showing the mechanism by which the semiconductor optical element emits light with an intensity corresponding to the potential difference between outer space and a charged body. FIG. 13 is a view showing the mechanism by which a semiconductor optical element different from the semiconductor optical element shown in FIG. 5 emits light with an intensity corresponding to the potential difference between outer space and a charged body. FIG. 14 is a front view showing the configuration of an optical element module including the semiconductor optical element according to a modification of the first embodiment. FIG. 15 is a graph showing the relationship between the intensity of emitted light of the semiconductor optical element in the optical element module and the potential difference. FIG. 16 is a block diagram showing the configuration of a potential difference measuring device according to the first embodiment of the present invention. FIG. 17 is a graph showing the relationship between the intensity of emitted light of the semiconductor optical element in the optical element module and the potential difference in a form different from that of FIG.

[0012] [First Embodiment] Hereinafter, a first embodiment of the present invention will be described in detail.

[0013] <Configuration of Potential Difference Measuring Device> Fig. 1 is a block diagram showing the configuration of a potential difference measuring device 101 according to this embodiment. Fig. 2 is a perspective view showing the external configuration of an artificial satellite 200 on which the potential difference measuring device 101 is mounted.

[0014] As shown in Fig. 1, the potential difference measuring device 101 includes a light source 1, an incident optical fiber 2 (first optical fiber), a semiconductor optical element 3, an output optical fiber 4 (second optical fiber), a detector 5 (photodetector), and a calculation unit 6. The potential difference measuring device 101 is mounted on a spacecraft such as an artificial satellite 200 shown in Fig. 2. Other spacecraft on which the potential difference measuring device 101 may be mounted include a planetary probe and a space transport vehicle. The artificial satellite 200 includes, as main components, a structure 201 that forms the structure of the main body of the artificial satellite 200, a solar cell panel 202, an antenna 203, and the like.

[0015] The potential difference measuring device 101 may be mounted on a space suit worn by an astronaut performing extravehicular activity, in addition to a spacecraft.

[0016] The light source 1 is a light source that emits light that is incident on the semiconductor optical element 3. As the light source 1, a light source element such as a semiconductor laser or a super luminescent diode is used. A super luminescent diode is a light source element that has a broad spectrum like an LED and emits high-intensity light like a semiconductor laser. The light source 1 is arranged inside the structure 201 of the satellite 200, like other electronic devices, so as to be protected from radiation and the like. Alternatively, the light source 1 may be arranged outside the structure 201 as long as measures are taken to protect it from radiation and the like.

[0017] When the detector 5 described below is disposed outside the structure 201, measures are taken to protect it from radiation, etc., just like the light source 1. Furthermore, radiation-resistant products exist for the incident optical fiber 2 and the output optical fiber 4 described below, so such products may be used. However, if such products are not available, measures are taken to protect the incident optical fiber 2 and the output optical fiber 4 from radiation, etc.

[0018] The incident optical fiber 2 is a light guide path that guides the light emitted from the light source 1 to the semiconductor optical element 3. One end of the incident optical fiber 2 is connected to the light source 1, and the other end is connected to the semiconductor optical element 3. The incident optical fiber 2 is arranged inside or outside the structure 201 depending on the positions at which the light source 1 and the semiconductor optical element 3 are arranged.

[0019] The semiconductor optical device 3 is an optical device that reduces the intensity of outgoing light relative to the intensity of incident light by absorbing incident light guided from the light source 1 by the input optical fiber 2 using free carriers generated by the presence of charged particles. The semiconductor optical device 3 reduces the intensity of outgoing light relative to the intensity of incident light by utilizing a phenomenon called free carrier absorption, in which free carriers (electrons or holes) in a semiconductor absorb light.

[0020] The semiconductor optical device 3 is fabricated by a semiconductor process, and is preferably fabricated using silicon photonics technology, which is particularly capable of forming minute optical elements. Alternatively, the semiconductor optical device 3 may be fabricated using a semiconductor material other than silicon. For example, wide-gap semiconductors using semiconductor materials such as silicon carbide and gallium nitride are resistant to radiation, and therefore the semiconductor optical device 3 may be fabricated using these semiconductor materials.

[0021] The semiconductor optical element 3 is disposed on a charged body 10. The charged body 10 is any part on the outer surface of the artificial satellite 200 as a spacecraft. Specifically, as shown in Fig. 2, the semiconductor optical element 3 is attached to the surface of a structure 201, a solar cell panel 202 (cover glass), an antenna 203, etc. The semiconductor optical element 3 is attached to the surface of the object to be attached by being stuck with a conductive adhesive material such as conductive tape, or by being fixed with screws, etc.

[0022] Depending on the structure of the satellite 200, the plasma may enter the internal space of the structure 201 through an opening, a ventilation structure, etc. In such a case, the semiconductor optical device 3 may be attached to the surface of the charged body 10 exposed inside the structure 201. This makes it possible to detect the potential difference with the plasma present in the internal space, just like the outside.

[0023] The artificial satellite 200 is electrically floating relative to the potential of the plasma that fills outer space. The charged body 10 is charged by charged particles in the plasma, electromagnetic radiation such as gamma rays, high-energy particle rays, and the photoelectric effect caused by sunlight. The potential difference between the charged body 10 and the plasma (outer space) is determined by the balance with the charge supplied by the plasma and taken in by the charged body 10.

[0024] The surface of the structure 201 is made of metal. The surface potential of the metal parts of the satellite 200 made of metal is generally set to a negative potential, the same as the potential of the negative electrode of the electronic circuit installed inside the structure 201. The surface (cover glass) of the solar panel 202 and the antenna 203 include components made of metal and components made of insulators. The metal parts (charged bodies 10) are charged by charged particles in the plasma, resulting in a potential difference called absolute charging between them and the plasma. Furthermore, the insulator parts (charged bodies 10) made of insulators and the metal parts (non-conducting metal parts) that are not electrically connected to the negative electrode of the electronic circuit due to the insulator have different capacitances and the balance between the various charging factors described above. As a result, the insulator parts and non-conducting metal parts generate a potential difference called local charging between them and the metal parts.

[0025] By attaching the semiconductor optical element 3 to the surface of each location on the satellite 200 as described above, the surface potential of the semiconductor optical element 3 becomes the same as the potential of the charged body 10. Alternatively, the surface potential of the semiconductor optical element 3 becomes a potential with a predetermined potential difference from the potential of the charged body 10 due to the electrostatic capacitance of the adhesive material and the influence of the electric field from structures around the semiconductor optical element 3 (for example, a module housing incorporating the semiconductor optical element 3).

[0026] The output optical fiber 4 is a light guide path that guides the light output from the semiconductor optical element 3 to the detector 5. One end of the output optical fiber 4 is connected to the semiconductor optical element 3, and the other end is connected to the detector 5. The output optical fiber 4 is arranged inside or outside the structure 201 depending on the positions at which the semiconductor optical element 3 and the detector 5 are arranged.

[0027] The detector 5 is configured with, for example, a photodiode so as to receive the emitted light from the semiconductor optical element 3 guided by the emission optical fiber 4 and detect the intensity of the emitted light. The detector 5 may be disposed inside the structure 201, like the light source 1, or may be disposed outside the structure 201.

[0028] The calculation unit 6 measures the potential difference between the charged body 10 and the plasma (outer space) by calculating the difference in intensity before and after the decrease in the emitted light from the semiconductor optical device 3 detected by the detector 5. Specifically, the calculation unit 6 has a storage unit and a calculation unit. The storage unit is configured, for example, by a memory, and stores the intensity of the emitted light before and after the decrease. The calculation unit subtracts the intensity of the emitted light after the decrease from the intensity of the emitted light before the decrease, which is stored in the storage unit. The calculation unit may be configured by an arithmetic circuit such as a CPU (Central Processing Unit) or a logic circuit.

[0029] Calculation unit 6 may be located on satellite 200, or may be located at a predetermined location on Earth. If calculation unit 6 is located on Earth, the light intensity detected by detector 5 needs to be transmitted to calculation unit 6. Therefore, in this case, potential difference measuring device 101 includes a transmission unit (not shown) that transmits the light intensity to Earth. Alternatively, in this case, detector 5 may have a transmission function for transmitting the light intensity to Earth.

[0030] The semiconductor optical element 3 outputs emitted light of maximum intensity in the absence of charged particles. The semiconductor optical element 3 changes the intensity of the emitted light so that the difference between this maximum intensity (intensity before reduction) and the intensity of the emitted light in the presence of charged particles (intensity after reduction), i.e., the intensity difference, changes in response to the potential difference. Therefore, by having the surface potential of the semiconductor optical element 3 be the same as the potential of the charged body 10 or have a predetermined potential difference from the potential of the charged body 10, the semiconductor optical element 3 reduces the intensity of the emitted light in response to the potential of the charged body 10 charged by the charged particles. Therefore, by having the detector 5 detect the difference in intensity of the emitted light from the semiconductor optical element 3 before and after reduction, the potential difference, which changes in response to the intensity difference, can be obtained.

[0031] <Details of Semiconductor Optical Device> Fig. 3 is a plan view showing the configuration of the semiconductor optical device 3. Fig. 4 is a plan view showing the configuration of another semiconductor optical device 3.

[0032] The semiconductor optical device 3 shown in FIG. 3 is a semiconductor optical resonator 3A (optical resonator). The semiconductor optical resonator 3A is made of a semiconductor material such as silicon and has a rectangular semiconductor substrate 3a. The semiconductor optical resonator 3A is a photonic crystal optical circuit equipped with a two-dimensional photonic crystal structure. The two-dimensional photonic crystal structure is a structure in which a two-dimensional photonic crystal is formed by periodically arranging a large number of air holes 3b in the semiconductor substrate 3a. The photonic band structure of the two-dimensional photonic crystal structure formed in the semiconductor substrate 3a is controlled by the diameter, distance (lattice constant of the photonic crystal), shape, etc. of the air holes 3b.

[0033] The semiconductor substrate 3a is provided with an incident portion 3c and an output portion 3d. The incident portion 3c is a portion where light (incident light L1) from the input optical fiber 2 is incident. The output portion 3d is a portion where light (output light L2) is output to the output optical fiber 4. The incident light L1 enters the incident portion 3c, propagates through the semiconductor substrate 3a in a propagation direction PD along the long side direction of the semiconductor substrate 3a, and is output from the output portion 3d as output light L2.

[0034] The semiconductor substrate 3 a has a waveguide region 31 and a pair of hole regions 32 .

[0035] The waveguide region 31 is formed to guide light from the incident portion 3 c to the exit portion 3 d. The waveguide region 31 is a linear portion having a certain width where the holes 3 b are filled. The waveguide region 31 is disposed at the center of the semiconductor substrate 3 a in the direction perpendicular to the propagation direction PD (the direction of the short side of the semiconductor substrate 3 a) within the plane of the semiconductor substrate 3 a.

[0036] The pair of void regions 32 are regions in which the numerous voids 3b are formed. The pair of void regions 32 are arranged side by side in the short side direction of the semiconductor substrate 3a, with the waveguide region 31 between them. Each of the pair of void regions 32 has a first region 32a and a second region 32b. One first region 32a is provided on the entrance portion 3c side and one on the exit portion 3d side. The second region 32b is provided between the two first regions 32a.

[0037] The waveguide region 31 includes a waveguide 31a, a resonator 31b, and a waveguide 31c. The waveguide region 31 is formed as a portion where the holes 3b are closed.

[0038] The waveguide 31a is formed between a pair of first regions 32a on the side of the incident portion 3c so as to extend from the incident portion 3c in the propagation direction PD. The resonator 31b is formed between a pair of second regions 32b as a portion where the holes 3b are blocked so as to extend from the end of the waveguide 31a in the propagation direction PD. The waveguide 31c is formed between a pair of first regions 32a on the side of the output portion 3d so as to extend from the end of the resonator 31b to the output portion 3d in the propagation direction PD.

[0039] The resonator 31b resonates at a specific wavelength λ(R) (excitation wavelength). The resonator 31b has a resonance mode (excitation resonance mode) for the wavelength λ(E) of the incident light L1 and a resonance mode for the resonant light emitted by the resonator 31b. As a result, the resonator 31b is excited by the wavelength λ(E) and emits resonant light having the wavelength λ(R).

[0040] In the first region 32a, the holes 3b have a diameter D1, and two adjacent holes 3b are spaced a distance R1 apart. In the second region 32b, the holes 3b have a diameter D2, and two adjacent holes 3b are spaced a distance R2 apart.

[0041] The wavelength bands of light propagating in the waveguides 31a and 31c depend on the diameter D1 and distance R1 of the air hole regions 32 adjacent to the waveguides 31a and 31c, respectively. The wavelength band of light propagating in the resonator 31b depends on the diameter D2 and distance R2 of the second region 32b adjacent to the resonator 31b.

[0042] When the distance R2 is longer than the distance R1, a difference in the wavelength band of the propagating light occurs between the waveguides 31a and 31c and the resonator 31b. As a result, light included in a specific wavelength band is reflected at the boundary between the waveguides 31a and 31c and the resonator 31b. Therefore, a portion of the incident light L1 entering the waveguide region 31 propagates back and forth between the two boundaries to become resonant light L3 that resonates in the resonator 31b. Furthermore, a portion of the resonant light L3 passes through the boundary on the side of the exit portion 3d in the propagation direction PD and becomes output light L2 that is output from the exit portion 3d to the outside of the semiconductor substrate 3a.

[0043] In this way, the waveguide region 31 generates resonant light L3 by the two waveguides 31a and 31c and the resonator 31b therebetween.

[0044] As long as the waveguides 31a, 31b and the resonator 31b are properly formed, the specific arrangement and diameter of the holes 3b in the semiconductor substrate 3a are not limited to the configuration shown in Figure 3. For example, when the distances R1 and R2 are different from each other as described above, the diameters D1 and D2 may be the same. Alternatively, when the diameters D1 and D2 are different from each other, the distances R1 and R2 may be the same. In these cases, as in the above case, the waveguide region 31 generates resonant light L3 by the two waveguides 31a, 31c and the resonator 31b therebetween.

[0045] The semiconductor optical resonator 3A described above has waveguides 31a and 31c and resonator 31b arranged in a straight line. Alternatively, the semiconductor optical resonator 3A may have waveguides 31a and 31c and resonator 31b arranged at positions offset from each other in the short-side direction of the semiconductor substrate 3a. Specifically, in the semiconductor substrate 3a, the waveguide 31a is arranged near an edge on one long side, the waveguide 31c is arranged near an edge on the other long side, and the resonator 31b is arranged at approximately the center in the short-side direction.

[0046] 4 is a semiconductor optical waveguide 3B (optical waveguide). The semiconductor optical waveguide 3B is made of a semiconductor material such as silicon and has a rectangular semiconductor substrate 3a. The semiconductor optical waveguide 3B is a photonic crystal optical circuit equipped with the above-mentioned two-dimensional photonic crystal structure.

[0047] The semiconductor substrate 3 a is provided with an incident portion 3 c and an exit portion 3 d. The incident light L1 enters the incident portion 3 c, propagates through the semiconductor substrate 3 a in a propagation direction PD along the long side direction of the semiconductor substrate 3 a, and exits from the exit portion 3 d as the exit light L2.

[0048] Unlike the semiconductor optical resonator 3A, the semiconductor optical waveguide 3B does not have a resonator 31b. Therefore, the semiconductor substrate 3a in the semiconductor optical waveguide 3B has a different structure from the semiconductor substrate 3a in the semiconductor optical resonator 3A, as will be described below.

[0049] The semiconductor substrate 3 a has a waveguide 34 and a pair of void regions 35 .

[0050] The waveguide 34 is formed to guide light from the incident portion 3 c to the exit portion 3 d. The waveguide 34 is a linear portion having a certain width, where the air holes 3 b are filled. The waveguide 34 is disposed at the center of the semiconductor substrate 3 a in the plane thereof in the direction of the short side of the semiconductor substrate 3 a.

[0051] The pair of void regions 35 are regions in which the numerous voids 3b are formed. The pair of void regions 35 are arranged side by side in the short side direction of the semiconductor substrate 3a, with the waveguide 34 between them. In the pair of void regions 35, all of the voids 3b have the same diameter, and the spacing between adjacent voids 3b is the same. However, the diameters of all of the voids 3b do not have to be the same. Furthermore, the spacing between adjacent voids 3b does not have to be the same.

[0052] The semiconductor optical element 3 described above is exemplified by a semiconductor optical resonator 3A or a semiconductor optical waveguide 3B that includes a photonic crystal structure having holes 3b. In contrast, the semiconductor optical element 3 may be a semiconductor optical waveguide that does not include a photonic crystal structure. Examples of such semiconductor optical waveguides include optical waveguides that have a structure in which a waveguide pattern is formed on a semiconductor material substrate by photolithography, etching, ion implantation, or the like. It goes without saying that semiconductor optical waveguides that do not include a photonic crystal structure may have other structures.

[0053] <Measurement of Potential Difference Using Potential Difference Measuring Device> Fig. 5 is a diagram showing the mechanism by which the semiconductor optical element 3, particularly the semiconductor optical resonator 3A or the semiconductor optical waveguide 3B, emits light with an intensity corresponding to the potential difference between outer space and a charged body 10. Fig. 6 is a diagram showing the mechanism by which the semiconductor optical element 3, particularly the semiconductor optical waveguide 3C, emits light with an intensity corresponding to the potential difference between outer space and a charged body.

[0054] When the charged body 10 is not placed in space and no charged particles are present, the semiconductor optical device 3 emits output light without reducing its intensity in response to the incident light from the light source 1. As a result, the memory unit of the calculation unit 6 previously stores the intensity of the output light in a state where no charged particles are present as the intensity before reduction. Because the intensity of the output light does not change from the intensity before reduction, the calculation unit 6 outputs the difference in intensity of the output light before and after reduction as 0. As a result, the potential difference between the charged body 10 and the space where no charged particles are present becomes 0.

[0055] In contrast, when the charged body 10 is placed in outer space, a potential difference occurs between outer space and the charged body 10, and there is no plasma around the charged body 10, the surface potential of the semiconductor optical device 3 becomes the same as the potential of the charged body 10 or a potential that has a predetermined potential difference from the potential of the charged body 10. When the charged body 10 is a spacecraft, the charged body 10 generally has a negative potential with respect to outer space.

[0056] FIG. 5 shows a case where a charged body 10 is placed in outer space, a potential difference exists between outer space and the charged body 10, and plasma (electrons E and positive ions I) exists around the charged body 10. In this case, because the potential of the semiconductor optical device 3 is negative, the positive ions I are attracted to the surface of the semiconductor optical device 3. In this state, the positive ions I become atoms A or molecules M by stealing electrons from the semiconductor optical device 3. The positive ions I are attracted to the surface of the semiconductor optical device 3 until the potential difference between outer space and the surface potential of the semiconductor optical device 3 is eliminated or an equilibrium state is reached. As a result, holes H are generated within the semiconductor optical device 3 in an amount corresponding to the potential difference. Since the positive holes H function as free carriers, light passing through the semiconductor optical device 3 is absorbed by the free carriers. In this way, when free carrier absorption occurs in the semiconductor optical device 3, the intensity of the light emitted from the semiconductor optical device 3 decreases.

[0057] The reduction in the intensity of the emitted light described above is due to the semiconductor optical resonator 3 A or the semiconductor optical waveguide 3 B. In contrast, the reduction in the intensity of the emitted light also occurs in the semiconductor optical waveguide 3 C shown in Fig. 6, which is the semiconductor optical waveguide not provided with the photonic crystal structure described above, due to the same mechanism as that of the semiconductor optical resonator 3 A or the semiconductor optical waveguide 3 B.

[0058] Furthermore, when the charged body 10 is positively charged with respect to space, electrons in space are attracted to the surface of the semiconductor optical device 3 and injected into the semiconductor optical device 3. As a result, light is absorbed by the electrons through free carrier absorption. Therefore, just as in the case of a negative potential difference, even with a positive potential difference, the light intensity decreases due to the generation of free carriers.

[0059] The calculation unit 6 stores the intensity of the emitted light after the decrease in the memory unit, and calculates the difference in intensity of the emitted light before and after the decrease stored in the memory unit using the calculation unit. Since the difference in intensity changes depending on the potential difference between the charged body 10 and outer space, the potential difference is measured.

[0060] The potential difference between the potential (V1) of the structure 201, which is the metal part of the satellite 200, and the potential (V0) of outer space is V0-V1. Therefore, the semiconductor optical device 3 disposed in the structure 201 provides this potential difference V0-V1 as the above-mentioned absolute charge. Also, the potential difference with respect to the potential (V2) of the solar cell panel 202 (cover glass), which is an insulating part of the satellite 200, is V0-V2. Therefore, by subtracting the potential difference (V0-V1) from the potential difference (V0-V2), the potential difference (V1-V2) is provided as the above-mentioned local charge.

[0061] <Effects of the Potential Difference Measuring Device> As described above, the potential difference measuring device 101 according to this embodiment includes the light source 1, the semiconductor optical element 3, the detector 5, and the calculation unit 6. The semiconductor optical element 3 reduces the intensity of the outgoing light relative to the incident light by absorbing the incident light from the light source 1 due to free carriers generated by the presence of charged particles. The semiconductor optical element 3 is disposed on the charged body 10, and its surface potential is set to the same potential as the charged body 10 or to a potential having a predetermined potential difference from the potential of the charged body 10. The detector 5 detects the intensity of the outgoing light. With the semiconductor optical element 3 disposed and its surface potential set as described above, the calculation unit 6 calculates the difference in intensity of the outgoing light detected by the detector 5 before and after the reduction, thereby measuring the potential difference between the charged body 10 charged by the charged particles and outer space, which changes in accordance with the difference in intensity.

[0062] In the above configuration, free carriers due to charged particles in the semiconductor optical element 3 absorb incident light, thereby reducing the intensity of the emitted light. The difference in intensity before and after the reduction varies depending on the potential difference between the charged body 10 and outer space, so the potential difference between the charged body 10 and outer space can be measured by calculating this intensity difference. Furthermore, the light source 1, the semiconductor optical element 3, the detector 5, and the calculation unit 6 can be fabricated using semiconductor processes. Therefore, the potential difference measuring device 101 to be mounted on the artificial satellite 200 can be made smaller.

[0063] Furthermore, the calculation unit 6 may be located on the Earth as described above, which allows the portion of the potential difference measuring device 101 mounted on the artificial satellite 200 to be made even smaller.

[0064] The semiconductor optical device 3 may be a semiconductor optical resonator 3A formed of a photonic crystal, or may be a semiconductor optical waveguide 3B formed of a photonic crystal.

[0065] According to the above configuration, the photonic crystal does not include a PN junction, making it possible to provide a potential difference measuring device 101 that is resistant to electrostatic breakdown. Furthermore, because it does not include a PN junction, it is possible to provide a potential difference measuring device 101 that is less susceptible to malfunctions due to cosmic radiation such as gamma rays, for example, malfunctions due to the formation of lattice defects, and single event effects.

[0066] The sensitivity of measuring the potential difference can be increased by increasing the Q value of the semiconductor optical resonator 3A, and the sensitivity of the semiconductor optical waveguide 3B can be increased by forming the waveguide 34 longer.

[0067] The charged body 10 may be made of metal, which makes it possible to measure the absolute charge due to the potential difference between the metallic structure 201 of the satellite 200 and outer space.

[0068] The charged body may be made of an insulator. As a result, when a potential difference between a part of the satellite 200 made of an insulator, such as the cover glass of the solar panel 202, and outer space is obtained, a local charge can be obtained by subtracting the potential difference between the structure 201 and outer space from the potential difference.

[0069] <Modification 1> A modification of the present embodiment will now be described. Fig. 7 is a front view showing the configuration of an optical element module 7 including a semiconductor optical element 3 according to this modification.

[0070] The potential difference measuring device 101 may include an optical element module 7 shown in Fig. 7. The optical element module 7 includes a base 71 (support), a conductive adhesive layer 72, a semiconductor layer 73, an insulating layer 74, a connecting conductor 75, and a semiconductor optical element 3.

[0071] The base 71 is attached to the surface of the charged body 10, thereby electrically connecting to the charged body 10. The base 71 is made of a conductive material such as metal. The base 71 is attached to the charged body 10 in the same manner as the above-mentioned semiconductor optical device 3 is attached to the charged body 10. The conductive adhesive layer 72 is formed from an adhesive material such as a conductive adhesive.

[0072] The semiconductor layer 73 is formed on the base 71 and is made of, for example, P-type silicon. The insulating layer 74 is formed on the semiconductor layer 73 and is made of, for example, SiO ２ The semiconductor optical device 3 is formed on the insulating layer 74.

[0073] Here, the semiconductor optical element 3 is made of P-type silicon. The semiconductor optical element 3 and the semiconductor layer 73 may be made of N-type silicon or non-doped silicon.

[0074] In the optical element module 7 configured as described above, the potential of the base 71 is assumed to be the same as the potential of the charged body 10, for example, −200 V. When the optical element module 7 is exposed to plasma in outer space, positively charged particles (cations) of the plasma are attracted to the base 71 and the semiconductor layer 73. As a result, electrons are stolen from the base 71 and the semiconductor layer 73 by the positively charged particles. In this state, the potential of the base 71 and the semiconductor layer 73 may not strictly be the same as that of the charged body 10, but for convenience, it is assumed here that the potential is the potential of the charged body 10.

[0075] On the other hand, when the positively charged particles of the plasma are attracted to the semiconductor optical element 3, the positively charged particles also steal electrons from the semiconductor optical element 3. Because the insulating layer 74 is interposed between the semiconductor optical element 3 and the semiconductor layer 73, even if electrons are stolen from the semiconductor optical element 3, no charge moves between the semiconductor optical element 3 and the semiconductor layer 73. Therefore, the number of holes in the semiconductor optical element 3 increases as electrons are stolen by the positively charged particles until the potential difference of −200 V is eliminated or an equilibrium state is reached.

[0076] In this state, the semiconductor optical device 3 has holes in the same number as the stolen electrons. These holes become free carriers and absorb light within the semiconductor optical device 3. The greater the potential difference between the semiconductor optical device 3 (charged body 10) and outer space, the more electrons are stolen from the semiconductor optical device 3, and the stronger the free carrier absorption by the corresponding number of holes. This results in a greater decrease in the intensity of the emitted light, and therefore a larger difference in the intensity of the light emitted from the semiconductor optical device 3 before and after the decrease.

[0077] Here, we will explain the results of an experiment conducted to confirm the decrease in intensity of the emitted light from the semiconductor optical element 3 when the potential difference is changed using the optical element module 7. Fig. 8 is a graph showing the relationship between the intensity of the emitted light from the semiconductor optical element 3 in the optical element module 7 and the potential difference. Fig. 10 is a graph showing the relationship between the intensity of the emitted light from the semiconductor optical element 3 in the optical element module 7 and the potential difference in a form different from that of Fig. 8.

[0078] In this experiment, a cubic structure, which is assumed to be a small artificial satellite, was placed in a vacuum chamber as a charged body 10. An optical element module 7 was attached to the surface of the charged body 10. The optical element module 7 was attached with a 1 mm ２ The test specimen used was a specimen mounted with a semiconductor optical element 3 having a size of about 1000 nm. By injecting plasma into the vacuum chamber using a plasma injection device, the vacuum chamber was filled with the plasma of electrons E and positive ions I as described above, and the voltage applied to the charged body 10 was changed. It was confirmed that the intensity of the light emitted from the semiconductor optical element 3 changed as the potential difference was changed.

[0079] 8, it was confirmed that the intensity of the emitted light decreased significantly when the potential difference was changed from 50 V to 100 V, 150 V, and 200 V. The reason why the intensity of the emitted light was disturbed at a potential difference of 200 V was due to the discharge that occurred in the vacuum chamber, and the potential difference decreased the moment the discharge occurred.

[0080] In Fig. 10, the minimum emitted light intensity observed for each applied potential difference (50 V, 100 V, 150 V, 200 V) in the measurement results shown in Fig. 8 is plotted by black circles. The emitted light intensity decreases as the potential difference increases, which is a phenomenon that occurs based on the theory explained below.

[0081] Ideally, the density N of free carriers induced in the semiconductor optical device 3 is proportional to the potential difference ΔV. However, depending on the structure of the optical device module 7 and the measurement environment, this proportional relationship may not hold, particularly in regions where the potential difference is small. The absorption coefficient α, which represents the strength of absorption by free carriers, is proportional to the free carrier density N, and the proportionality coefficient for electrons is approximately twice as large as that for holes.

[0082] Due to such free carrier absorption, the intensity of the light emitted from the semiconductor optical device 3 is attenuated in accordance with the following relational expression:

[0083] Iout ∝ exp(-Aα), where A is a constant that depends on factors such as the light path length within the semiconductor optical device 3. The dashed line in Figure 10 shows the curve resulting from fitting the experimental data based on the above-mentioned exponential decay model. In particular, in the region above 100 V, the experimental data and the fitting curve match closely, indicating that good experimental data were obtained. On the other hand, in the region from 0 V to 100 V, there is a slight deviation between the experimental data and the fitting curve. By measuring the decrease in the output light intensity in this region in advance, it is possible to adequately detect potential differences of less than 100 V.

[0084] As described above, it was confirmed that the optical element module 7 mounting the sufficiently miniaturized semiconductor optical element 3 exhibits a difference in the decrease in the intensity of the emitted light from the semiconductor optical element 3 depending on the potential difference. Therefore, it is possible to measure the potential difference between the charged body 10 and outer space using such an optical element module 7.

[0085] As described above, the optical element module 7 supports the semiconductor optical element 3 on the base 71 via the semiconductor layer 73. This makes it easier to improve the measurement accuracy of the optical element module 7 as a sensor and to provide functions for stabilizing operation, and allows for a robust configuration, compared to a case in which only the semiconductor optical element 3 is attached to the charged body 10. Furthermore, the optical element module 7 allows the semiconductor optical element 3 to be placed close to the surface of the artificial satellite 200, for example, within a few centimeters.

[0086] The semiconductor optical device 3 may be connected to the negative or positive terminal of an electronic circuit via wiring having a switch. The semiconductor optical device 3 may also be connected to a device that controls the potential via the wiring. This allows the charge accumulated in the semiconductor optical device 3 to be released from the semiconductor optical device 3. Therefore, the semiconductor optical device 3 can be initialized to a state where it is not charged by the charged particles of the plasma. Furthermore, by turning off the switch after initialization and observing the rate at which the light intensity changes again, it is possible to use the difference in the generation rates of electrons and holes to determine whether the potential difference with outer space is positive or negative.

[0087] Alternatively, a wire may be connected to the base 71, and the wire may be connected to a device for controlling the potential, thereby setting the potential of the base 71 to a desired value. This makes it possible to control the reference potential of the semiconductor optical device 3. Furthermore, by observing the rate of change in light intensity that occurs when the potential of the base 71 is changed, it is possible to determine the polarity of the potential difference with outer space by utilizing the difference in the generation rate of electrons and holes.

[0088] The configuration of this modification can also be applied to Modification 2 and Embodiment 2, which will be described later. In particular, the experimental results described above were obtained in a vacuum chamber similar to the vacuum chamber of Modification 2. Therefore, the potential difference measuring device 101 can be applied not only to devices in space but also to devices on the ground.

[0089] <Modification 2> Next, another modification of this embodiment will be described.

[0090] In this modification, a configuration in which a potential difference measuring device 101 is applied to a plasma device (not shown) will be described.

[0091] The plasma device is a device that performs various processes, such as etching and deposition, on a workpiece, such as a wafer, placed in a vacuum chamber in a reactive gas atmosphere using plasma generated in the vacuum chamber. The potential difference measuring device 101 measures the potential difference between the workpiece, which serves as a charged body 10, and the plasma space. For this purpose, the semiconductor optical element 3 is attached to the surface of the workpiece. This sets the semiconductor optical element 3 to a potential that is the same as the potential of the charged body 10 or that has a predetermined potential difference from the potential of the charged body 10.

[0092] In the semiconductor optical device 3, free carriers due to charged particles from the plasma space absorb incident light, thereby reducing the intensity of the emitted light. The difference in intensity before and after the reduction varies depending on the potential difference between the charged body 10 and the plasma space, so the potential difference between the charged body 10 and the plasma space can be measured by calculating this intensity difference using the calculation unit 6. Therefore, before processing, the potential difference between the charged body 10 and the plasma space can be measured, and measures such as removing electricity from the object to be processed can be taken as necessary.

[0093] Furthermore, the light source 1, detector 5, and calculation unit 6 other than the semiconductor optical element 3 may be disposed outside the vacuum chamber, and therefore do not disturb the plasma space. Furthermore, the semiconductor optical element 3 is fabricated to be extremely small and attached to the surface of the workpiece, so that it has almost no effect on the plasma space. Furthermore, the input optical fiber 2 and the output optical fiber 4, which are drawn into the vacuum chamber, may be disposed so as to disturb the plasma space as little as possible.

[0094] If the plasma device is an etching device, a blocking structure is provided to block the plasma flow incident on the semiconductor optical element 3 so as to prevent etching of the semiconductor optical element 3. Alternatively, the semiconductor optical element 3 may have a protective function to block the plasma flow.

[0095] The configuration of this modified example can also be applied to the second embodiment described later.

[0096] [Embodiment 2] A description will be given of embodiment 2 of the present invention. For the sake of convenience, components having the same functions as those described in embodiment 1 will be denoted by the same reference numerals, and the description thereof will be omitted.

[0097] FIG. 9 is a block diagram showing the configuration of a potential difference measuring device 102 according to the second embodiment.

[0098] 9, the potential difference measuring device 102 includes a light source 1, similar to the potential difference measuring device 101 of the first embodiment. The potential difference measuring device 102 also includes a plurality of input optical fibers 2, semiconductor optical elements 3, output optical fibers 4, and detectors 5. Only one calculation unit 6 is provided. Similar to the semiconductor optical elements 3, output optical fibers 4, and detectors 5, multiple calculation units 6 may be provided. However, in order to simplify the configuration of the potential difference measuring device 101, it is preferable to provide multiple calculation units 6 as a single device with the calculation functions. Furthermore, the potential difference measuring device 102 includes an optical branching unit 8.

[0099] The light source 1 and the optical splitter 8 are connected by a single incident optical fiber 2. The optical splitter 8 and the multiple semiconductor optical elements 3 are connected by the same number of incident optical fibers 2 as the semiconductor optical elements 3. The optical splitter 8 is an optical device that splits the single incident optical fiber 2 connected to the light source 1 into multiple optical fibers to be connected to the multiple semiconductor optical elements 3. The incident optical fiber 2 is configured so that the optical splitter 8 guides light from the single light source 1 to the multiple semiconductor optical elements 3.

[0100] The semiconductor optical elements 3 are arranged at various locations on the satellite 200 where potential differences are to be measured. The semiconductor optical elements 3 and the detectors 5 are individually connected by a plurality of output optical fibers 4. The detectors 5 are preferably arranged in a concentrated manner so that the difference in intensity before and after the decrease in output light from the semiconductor optical elements 3 can be confirmed at one location.

[0101] In the potential difference measuring device 102 configured as described above, light from the light source 1 is transmitted to each semiconductor optical element 3 via multiple incident optical fibers 2 branched from a single incident optical fiber 2 by an optical brancher 8. The intensity of the emitted light generated by each semiconductor optical element 3 is detected by a detector 5 corresponding to the semiconductor optical element 3. The calculation unit 6 has multiple storage units and arithmetic units, and thereby individually calculates the difference in intensity of the emitted light before and after the decrease detected by the multiple detectors 5. Furthermore, when multiple calculation units 6 are provided, each calculates the difference in intensity of the emitted light before and after the decrease detected by the multiple detectors 5.

[0102] This makes it possible to measure the potential difference between a plurality of charged bodies 10 and outer space using a single light source 1. For example, in a spacecraft such as an artificial satellite 200, it is possible to measure the potential difference between charged bodies 10 at various locations (such as the structure 201 of the artificial satellite 200 and an insulator such as the cover glass of a solar panel 202) and outer space. This makes it possible to grasp the charging status at various locations on the spacecraft and take measures against charging.

[0103] [Notes] The present invention is not limited to the above-described embodiments, and various modifications are possible within the scope of the claims. Furthermore, embodiments obtained by appropriately combining the technical means disclosed in different embodiments are also included in the technical scope of the present invention.

[0104] REFERENCE SIGNS LIST 1 Light source 2 Incident optical fiber (first optical fiber) 3 Semiconductor optical element 3A Semiconductor optical resonator (optical resonator) 3B Semiconductor optical waveguide (optical waveguide) 4 Output optical fiber (second optical fiber) 5 Detector 6 Calculation unit 10 Charged body 71 Base (support) 73 Semiconductor layer 74 Insulating layer 101, 102 Potential difference measuring device L1 Incident light L2 Output light

Claims

1. A potential difference measuring device comprising: a light source; a semiconductor optical element that reduces the intensity of outgoing light relative to the intensity of incident light by absorbing incident light from the light source with free carriers generated by the presence of charged particles; a detector that detects the intensity of the outgoing light; and a calculation unit that calculates the difference in intensity before and after the reduction in the outgoing light, wherein the intensity difference changes depending on the potential difference between a charged body charged by the charged particles and outer space, and the calculation unit measures the potential difference by calculating the intensity difference when the semiconductor optical element is placed on the charged body and the surface potential of the semiconductor optical element is set to the same potential as the charged body or to a potential that has a predetermined potential difference from the charged body.

2. A potential difference measuring device comprising: a light source; a semiconductor optical element that reduces the intensity of outgoing light relative to the intensity of incident light by absorbing incident light from the light source with free carriers generated by the presence of charged particles; a detector that detects the intensity of the outgoing light; and a calculation unit that calculates the difference in intensity before and after the reduction in the outgoing light, wherein the intensity difference changes depending on the potential difference between a charged body charged by the charged particles and plasma space, and the calculation unit measures the potential difference by calculating the intensity difference when the semiconductor optical element is placed on the charged body and the surface potential of the semiconductor optical element is set to the same potential as the charged body or to a potential that has a predetermined potential difference from the potential of the charged body.

3. The potential difference measuring device according to claim 1 or 2, further comprising: a support made of a conductive material and electrically connected to the charged body; a semiconductor layer formed on the support; and an insulating layer formed on the semiconductor layer, wherein the semiconductor optical element is formed on the insulating layer.

4. The potential difference measuring device according to claim 1 or 2, wherein the semiconductor optical element is an optical resonator formed by a photonic crystal.

5. The potential difference measuring device according to claim 1 or 2, wherein the semiconductor optical element is an optical waveguide formed by a photonic crystal.

6. The potential difference measuring device according to claim 1 or 2, wherein the charged body is made of metal.

7. The potential difference measuring device according to claim 1 or 2, wherein the charged body is made of an insulating material.

8. The potential difference measuring device according to claim 1 or 2, wherein a plurality of the semiconductor optical elements and a plurality of the detectors are provided, and further comprising: a first optical fiber that guides light from the single light source to a plurality of the semiconductor optical elements; and a plurality of second optical fibers that guide the emitted light from the plurality of the semiconductor optical elements to a plurality of the detectors.

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