Optical absorber, and beam splitter and optical communication device including the same

Abstract

To provide a technique that removes unnecessary light separated from a light beam and can be applied to sufficiently reduce noise in optical wireless communication.SOLUTION: An optical absorber (32) provided in an optical communication device (11) includes: a light-absorbing glass block that absorbs incident light; a first cavity in which light that has passed through the light-absorbing glass block is reflected in a multiple manner; and a second cavity in which light reflected by the light-absorbing glass block is reflected in a multiple manner.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to an optical absorber, and a beam splitter and an optical communication device including the optical absorber. [Background technology]

[0002] With the development of the information society, there has been a demand for high-speed communication. Radio waves are used for such high-speed communication. Meanwhile, light is an electromagnetic wave, but its use is not restricted by the Radio Law. Therefore, optical wireless communication (also called "space optical communication") is currently attracting attention, and devices for optical wireless communication are also being developed.

[0003] In optical wireless communication, it is required to remove optical components that become noise from the signal light. For example, when a free-space optical wireless communication using a laser is constructed in a coaxial optical system and the signal light is separated from the transmitted light or received light, it is possible that nearly half of the transmitted light or received light is emitted to the outside from the coaxial optical system. The light emitted from the optical system (hereinafter also referred to as "unwanted light") can become noise in the signal light. In order to sufficiently reduce the noise in the signal light, it is necessary to appropriately process such unwanted light.

[0004] Known as a technology capable of removing unnecessary light separated from a light beam is a diffuser that forms a tapered cavity using a member that reflects the unnecessary light, introduces the unnecessary light into the cavity, and attenuates it by multiple reflection (see, for example, Patent Document 1). Also known as a technology capable of removing unnecessary light separated from a light beam is a technology that uses an anti-reflection material containing carbon nanofibers on the surface of a light-reducing plate used in an exposure device (see, for example, Patent Document 2). [Prior art documents] [Patent documents]

[0005] [Patent Document 1] JP 2005-140964 A [Patent Document 2] International Publication No. 2010 / 150550 Summary of the Invention [Problem to be solved by the invention]

[0006] The above-mentioned conventional techniques may not be able to sufficiently remove unnecessary light, and therefore may not be able to sufficiently reduce noise in signal light when applied to optical wireless communication.

[0007] An object of one embodiment of the present invention is to provide a technique for removing unnecessary light separated from a light beam, which can be applied to sufficiently reduce noise in optical wireless communication. [Means for solving the problem]

[0008] In order to solve the above problems, a light absorber according to one embodiment of the present invention comprises a light-absorbing glass block that absorbs incident light, a first cavity in which light that has passed through the light-absorbing glass block is multiple-reflected, and a second cavity in which light reflected by the light-absorbing glass block is multiple-reflected.

[0009] In addition, in order to solve the above-mentioned problems, a beam splitter equipped with an optical absorber according to one aspect of the present invention is a beam splitter equipped with the above-mentioned optical absorber, wherein the optical absorber is arranged in any one of two or more optical paths branched by the beam splitter.

[0010] In order to solve the above problem, an optical communication device according to an aspect of the present invention is an optical communication device including the above beam splitter, and uses the beam splitter to branch an optical path of signal light. Effect of the Invention

[0011] According to one aspect of the present invention, it is possible to remove unnecessary light separated from a light beam so as to be applicable to sufficient noise reduction in optical wireless communication. [Brief description of the drawings]

[0012] [Figure 1] FIG. 1 is a diagram illustrating a configuration of an optical communication device according to a first embodiment of the present invention. [Diagram 2] FIG. 1 is a diagram illustrating a schematic configuration of an optical absorber according to a first embodiment of the present invention. [Diagram 3] 3 is a diagram for explaining the behavior of transmitted light in the optical absorber shown in FIG. 2. [Figure 4] 3 is a diagram for explaining the behavior of reflected light in the optical absorber shown in FIG. 2. [Diagram 5] 3 is a diagram illustrating a schematic configuration for measuring return light from the optical absorber illustrated in FIG. 2. FIG. [Figure 6] FIG. 10 is a diagram illustrating a schematic configuration of an optical communication device according to a second embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] [Embodiment 1] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will now be described in detail below. The configuration of an optical communication device according to this embodiment is shown in FIG.

[0014] [Configuration of optical communication device] 1, the optical communication device 11 is a transmitter in optical wireless communication, and includes an optical fiber 2, a beam splitter unit 3, and an adjustment device 4. The optical communication device 11 has the above-mentioned components in, for example, a sealed housing, and the above-mentioned components are appropriately arranged within the housing to form a coaxial optical system.

[0015] The optical fiber 2 is optically connected to a light source of signal light, and the signal light from the light source is transmitted to the optical fiber 2. The optical fiber 2 is, for example, a single mode optical fiber, and has a core and a cladding.

[0016] The beam splitter unit 3 includes a beam splitter 31 and a light absorber 32. The light absorber 32 has a light absorbing member 331.

[0017] The beam splitter 31 is an optical element that separates a specific polarization component of the light beam from the optical fiber 2 as signal light and reflects it toward the adjustment device 4. The signal light travels along an optical axis OA1. Light components other than the signal light pass through the beam splitter 31 and travel straight toward the optical absorber 32. In other words, the beam splitter 31 splits the incident light beam into signal light traveling in a direction perpendicular to the optical axis OA2 of the optical fiber 2 and other unnecessary light traveling straight along the optical axis OA2.

[0018] The optical absorber 32 is disposed at a position where the optical axis OA2 passes through the center of the entrance of the beam splitter 31. The distance between the optical absorber 32 and the beam splitter 31 in the direction along the optical axis OA2 is appropriately determined within a range where substantially all of the unnecessary light is introduced into the optical absorber 32. From the viewpoint of miniaturization of the optical communication device 11, the shorter the distance, the more preferable it is. The beam splitter 31 and the optical absorber 32 are held by a holding member such as a frame in the above-mentioned specific positional relationship. In this manner, the optical communication device 11 includes the beam splitter 31 and the optical absorber 32. In the beam splitter unit 3, the optical absorber 32 is disposed on one of the two optical paths split by the beam splitter 31, that is, on the optical path of the unnecessary light to be attenuated. As a result, in the optical communication device 11, the optical path of the signal light is split by the beam splitter 31. The configuration of the optical absorber 32 will be described later.

[0019] The adjustment device 4 includes a wedge prism 41 rotatably arranged on the optical path of the signal light, a branching section 42 arranged on the optical path of the signal light to branch the signal light, a split photodiode 43 that receives light branched from the signal light (also called "branched light"), and a control section 44 that controls the actuator of the wedge prism 41 so that the difference detected by the split photodiode 43 is reduced.

[0020] The wedge prism 41 is a set of two pairs (four pieces) of wedge prisms. The splitter 42 is, for example, a cube-shaped beam splitter, and splits the signal light in a direction perpendicular to the traveling direction of the signal light. The control unit 44 is, for example, a CPU (Central Processing Unit). The adjustment device 4 further includes an actuator that rotates the wedge prism 41. The actuator is, for example, a voice coil motor.

[0021] Each pair (two pieces) of wedge prisms 41 rotates in a counter direction relative to each other, thereby controlling the component of a specific direction (for example, the direction of one axis and the direction of the other axis in the vertical coordinate system) perpendicular to the optical axis OA1 of the signal light after separation by the beam splitter 31. The voice coil motor has a small and lightweight configuration, and rotates each wedge prism to a precise position at high speed.

[0022] [Configuration of optical absorber] The configuration of the light absorber 32 according to this embodiment is shown in FIG 2. As shown in FIG 2, the light absorber 32 has a housing 321 and a light absorbing member 331.

[0023] The housing 321 is a hollow, approximately rectangular parallelepiped that is composed of a bottom plate, a top plate, and wall plates that surround the four sides between them. The housing 321 has heat conductivity overall and is made of, for example, aluminum. An opening is provided in one wall plate of the housing 321, and an opening tube portion 322 is disposed in the opening. The opening tube portion 322 is an approximately cylindrical member, and therefore the opening shape of the opening tube portion 322 is circular. The opening diameter of the opening tube portion 322 may be an appropriate size according to the beam diameter of the unwanted light to be targeted. One end of the opening tube portion 322 protrudes into the housing 321. The protruding height of one end of the opening tube portion 322 from the inner surface of the wall plate of the housing 321 is, for example, 1.1 times or more the opening diameter of the opening tube portion 322.

[0024] Light absorbing member 331 is a plate-like member having a circular opening 332, and at least its surface facing housing 321 is made of a light absorbing material. In this manner, light absorbing member 331 has light absorption properties at least in the portion facing housing 321. Light absorbing member 331 may be made of, for example, a light absorbing material, and may have light absorption properties as a whole.

[0025] The light absorbing member 331 is disposed at a position where the optical axis OA2 passes through the center of the opening 332 of the light absorbing member 331. In addition, in the direction along the optical axis OA2, the light absorbing member 331 may be disposed so as not to be too close to the opening tube portion 322, for example, so that the distance to the outer end of the opening tube portion 322 is about 2 to 4 times the opening diameter of the opening tube portion 322. The diameter of the opening 332 of the light absorbing member 331 can be appropriately determined from the viewpoint of blocking return light from the opening tube portion 322, which will be described later, and may be, for example, equal to or larger than the opening diameter of the opening tube portion 322.

[0026] Three plate-like members, namely, frame plate 323, first reflecting plate 327, and second reflecting plate 328, are arranged inside housing 321. All of these plate-like members stand vertically from the bottom plate to the top plate of housing 321. All of these plate-like members are made of aluminum, just like housing 321.

[0027] In plan view, frame plate 323 extends from one corner of the bottom plate in a direction oblique to the wall plate in which opening tube portion 322 is disposed. Frame plate 323 has a rectangular opening, in which a rectangular light-absorbing glass block 324 is fixed. Light-absorbing glass block 324 is bonded and fixed without any gaps to the opening of frame plate 323 with a thermally conductive adhesive (for example, an adhesive in which a thermally conductive filler is dispersed).

[0028] The light-absorbing glass block 324 is a glass member having a characteristic of absorbing incident unwanted light. For example, the light-absorbing glass block 324 has an anti-reflection film on the surface on the side where the unwanted light is incident. The light-absorbing glass block 324 may be made of glass having an appropriate material according to the signal light. For example, when the signal light is infrared light, the light-absorbing glass block 324 may be made of glass having an absorption band in the C-band. Examples of such light-absorbing glass blocks 324 include SCHOTT (registered trademark) KG5, and ISK171, ISK167, and ISK153 manufactured by Isuzu Precision Glass Co., Ltd.

[0029] Light-absorbing glass block 324 is disposed at a position facing opening cylindrical portion 322 in the direction along optical axis OA2. Light-absorbing glass block 324 is disposed parallel to frame plate 323, and therefore light-absorbing glass block 324 is also disposed at an angle to optical axis OA. In this manner, the incident surface (incident surface) of light-absorbing glass block 324 extends at an angle to the optical axis OA2 of the incident light. The angle α that the incident surface of light-absorbing glass block 324 makes with the wall plate having the opening is, for example, 30°.

[0030] When the incident direction of the unwanted light to the light-absorbing glass block 324 is taken as a reference, the frame plate 323 divides the inside of the housing 321 into a space in front of the light-absorbing glass block 324 and a space behind the light-absorbing glass block 324. The front space is a space from which reflected light exits when the unwanted light enters the light-absorbing glass block 324, and the rear space is a space from which transmitted light exits when the unwanted light enters the light-absorbing glass block 324. Of the spaces inside the housing 321, the space behind the light-absorbing glass block 324 from which the transmitted light exits is also referred to as a "first cavity" (reference number 325), and the space in front of the light-absorbing glass block 324 from which the reflected light exits is also referred to as a "second cavity" (reference number 326).

[0031] The first reflecting plate 327 is a plate-like member that reflects light transmitted through the light-absorbing glass block 324, and is disposed in the first cavity 325. The first reflecting plate 327 is disposed at a position facing the light-absorbing glass block 324 in a direction along the optical axis OA2. In addition, the first reflecting plate 327 extends in a diagonal direction with respect to the wall plate on which the opening cylinder portion 322 is disposed, and further extends in a diagonal direction with respect to the frame plate 323, when viewed from above. The angle β that the first reflecting plate 327 makes with respect to the frame plate 323 is, for example, 30°. Therefore, the angle (α+β) that the incident surface of the first reflecting plate 327 makes with respect to the wall plate on which the opening cylinder portion 322 is disposed is, for example, 60°. In this way, the incident surface of the first reflecting plate 327 extends diagonally with respect to the optical axis OA2 of the light incident on the light-absorbing glass block 324.

[0032] The second reflecting plate 328 is a plate-like member that reflects light reflected by the light-absorbing glass block 324. The second reflecting plate 328 extends in a direction along the optical axis OA2 in a plan view, and is connected to an end of the frame plate 323 and the rear wall plate of the housing 321. That is, the second reflecting plate 328, together with the frame plate 323, divides the internal space of the housing 321 into two. In this manner, the first cavity 325 and the second cavity 326 are separated by the second reflecting plate 328. In the second cavity 326, there is no surface parallel to the surface of the light-absorbing glass block 324, and all of the planes of the inner surfaces of the second cavity 326 are oblique to the surface of the light-absorbing glass block 324.

[0033] The surfaces inside the housing 321 other than the light absorbing glass block 324 are covered with a light absorbing layer that has light absorbing properties for both transmitted light and reflected light of the light absorbing glass block 324. The reflectance of unnecessary light on the inner surface of the housing 321 is preferably low from the viewpoint of suppressing return of unnecessary light (emission from the light absorber 32 to the outside), and is preferably 1% or less, for example. Examples of the light absorbing layer include a sheet-like light absorbing material attached to the inner surface and a coating film of a light absorbing component. The light absorbing material or light absorbing component can be appropriately determined depending on the type of light beam (signal light). For example, when the light beam is infrared, a material that absorbs near infrared rays can be used as the light absorbing material or light absorbing component.

[0034] [Attenuation of unwanted light in the optical absorber] The unwanted light that has passed through the tubular opening portion 322 reaches the light absorbing glass block 324, where part of it is absorbed by the light absorbing glass block 324, part of it transmits through the light absorbing glass block 324, and part of it is reflected by the light absorbing glass block 324. The light absorbing glass block 324 absorbs part of the unwanted light and attenuates it by converting it to heat. Hereinafter, the component of the unwanted light that has transmitted through the light absorbing glass block 324 is also referred to as "unwanted transmitted light", and the component of the unwanted light that has been reflected by the light absorbing glass block 324 is also referred to as "unwanted reflected light".

[0035] The behavior of the unwanted transmitted light in the light absorber 32 shown in Fig. 2 will be described with reference to Fig. 3. In the figure, the traveling direction of the unwanted (transmitted) light at the beam center is indicated by an arrow. For example, as shown in Fig. 3, the unwanted transmitted light is reflected by the surface of the first reflector 327, then by the second reflector 328, by the rear wall panel of the housing 321, and repeatedly reflected by the tapered cavity formed between the side wall panel of the housing and the rear surface of the first reflector 327. Thus, in the first cavity 325, the light transmitted through the light absorbing glass block 324 is multiple-reflected.

[0036] In this specification, "multiple reflections" means that light is reflected two or more times on different surfaces, and when the direction of travel at the center of the light beam is represented by an arrow, the arrow successively reaches two or more inner surface portions of the housing 321.

[0037] The behavior of unwanted reflected light in the light absorber 32 shown in Fig. 2 will be described with reference to Fig. 4. The arrows in the figure indicate the traveling direction of the unwanted (reflected) light at the beam center. For example, as shown in Fig. 4, the unwanted reflected light is reflected multiple times by the surfaces of the front, side, and rear wall panels of the housing 321 and the surface of the second reflector 328. In this way, the light reflected by the light absorbing glass block 324 is also reflected multiple times in the second cavity 326.

[0038] As described above, the surfaces inside the housing 321 other than the light absorbing glass block 324 are covered with a light absorbing layer. Therefore, each time the unwanted transmitted light and the unwanted reflected light are reflected, a part of them is absorbed by the light absorbing layer and converted into heat, and is attenuated. In this way, in both the first cavity 325 and the second cavity 326, the unwanted light is attenuated each time it is reflected.

[0039] In addition, a part of the opening tube portion 322 protrudes inward of the housing 321. Therefore, even if unwanted reflected light is reflected toward the opening tube portion 322 due to multiple reflections in the second cavity 326, the outer peripheral surface of the opening tube portion 322 attenuates the unwanted reflected light and reflects it toward the inside of the second cavity 326.

[0040] In this way, the unwanted light that reaches the inside of the light absorber 32 is first absorbed and attenuated by the light absorbing glass block 324, the unwanted transmitted light is multiple-reflected in the first cavity 325, and the unwanted reflected light is multiple-reflected in the second cavity 326, and each of these unwanted lights is attenuated with each reflection. Therefore, the unwanted light introduced into the inside of the light absorber 32 is substantially attenuated and disappears.

[0041] A part of the unwanted light may be emitted to the outside from the housing 321 through the opening tube portion 322 due to multiple reflections. When the unwanted light emitted in this way is blocked by the light absorbing member 331, it is absorbed by the light absorbing member 331. Therefore, the amount of unwanted light returning from inside the optical absorber 32 to the outside (hereinafter also referred to as "return light") is extremely small, and can be substantially ignored as noise in optical wireless communication.

[0042] [Major effects] In this embodiment, the optical absorber 32 includes a light absorbing glass block 324 arranged on the optical path of the incident unwanted light, a first cavity 325 behind the light absorbing glass block 324 for multiple reflection of the transmitted light through the light absorbing glass block 324, and a second cavity 326 in front of the light absorbing glass block 324 for multiple reflection of the reflected light from the light absorbing glass block 324. Therefore, in optical communications, for example, optical communications using infrared rays, the optical absorber 32 can be a high-performance optical attenuator in which substantial light leakage that would affect the optical communications does not occur.

[0043] In this embodiment, the light-absorbing glass block 324 is disposed so that its surface is inclined with respect to the optical axis of the incident unwanted light. More specifically, in this embodiment, the light-absorbing glass block 324 is attached so that its surface is inclined with respect to the optical axis at an angle of 30°. Therefore, reflected light of the unwanted light that reaches the light-absorbing glass block 324 is likely to be introduced into the second cavity 326 and is likely to be multiple-reflected in the second cavity 326.

[0044] In this embodiment, a first reflector 327 is further disposed behind the light-absorbing glass block 324. The first reflector 327 is disposed at an angle to the optical axis OA2, so that no reflection of the reflected transmitted light toward the cylindrical opening portion 322 occurs. This is advantageous from the viewpoint of suppressing the occurrence of return light. The first reflector 327 is disposed at an even further angle to the light-absorbing glass block 324. This is advantageous from the viewpoint of introducing the unwanted transmitted light deeper into the first cavity 325 and promoting multiple reflections.

[0045] In this embodiment, the first cavity 325 and the second cavity 326 are separated by a second reflector 328. Therefore, the unwanted transmitted light that is multiple-reflected in the first cavity 325 does not reach the second cavity 326, and the unwanted reflected light that is multiple-part-reflected in the second cavity 326 does not reach the first cavity 325. Since the unwanted light is prevented from going around between the cavities in this way, the occurrence of unintended unwanted light escaping (return light) is further suppressed. Therefore, it is more advantageous in terms of reliably attenuating each unwanted light in each cavity, and is also more advantageous in suppressing the occurrence of return light.

[0046] The light absorber 32 has a two-stage structure consisting of a housing 321 that absorbs unnecessary light and attenuates it by multiple reflection, and a light absorbing member 331 that absorbs the return light from the housing 321. Therefore, the slight return light from the housing 321 is absorbed by the light absorbing member 331 and further reduced.

[0047] In this embodiment, the inner surface of the housing 321 is formed of a light absorbing layer. Therefore, the unwanted light is absorbed and attenuated with each reflection due to multiple reflections in the first cavity 325 or the second cavity 326. In this way, the light absorber of this embodiment repeatedly reflects the unwanted light on the anti-reflection wall and attenuates it each time, thereby further suppressing the generation of return light.

[0048] In this embodiment, the housing 321, the first reflector 327, the second reflector 328, and the frame plate 323 are all made of aluminum. The light absorbing glass block 324 is fixed to the frame plate via a heat conductive adhesive. Therefore, the heat generated by the absorption of unnecessary light is transmitted by these aluminum members and adhesive, and can be dissipated from the housing 321 by radiative heat dissipation (blackbody radiation). This prevents local overheating of the housing 321, and also allows proper heat dissipation from the housing 321. In particular, since heat is transmitted between members that are in direct contact with each other, it is expected that the above-mentioned prevention of local overheating and proper heat dissipation can be achieved even in a vacuum.

[0049] In the present embodiment, a light guide path in which the distance between opposing wall surfaces gradually decreases is formed in first cavity 325 between first reflecting plate 327 and the side wall of housing 321. Therefore, unwanted light introduced into the light guide path by multiple reflection is multiple-reflected in the light guide path, and is easily attenuated and disappeared (converted into heat) in the light guide path.

[0050] The light absorber 32 of this embodiment can attenuate and substantially eliminate incident unwanted light with a simple configuration including a housing 321 having a simple rectangular shape in a plan view, a frame plate 323 for fixing a light-absorbing glass block 324, and two reflecting plates 327 and 328. There are many possible configurations, including the shape of the housing, for causing multiple reflection of unwanted light in each of the first cavity 325 behind the light-absorbing glass block 324 and the second cavity 326 in front of the light-absorbing glass block 324. This embodiment is one example of the present invention, and it is clear from the above description that the present invention can include many such configurations.

[0051] [Verification experiment example] The unnecessary light to the optical absorber and the return light from the optical absorber were measured with a power meter, and the degree of attenuation of the return light by the optical absorber was measured. For the optical absorber, the optical absorber of this embodiment shown in FIG. 2 was used, and a commercially available optical diffuser was used as a comparative example. The commercially available optical diffuser has a mechanism for attenuating the introduced light by reflecting it. FIG. 5 is a schematic diagram showing a configuration for measuring the return light from the optical absorber shown in FIG. 2.

[0052] The intensity of the unwanted light entering the optical absorber is measured by placing a power meter 50 on the optical axis of the unwanted light instead of the optical absorber 32, and the return light from the optical absorber is measured by placing the power meter 50 on the optical path of the return light reflected by the beam splitter 31.

[0053] As a result, it was confirmed that the optical absorber of this embodiment attenuated the unwanted light by -70 dB. In contrast, the attenuation of the unwanted light in the above-mentioned commercially available optical diffuser was -45 dB. Thus, the optical absorber of this embodiment has a higher optical attenuation effect than the commercially available optical diffuser that attenuates light only by reflection.

[0054] In this way, the optical absorber 32 of this embodiment is configured in a closed space, can be constructed in a limited space, and can substantially prevent the occurrence of return light. Therefore, it is possible to attenuate the return light of unwanted light to -70 dB or less. Therefore, an optical communication device equipped with the optical absorber 32 of this embodiment can transmit and receive signal light that is substantially free of noise, enabling stable optical communication.

[0055] [Embodiment 2] Another embodiment of the present invention will be described below. For ease of explanation, the same reference numerals are given to components having the same functions as those described in the above embodiment, and the explanations thereof will not be repeated. FIG. 6 is a diagram showing a schematic configuration of an optical communication device according to a second embodiment of the present invention. The optical communication device 12 shown in FIG. 6 is a receiver in optical wireless communication. The optical communication device 12 is basically configured in the same way as the optical communication device 11 described above, except that it is configured to receive an optical beam.

[0056] The optical communication device 12 includes an adjustment device 5 , a beam splitter unit 6 and an optical fiber 2 .

[0057] The adjustment device 5 has a branching section 51, a wedge prism 52, a segmented photodiode 53, and a control section 54, and is similar to the adjustment device 4 of the first embodiment except that a light beam is transmitted from the branching section 51.

[0058] The beam splitter unit 6 includes a beam splitter 61 and an optical absorber 62, and the optical absorber 62 has a light absorbing member 631. The beam splitter 61 splits the light beam into signal light and unwanted light, reflects the signal light toward the optical fiber 2, and causes the unwanted light to travel straight. The optical absorber 62 is disposed on the optical axis OA2 of the unwanted light traveling straight.

[0059] The optical fiber 2 is disposed at a position where it receives the signal light from the beam splitter 61 .

[0060] In the optical communication device 12 as well, unnecessary light other than the signal light is attenuated and essentially eliminated by the optical absorber 62. In the optical communication device 12 as well, the return light from the optical absorber 62 can be attenuated to -70 dB or less. Thus, in a receiving device in optical wireless communication as well, the optical absorber 62 of this embodiment can sufficiently remove noise in the signal light to enable stable optical communication.

[0061] [Modifications] In the present invention, the optical absorber and the beam splitter do not have to be integrally configured. For example, the optical absorber and the beam splitter may be directly fixed to the casing of the optical communication device separately so as to have a specific desired positional relationship.

[0062] In the present invention, when the beam splitter splits the light beam into the signal light and two or more unwanted lights, two or more optical absorbers may be disposed relative to the beam splitter according to the amount of the separated unwanted lights.

[0063] In the present invention, the shape of the housing of the optical absorber can be appropriately determined within a range in which unwanted light can be multiple-reflected in each of the first cavity and the second cavity. For example, the shape of the inside of the housing when viewed from above may be a polygon other than a rectangle.

[0064] Furthermore, the shape of the light-absorbing glass block when viewed in a plan view does not have to be rectangular as long as it is capable of absorbing unnecessary light. For example, the light-absorbing glass block may have a circular or annular shape when viewed in a plan view. Such circular or annular light-absorbing glass blocks are preferable from the viewpoint of heat uniformity in the light-absorbing glass block and weight reduction due to a small surplus portion relative to the diameter of the incident beam. Light-absorbing glass blocks having a rectangular shape when viewed in a plan view are preferable from the viewpoints of relatively low cost and improved heat dissipation from the light-absorbing glass block due to a larger contact area with the frame plate.

[0065] At least one of the opposing main surfaces of the light-absorbing glass block may be roughened like ground glass (or frosted glass). The light-absorbing glass block does not have to be one piece, but may be two or more pieces. When a plurality of light-absorbing glass blocks are used, the arrangement of the light-absorbing glass blocks may be devised so that attenuation by absorption or reflection is possible.

[0066] In the present invention, the optical absorber may further include a cooling structure for suppressing heat generation due to absorption of unnecessary light. Examples of the cooling structure include a heat dissipation fin structure, an air-cooled fan, a water-cooled structure, and a heat pipe. A heat pipe is a compact structure and is not dependent on the environment, so it is suitable as a cooling structure for an optical absorber mounted on an optical communication device in an artificial satellite.

[0067] In the present invention, the first cavity and the second cavity may be in communication with each other, which makes it possible to further utilize the second cavity or the first cavity for attenuating transmitted or reflected light.

[0068] Furthermore, in the present invention, the transmitted light of the light-absorbing glass block may be reflected by a surface of a wall plate of the first cavity within a range in which the unwanted light is multiple-reflected in the first cavity. In this case, the wall surface is preferably a surface that is oblique to the optical axis OA2 of the unwanted light. The reflections on the first reflecting plate and the wall surface described above are both aspects of a reflecting surface that reflects the transmitted light of the light-absorbing glass block in the first cavity.

[0069] In addition, in the present invention, the arrangement of the reflector is not limited to the above-mentioned embodiment. For example, the second reflector may be arranged obliquely with respect to the optical axis OA2 to form a tapered cavity in the second cavity. This configuration is expected to have the effect of further attenuating unnecessary light in the second cavity.

[0070] The optical absorber and beam splitter according to the present invention can be applied to various optical devices in which unwanted light can become noise, in addition to optical communication devices, and can achieve the same effects as those of this embodiment in such optical devices.

[0071] 〔summary〕 Radio wave communication technologies such as 5G have achieved dramatic increases in speed, but it can also be said that the communication speed has reached its theoretical upper limit. Possible methods for further increasing communication speeds include increasing the number of bands and increasing the frequency. However, radio wave bands are in short supply internationally, making it difficult to increase the number of bands. In addition, increasing the frequency increases the directionality of radio waves, which can make diffraction, an advantage of radio waves, ineffective.

[0072] In 5G, the price of faster speeds is increased directionality, and radio waves cannot travel inside or behind buildings, making it difficult to ensure communication quality. This requires the installation of many base stations. Relaying signals from base stations is also difficult for the same reason. Even if cables are laid, the increased construction costs due to the large number of base stations and the problem of laying the cables are difficult.

[0073] In contrast, optical communications has a high degree of directivity but does not propagate over a wide area like radio waves. This gives it an advantage in terms of security. At this stage, optical communications from moving objects is difficult, but optical communications is thought to be suitable for connections from fixed base stations to relay points. Optical communications is also thought to be advantageous as a technology to complement radio waves in space communications between artificial satellites. Therefore, as the speed and distance of optical wireless communications increases, even more precise optical technology is thought to be required.

[0074] According to the above-mentioned embodiment, it is possible to attenuate unnecessary light applicable to optical communications. The present invention, which has such an effect, is expected to contribute to the achievement of, for example, Goal 9 "Build resilient infrastructure, promote inclusive and sustainable industrialization and innovation" of the Sustainable Development Goals (SDGs) proposed by the United Nations.

[0075] A first aspect of the present invention is an optical absorber (32) including a light absorbing glass block that absorbs incident light, a first cavity (325) in which light transmitted through the light absorbing glass block (324) undergoes multiple reflections, and a second cavity (326) in which light reflected by the light absorbing glass block undergoes multiple reflections. According to the first aspect, it is possible to provide a technique for removing unnecessary light separated from an optical beam, which is applicable to sufficient noise reduction in optical wireless communication.

[0076] In the second aspect of the present invention, the incident surface of the light-absorbing glass block in the first aspect extends obliquely with respect to the optical axis of the incident light. The second aspect is even more effective in terms of promoting attenuation of unwanted light by multiple reflections.

[0077] The third aspect of the present invention is the first or second aspect, in which the first cavity includes a reflecting surface (first reflector 327) that reflects light transmitted through the light-absorbing glass block, and the reflecting surface extends obliquely with respect to the optical axis of the light incident on the light-absorbing glass block. The third aspect is even more effective in terms of promoting attenuation of unwanted light by multiple reflections in the first cavity.

[0078] In a fourth embodiment of the present invention, in any one of the first to third embodiments, the first and second cavities that communicate with each other are separated by a reflector (second reflector 328) that reflects light that has passed through the light-absorbing glass block and light that has been reflected by the light-absorbing glass block. The fourth embodiment is even more effective in terms of preventing unwanted light from leaking between the first and second cavities.

[0079] A fifth aspect of the present invention is any one of the first to third aspects, in which the inner surfaces of the first and second cavities are covered with a light absorbing layer that absorbs light transmitted through the light absorbing glass block and light reflected by the light absorbing glass block. The fifth aspect is even more effective in terms of promoting attenuation of unnecessary light by anti-reflection.

[0080] A sixth aspect of the present invention is a beam splitter (31) including an optical absorber according to any one of the first to fifth aspects, the optical absorber being disposed in any one of two or more optical paths branched by the beam splitter. As with the first aspect, the sixth aspect can provide a technique for removing unnecessary light separated from an optical beam, which is applicable to sufficient noise reduction in optical wireless communication.

[0081] A seventh aspect of the present invention is an optical communication device (11) including the beam splitter of the sixth aspect, which uses the beam splitter to branch an optical path of a signal light. According to the seventh aspect, in optical wireless communication, unnecessary light that becomes noise and is separated from the light beam can be sufficiently removed.

[0082] The present invention is not limited to the above-described embodiments, and various modifications are possible within the scope of the claims. Embodiments obtained by appropriately combining the technical means disclosed in the different embodiments are also included in the technical scope of the present invention. [Explanation of symbols]

[0083] 11, 12 Optical communication devices 2. Optical Fiber 3, 6 Beam splitter unit 4, 5 Adjustment device 31, 61 Beam splitter 32, 62 Light absorber 331, 631 Light absorbing material 41, 52 Wedge Prism 42, 51 Branch 43, 53 Segmented photodiode 44, 54 Control section 50 Power Meter 321 Case 322 Open cylinder part 323 323 324 Light-absorbing glass block 325 First cavity 326 Second cavity 327 First reflective plate (reflective surface) 328 Second reflector (reflector) 332 Aperture OA1, OA2 optical axis

Claims

1. A light absorber comprising: a light-absorbing glass block that absorbs incident light; a first cavity in which light transmitted through the light-absorbing glass block is multiple-reflected; and a second cavity in which light reflected by the light-absorbing glass block is multiple-reflected.

2. 2. The light absorber according to claim 1, wherein the entrance surface of said light absorbing glass block extends obliquely with respect to the optical axis of the incident light.

3. the first cavity includes a reflective surface that reflects light transmitted through the light-absorbing glass block; 2. The light absorber of claim 1, wherein said reflecting surface extends obliquely with respect to an optical axis of light incident on said light absorbing glass block.

4. 2. The light absorber of claim 1, wherein the first cavity and the second cavity, which are in communication with each other, are separated by a reflector that reflects light transmitted through the light-absorbing glass block and light reflected by the light-absorbing glass block.

5. 2. The light absorber of claim 1, wherein inner surfaces of the first cavity and the second cavity are covered with a light absorbing layer that absorbs light transmitted through and reflected by the light absorbing glass block.

6. A beam splitter comprising an optical absorber according to any one of claims 1 to 5, wherein the optical absorber is disposed in any one of two or more optical paths branched by the beam splitter.

7. An optical communication device comprising the beam splitter according to claim 6, An optical communication device using the beam splitter to split the optical path of a signal light.

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