Receiving optical system, wireless optical receiving device, and wireless optical communication system

Abstract

To provide: a receiving optical system, capable of, while increasing the aperture of the receiving optical system, reducing an increase in size of a radio optical communication device while suppressing a decrease in sensitivity; a transmitting optical system transmitting to the receiving optical system; and a radio optical receiving / transmitting device and radio optical communication system including the same.SOLUTION: A receiving optical system receiving signal light includes a main mirror (L3) and a sub mirror (L2). The main mirror (L3) reflects the received signal light to the objective side of the receiving optical system, and the sub mirror (L2) reflects the signal light reflected by the main mirror (L3) to the image surface side of the receiving optical system. In the receiving optical system, the following relational expression is satisfied: 0.15<Ds / Dm<0.80...(1). In the expression, Ds represents an optical effective diameter of the sub mirror (L2) and Dm represents an optical effective diameter of the main mirror (L3).SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a receiving optical system and a wireless optical receiver including the same. The present invention also relates to a transmitting optical system and a wireless optical transmitter including the same. The present invention further relates to a wireless optical communication system including such a wireless optical receiver and a wireless optical transmitter.

Background Art

[0002] Communication technologies using radio waves such as the fifth generation have achieved dramatic high speeds, but their communication speeds are approaching their limits. To achieve further high speeds, there are methods such as increasing the number of bands and increasing the frequency. However, the frequency bands available for wireless communication are internationally in short supply, and it is difficult to increase the number of bands. In addition, when the frequency is increased, the directivity of radio waves increases. For this reason, radio waves do not reach inside buildings or in the shade, and communication quality cannot be ensured, so a large number of base stations must be installed. Therefore, wireless optical communication (also referred to as free space optical communication) has attracted attention. In wireless optical communication, optical axis alignment between the transmitting side and the receiving side is cited as one of the problems. The optical axis alignment becomes more difficult as the communication distance increases.

[0003] For example, Patent Document 1 discloses an optical space communication device in which an optical system is separated into a collimating optical system and a separating optical system, and the separating optical system can be driven at high speed with a light force. According to this, it is said that optical axis misalignment correction can be performed at low cost and at high speed.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] On the other hand, in wireless optical communication, as a method for facilitating optical axis alignment between the transmission side and the reception side, a method of increasing the aperture of the optical system on the reception side can be considered. This is because increasing the aperture of the optical system on the reception side makes it easier to detect misalignment of the optical axis on the reception side and then perform optical axis alignment. However, increasing the aperture of the optical system increases the size of the optical system, and consequently increases the size of the wireless optical communication device (wireless optical reception device).

[0006] One aspect of the present invention has been made in view of the above problems, and an object thereof is to realize a receiving optical system that reduces an increase in the size of a wireless optical communication device while suppressing a decrease in sensitivity while increasing the aperture of the receiving optical system, a transmitting optical system that transmits to the receiving optical system, and a wireless optical transceiver and a wireless optical communication system including these.

Means for Solving the Problems

[0007] A receiving optical system according to one aspect of the present invention is a receiving optical system that receives signal light, includes a main mirror and a sub-mirror, the main mirror reflects the received signal light to the object side of the receiving optical system, the sub-mirror reflects the signal light reflected by the main mirror to the image plane side of the receiving optical system, and satisfies the following relational expression. 0.15 < Ds / Dm < 0.80 ···(1) However,[[]] Ds: Optical effective diameter of the sub-mirror Dm: Optical effective diameter of the main mirror

[0008] Moreover, a transmitting optical system according to one aspect of the present invention is a transmitting optical system that transmits signal light, and includes an optical element that converts the intensity profile of the signal light in a direction orthogonal to the optical axis into a ring-shaped or top-hat-shaped intensity profile.

[0009] In addition, a wireless optical reception device according to one aspect of the present invention includes the above-described receiving optical system.

[0010] Moreover, a wireless optical transmission device according to one aspect of the present invention includes the above-described transmitting optical system.

[0011] In addition, a wireless optical communication system according to an aspect of the present invention includes the above-described wireless optical transmission device and wireless optical reception device.

Advantages of the Invention

[0012] According to an aspect of the present invention, it is possible to realize a receiving optical system that reduces an increase in the size of a wireless optical communication device while suppressing a decrease in sensitivity while increasing the aperture of the receiving optical system, a transmitting optical system that transmits to the receiving optical system, and a wireless optical transceiver and a wireless optical communication system including these.

Brief Description of the Drawings

[0013]

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Mode for Carrying Out the Invention

[0014] 〔Wireless Communication System〕 A wireless communication system according to an embodiment of the present invention will be described. The wireless optical communication system according to this embodiment is a communication system that transmits and receives information by light, and includes a wireless optical transmission device that transmits signal light and a wireless optical reception device that receives the signal light. Note that the signal light used in the wireless optical communication system according to this embodiment may be visible light with a wavelength of 360 nm or more and less than 760 nm, or infrared light with a wavelength of 760 nm or more and 1 mm or less. If the wavelength of the signal light is 1000 nm or more and 1650 nm or less (more preferably 1290 nm or more and 1610 nm or less), it becomes easy to generate signal light using a light source for optical communication such as a YAG laser.

[0015] 〔Receiving Optical System of Wireless Optical Reception Device〕 Hereinafter, the receiving optical system included in the wireless optical receiver according to an embodiment of the present invention will be described. Note that the receiving optical system described below is one aspect of the receiving optical system according to the present invention, and the receiving optical system according to the present invention is not limited to the following aspects. 1. Optical configuration The receiving optical system according to this embodiment is a reflecting telescope type optical system including a lens having a positive refractive power (for example, a convex lens), a secondary mirror, a primary mirror, an optical axis adjustment lens group, and a condenser lens. Note that the lens having a positive refractive power, the optical axis adjustment lens group, and the condenser lens can be omitted. The lens having a positive refractive power is the lens on which the signal light transmitted from the transmitting optical system is first incident, and the signal light is collected by the lens having a positive refractive power onto the primary mirror. The primary mirror is the first reflecting mirror, and reflects the received signal light to the object side of the receiving optical system. The secondary mirror is the second reflecting mirror, and reflects the signal light reflected by the primary mirror to the image plane side of the receiving optical system. The reflected signal light passes through the optical axis adjustment lens group and is condensed by the condenser lens. The lens having a positive refractive power, the secondary mirror, the primary mirror, the optical axis adjustment lens group, and the condenser lens are arranged on one optical axis in this order from the object side.

[0016] The optical axis adjustment lens group has a negative refractive power. The optical axis adjustment lens group is preferably arranged on the image plane side of the primary mirror and on the object side of the condenser lens. The optical axis adjustment lens group is a lens group for bringing the optical axis of the signal light passing through the receiving optical system closer to the optical axis of the receiving optical system when the optical axis of the signal light does not coincide with the optical axis of the receiving optical system. When the deviation of the optical axis of the signal light is detected, the optical axis adjustment lens group is driven in a direction orthogonal to the optical axis of the receiving optical system so that the optical axis of the signal light approaches the optical axis of the receiving optical system. Further, the receiving optical system includes a condenser lens on the image plane side of the optical axis adjustment lens group. The condenser lens is a lens for focusing the signal light on the incident end of, for example, an optical fiber.

[0017] The effective diameter of the lens having a positive refractive power is larger than the effective diameter of the primary mirror. By providing a lens having a positive refractive power with an effective diameter larger than that of the primary mirror on the object side of the primary mirror, it is possible to realize a larger aperture of the telescope without increasing the size of the primary mirror.

[0018] The signal light incident on the lens with positive refractive power has its central portion blocked by the secondary mirror, and the rest reaches the primary mirror. The signal light that reaches the primary mirror is reflected toward the secondary mirror on the object side of the receiving optical system and is reflected again by the secondary mirror in the direction of the primary mirror (the image plane side of the receiving optical system). The signal light reflected by the secondary mirror passes through the primary mirror and the optical axis adjustment lens group, and is focused on the incident end of the optical fiber by the condenser lens. The signal light incident on the incident end of the optical fiber is guided by the optical fiber to a receiving circuit including a light receiving element (photosensor), converted into an information signal, and processed. Alternatively, without using an optical fiber, a light receiving element may be disposed at the focal position of the condenser lens to directly convert the signal light into an electrical signal and process it in the receiving circuit. Note that the information acquired by the receiving circuit may be signalized by the transmitting circuit and transmitted toward the next wireless optical receiving device. In this specification, when an optical fiber is disposed, the incident end of the optical fiber becomes the image plane, and when a light receiving element is disposed without using an optical fiber, the light receiving element becomes the image plane.

[0019] Of the signal light incident on the lens with positive refractive power, the central portion of the signal light is blocked by the secondary mirror. Therefore, the central portion of the lens with positive refractive power where the signal light is blocked by the secondary mirror may have an aperture. Also, since the central portion of the primary mirror is the portion through which the signal light reflected by the secondary mirror passes, it is preferable that the primary mirror has an aperture through which the signal light passes in the central portion, or that the central portion of the primary mirror is a transmissive lens having no reflecting function. When the central portion of the primary mirror is a transmissive lens having no reflecting function, an optical design considering the refractive index of the lens can be made.

[0020] The primary mirror is a concave reflecting mirror, which may be a reflecting mirror having a reflecting surface on the back surface of the lens (the surface on the image plane side of the optical path of the incident optical system), or a reflecting mirror having a reflecting surface on the front surface of the lens (the surface on the object side of the optical path of the incident optical system). In the case of a reflecting mirror having a reflecting surface on the back surface of the lens, the refraction of the lens of the primary mirror can be included in the optical design, increasing the options for optical design. Also, the reflecting surface can be protected by the lens. In the case of a reflecting mirror (a simple concave mirror) having a reflecting surface on the front surface of the lens, it is not necessary to consider the influence of aberration due to the refraction of the lens. The reflecting surface formed on the front surface of the lens can be protected by coating or the like.

[0021] The secondary mirror is a convex reflecting mirror, which may be a reflecting mirror having a reflecting surface on the back surface of the lens (the surface on the image plane side of the optical path of the incident optical system), or a reflecting mirror having a reflecting surface on the front surface of the lens (the surface on the object side of the optical path of the incident optical system). In this embodiment, the signal light reflected by the secondary mirror passes through the optical axis adjustment lens group described later and is focused by the condenser lens. However, the optical axis adjustment lens group may not be used. Also, instead of using the condenser lens, a method of directly focusing by the secondary mirror may be employed.

[0022] Also, the signal light may be received only by the primary mirror without providing a lens having a positive refractive power. In this case, the lens configuration is simplified, and the aberration due to the refraction of the lens having a positive refractive power can be reduced. When a lens having a positive refractive power is not provided, if the primary mirror and the secondary mirror are configured as simple concave mirror reflecting mirrors (Cassegrain system), the overall configuration becomes simpler, and the influence of aberration due to refraction can be eliminated.

[0023] Note that in this specification, the "lens group" means a collection of one or more lenses. The lens group may be composed of a single lens, or may be composed of a plurality of single lenses. For example, the lens group may include a cemented lens in which a plurality of single lenses are integrated without an air gap, or may include a composite lens in which a single lens and resin are integrated without an air gap.

[0024] In the receiving optical system according to the present embodiment, the signal light is first reflected by the primary mirror in the object direction of the receiving optical system and then reflected again by the secondary mirror in the image plane direction of the receiving optical system. Therefore, while increasing the aperture of the receiving optical system (the aperture of the lens having positive refractive power), it is possible to reduce the increase in the size of the receiving optical system and thus the size of the wireless optical communication device. For this reason, for example, it becomes easy to carry in and install equipment at a desired location and align the optical axes.

[0025] 2. Optical Characteristics In the receiving optical system according to the present embodiment, when the optical effective diameter of the primary mirror is Dm and the optical effective diameter of the secondary mirror is Ds, 0.15 < Ds / Dm < 0.80 ··· (1) satisfies the following relational expression. Note that the lower limit value is preferably 0.20, more preferably 0.25. Also, the upper limit value is preferably 0.70, more preferably 0.60. Note that the combination of these lower limit and upper limit values is arbitrary. When the value of Ds / Dm is 0.15 or less, the primary mirror becomes relatively small with respect to the secondary mirror, so that a large amount of signal light is blocked by the secondary mirror, leading to a decrease in the sensitivity of the receiving optical system, which is not preferable. On the other hand, when the value of Ds / Dm is 0.80 or more, the primary mirror becomes relatively large with respect to the secondary mirror, so the sensitivity is improved, but the back focus (BF) becomes long, resulting in an increase in the overall length of the receiving optical system, and the receiving optical system and thus the wireless optical communication device become large, which is not preferable. By setting the value of Ds / Dm within the range of the above relational expression (1), it is possible to suppress a decrease in the sensitivity of the receiving optical system and to reduce an increase in the size of the receiving optical system and thus the wireless optical communication device. Note that when receiving signal light with a lens having positive refractive power, the effective diameter of the lens having positive refractive power becomes the optical effective diameter of the primary mirror, and when receiving signal light only with the primary mirror, the effective diameter of the primary mirror becomes the optical effective diameter of the primary mirror. The optical effective diameter means the maximum diameter with the center of the optical axis through which the lens light beam passes.

[0026] Further, in the present embodiment, when the overall optical length of the receiving optical system at infinity focus is L and the focal length of the receiving optical system (the focal length of the receiving optical system at infinity focus) is f, 0.10 < L / f < 0.80 ··· (2) It is preferable to satisfy the relational expression. The lower limit value is preferably 0.20, more preferably 0.25. The upper limit value is preferably 0.70, more preferably 0.60. The combination of these lower limit value and upper limit value is arbitrary. When the value of L / f is 0.10 or less, it is preferable for miniaturization of the receiving optical system, but the reflecting part composed of the main mirror, the sub-mirror, etc. becomes too small and the light receiving area becomes small, leading to a decrease in the sensitivity of the receiving optical system, so it is not preferable. On the other hand, when the value of L / f is 0.80 or more, the overall length of the receiving optical system becomes long, so the receiving optical system, and thus the wireless optical communication device, becomes large, so it is not preferable. By setting the value of L / f within the range of the relational expression (2), it is possible to suppress the decrease in the sensitivity of the receiving optical system as much as possible and to minimize the increase in the size of the receiving optical system, and thus the wireless optical communication device. Here, the overall optical length of the receiving optical system is the length from the object side surface of the lens arranged on the most object side to the image plane, and for the length from the image side surface of the lens arranged on the most image side to the image plane, it is the air equivalent length.

[0027] The receiving optical system of this embodiment includes an optical axis adjustment lens group. The optical axis adjustment lens group is provided on the image side of the sub-mirror and on the object side of the condenser lens, and is movable in a direction perpendicular to the optical axis of the signal light. The optical axis adjustment lens group has a negative refractive power.

[0028] Since the wireless optical receiver is far from the wireless optical transmitter that emits the signal light, even if the emission direction of the signal light from the wireless optical transmitter is slightly deviated, it is largely deviated in the wireless optical receiver. The optical axis adjustment lens group is a lens group for bringing the optical axis of the deviated signal light closer to the optical axis of the receiving optical system.

[0029] The optical axis adjustment lens group is controlled as follows, for example. First, the signal light is split by a beam splitter at an arbitrary position in the receiving optical system. The split signal light is detected by a position detector equipped with optical sensors such as a plurality of photodiodes. By analyzing which optical sensor detected how much signal light, it is possible to determine in which direction and by how much the optical axis of the signal light is deviated. Based on this information, the optical axis adjustment lens group is driven in a two-dimensional direction orthogonal to the optical axis using an actuator so that the optical axis of the signal light approaches the optical axis of the receiving optical system. By controlling in this way, the optical axis of the signal light can be made to approach the optical axis of the receiving optical system.

[0030] Alternatively, an acceleration sensor may be disposed at an arbitrary position in the receiving optical system or the wireless optical receiving device. Then, the amount of movement of the receiving optical system is calculated from the magnitude and direction of the acceleration detected by the acceleration sensor, and the optical axis adjustment lens group may be driven using an actuator so that the optical axis of the signal light approaches the optical axis of the receiving optical system. By bringing the optical axis of the signal light closer to the optical axis of the receiving optical system, the amount of light that can be received at the incident end of the optical fiber increases, so the reception sensitivity can be improved.

[0031] In this embodiment, assuming that the focal length of the optical axis adjustment lens group is f’’ and the focal length of the receiving optical system is f, 0.01 < |f’’ / f| < 2.00 ··· (3) It is preferable to satisfy the following relational expression. The lower limit value is preferably 0.20, more preferably 0.25. Also, the upper limit value is preferably 1.50, more preferably 1.20. Note that the combination of these lower limit values and upper limit values is arbitrary.

[0032] When the value of |f'' / f| is 0.01 or less, it is preferable in that the stroke required for optical axis adjustment can be reduced. However, it is not preferable because the back focus becomes too long and the overall length of the receiving optical system increases. On the other hand, when the value of |f'' / f| is 2.00 or more, it is preferable from the viewpoint of reducing the overall length of the receiving optical system. However, the optical axis adjustment stroke becomes too large, leading to an increase in the size of the actuator for driving the optical axis adjustment lens group, so it is not preferable. By setting the value of |f'' / f| within the range of the above relational expression (3), the stroke required for optical axis adjustment can be made as small as possible, and the effect of minimizing the increase in the size of the receiving optical system and thus the wireless optical communication device can be achieved.

[0033] [Embodiment of the receiving optical system] Hereinafter, an embodiment of the receiving optical system included in the wireless optical receiving device according to an embodiment of the present invention will be described.

[0034] [Example 1] Next, Example 1 of the present invention will be described below. FIG. 1 is a lens cross-sectional view of the receiving optical system according to Example 1. The receiving optical system of Example 1 includes, in order from the object side, a lens L1 having a positive refractive power, a secondary mirror (reflecting mirror) L2, a primary mirror (reflecting mirror) L3, an optical axis adjustment lens group L4, and a condenser lens L5. An aperture stop S is disposed between the secondary mirror L2 and the primary mirror L3. A cover glass CG is disposed on the image plane side of the condenser lens L5. Table 1 is a table of surface data of the receiving optical system of Example 1. In the following tables, the unit of length is all "mm", and the unit of the angle of view is all "°".

[0035] In Table 1, "No." represents the order of the lens surfaces counted from the object side, "R" represents the radius of curvature of the lens surface, "D" represents the interval on the optical axis of the lens surface, "Nd" represents the refractive index with respect to the d-line (wavelength λ = 587.56 nm), and "ABV" represents the Abbe number with respect to the d-line. Also, "STOP" in "No." represents the aperture stop. The aperture stop here refers to the aperture stop that defines the light beam diameter of the receiving optical system, that is, the aperture stop that defines the Fno of the receiving optical system.

[0036] [Table 1] No. R D Nd ABV 1 295.4252 9.0000 1.48749 70.44 2 1091.6854 79.0000 1.00000 3 -224.1973 11.0000 1.58913 61.25 4 -278.7805 -11.0000 -1.58913 61.25 5 -224.1973 -71.6384 -1.00000 6 -344.0568 -3.0000 -1.48749 70.44 7 -186.3418 3.0000 1.48749 70.44 8 -344.0568 0.0000 1.00000 9 STOP 0.0000 71.6385 1.00000 10 -224.1973 11.0000 1.58913 61.25 11 -278.7805 1.3000 1.00000 12 1404.5415 3.0000 1.51680 64.20 13 -170.9826 1.4500 1.00000 14 1639.9052 2.6996 1.75520 27.53 15 -36.3014 0.0000 1.00000 16 -36.3014 1.2000 1.74400 44.72 17 34.1037 31.1345 1.00000 18 399.9891 3.2161 1.48749 70.44 19 -63.5679 42.0000 1.00000 20 0.0000 2.0000 1.51633 64.14 21 0.0000 0.9999 1.00000

[0037] Table 2 shows the specifications of the receiving optical system of Example 1. In this specification table, in the said specification table, "F" represents the focal length of the receiving optical system at infinity focus, "Fno" represents the F-number, "W" represents the half field angle, Dm represents the effective optical diameter of the primary mirror, Ds represents the effective optical diameter of the secondary mirror, L represents the total optical length (air equivalent length) of the receiving optical system, and f’’ represents the focal length of the optical axis adjustment lens group respectively.

[0038] [Table 2] F 670.00 Fno 7.27 (Effective FNO) W 1.2139 Dm 92.41 Ds 32.38 L 187.32 f’’ -47.6

[0039] From Table 2, the numerical values of the relational expressions (1) to (3) in Example 1 are calculated as follows. That is, relational expression (1): Ds / Dm = 0.35, relational expression (2): L / f = 0.280, relational expression (3): |f’’ / f| = 0.07.

[0040] Also, Fig. 2 is a diagram showing the longitudinal aberration of the receiving optical system of Example 1 at infinity focus. In order from the left side toward the drawing, "LOMGITUDINAL SPHERICAL ABER." indicates the spherical aberration (mm), "ASTIGMATIC FIELD CURVES" indicates the astigmatic aberration (mm), and "DISTORTION" indicates the distortion aberration (%).

[0041] In the diagram representing the spherical aberration, the vertical axis is the ratio to the open F value, and the horizontal axis is the defocus (mm). In the diagram representing the spherical aberration, the solid line indicates the longitudinal aberration at the d-line (wavelength λ = 587.56 nm), and the dotted line indicates the longitudinal aberration at the C-line (wavelength λ = 656.28 nm).

[0042] In the diagram representing the astigmatic aberration, the vertical axis is the half field angle (°), and the horizontal axis is the defocus (mm). In the diagram representing the astigmatic aberration, the solid line indicates the sagittal image plane (S) with respect to the d-line, and the dotted line indicates the meridional image plane (T) with respect to the d-line.

[0043] In the figure showing the distortion aberration, the vertical axis represents the semi-field angle (°) and the horizontal axis represents %.

[0044] FIG. 3 is a diagram showing the lateral aberration of the receiving optical system of Example 1 before and after optical axis adjustment using the optical axis adjustment lens group. In the figure, "RELATIVE FIELD HEIGHT" indicates the ratio of the field angle, and "(0.371)" in the upper row 0 " is the lateral aberration at 0.3 field, and "(0.000)" in the lower row 0 " is the lateral aberration on the axis. Also, "Y-FAN" is the tangential direction, the vertical axis is the aberration amount, and the horizontal axis is the pupil ratio. "X-FAN" is the sagittal direction, the vertical axis is the aberration amount, and the horizontal axis is the pupil ratio. The solid line indicates the d-line and the dotted line indicates the g-line.

[0045] [Example 2] Next, Example 2 of the present invention will be described below. FIG. 4 is a lens cross-sectional view of the receiving optical system according to Example 2. The receiving optical system of Example 2 includes, in order from the object side, a lens L1 having a positive refractive power, a secondary mirror (reflective mirror) L2, a primary mirror (reflective mirror) L3, and an optical axis adjustment lens group L4. An aperture stop S is disposed between the secondary mirror L2 and the primary mirror L3. A cover glass CG is disposed on the image plane side of the optical axis adjustment lens group L4. Table 3 is a table of surface data of the receiving optical system of Example 2. Table 4 shows the specification table of the receiving optical system of Example 2. The meanings of the symbols in Tables 3 and 4 are as described in Tables 1 and 2.

[0046] [Table 3] No. R D Nd ABV 1 265.2704 9.0000 1.48749 70.44 2 737.1031 79.0000 1.00000 3 -225.1211 11.0000 1.58913 61.25 4 -283.0904 -11.0000 -1.58913 61.25 5 -225.1211 -72.9357 -1.00000 6 -334.2517 -3.0000 -1.48749 70.44 7 -179.8273 3.0000 1.48749 70.44 8 -334.2517 0.0000 1.00000 9 STOP 0.0000 72.9358 1.00000 10 -225.1211 11.0000 1.58913 61.25 11 -283.0904 22.7350 1.00000 12 45.1681 2.5010 1.75520 27.53 13 382.9183 1.2000 1.74400 44.72 14 29.2822 59.5639 1.00000 15 0 2.0000 1.51633 64.14 16 0 1.0001 1.00000

[0047] [Table 4] F 670.00 Fno 7.37 (Effective FNO) W 1.2135 Dm 92.54 Ds 32.87 L 187.32 f’’ -127.9

[0048] From Table 4, the numerical values of the relational expressions (1) to (3) in Example 2 are calculated as follows. That is, relational expression (1): Ds / Dm = 0.36, relational expression (2): L / f = 0.280, relational expression (3): |f’’ / f| = 0.19

[0049] Figure 5 is a diagram showing the longitudinal aberration at infinity focus of the receiving optical system of Example 2. The meanings of the symbols in the figure are as described in Figure 2

[0050] Figure 6 is a diagram showing the lateral aberration diagrams of the receiving optical system of Example 2 before and after optical axis adjustment using the optical axis adjustment lens group. The meanings of the symbols in the figure are as described in Figure 3

[0051] [Example 3] Next, Example 3 of the present invention will be described below. FIG. 7 is a lens cross-sectional view of the receiving optical system according to Example 3. The receiving optical system of Example 3 includes, in order from the object side, a lens L1 having a positive refractive power, a secondary mirror (reflective mirror) L2, and a primary mirror (reflective mirror) L3. An aperture stop S is disposed between the secondary mirror L2 and the primary mirror L3. A cover glass CG is disposed on the image plane side of the primary mirror L3. Table 5 is a table of surface data of the receiving optical system of Example 3. Table 6 shows the specifications table of the receiving optical system of Example 3. The meanings of the symbols in Tables 5 and 6 are as described in Tables 1 and 2. In Example 3, the secondary mirror L2 also serves as an optical axis adjustment lens. By using the secondary mirror L2 as an optical axis adjustment lens, the optical axis can be adjusted without providing a dedicated optical axis adjustment lens group.

[0052] [Table 5] No. R D Nd ABV 1 245.4700 9.0000 1.48749 70.44 2 682.2044 96.9798 1.00000 3 -229.7706 11.0000 1.58913 61.25 4 -315.8073 -11.0000 -1.58913 61.25 5 -229.7706 -90.9797 -1.00000 6 -205.7677 -3.0000 -1.48749 70.44 7 -150.9993 3.0000 1.48749 70.44 8 -205.7677 0.0000 1.00000 9 STOP 0.0000 90.9797 1.00000 10 -229.7706 11.0000 1.58913 61.25 11 -315.8073 89.1660 1.00000 12 0.0000 2.0000 1.51633 64.14 13 0.0000 0.9997 1.00000

[0053] [Table 6] F 700.00 Fno 7.13 (Effective FNO) W 1.1879 Dm 96.09 Ds 29.18 L 208.46 f’’ -

[0054] From Table 6, the numerical values of relational expressions (1) and (2) in Example 3 are calculated as follows. That is, relational expression (1): Ds / Dm = 0.30, relational expression (2): L / f = 0.298. In Example 3, since the optical axis adjustment lens is a convex mirror, the focal length f’’ and relational expression (3) are not calculated.

[0055] FIG. 8 is a diagram showing the longitudinal aberration at infinity focus of the receiving optical system of Example 3. The meanings of the symbols in the figure are as described in FIG. 2.

[0056] FIG. 9 is a diagram showing the lateral aberration diagrams of the receiving optical system of Example 3 before optical axis adjustment and after optical axis adjustment using the optical axis adjustment lens (secondary mirror L2). The meanings of the symbols in the figure are as described in FIG. 3.

[0057] [Example 4] Next, Example 4 of the present invention will be described below. FIG. 10 is a lens cross-sectional view of the receiving optical system according to Example 4. The receiving optical system of Example 4 has, in order from the object side, a secondary mirror (reflecting mirror) L2 and a primary mirror (reflecting mirror) L3. An aperture stop S is disposed between the secondary mirror L2 and the primary mirror L3. Table 7 is a table of surface data of the receiving optical system of Example 4. Also, Table 8 shows a table of specifications of the receiving optical system of Example 4. The meanings of the symbols in Tables 7 and 8 are as described in Tables 1 and 2.

[0058] [Table 7] No. R D Nd ABV 1 0.0000 308.3333 1.00000 2STOP -925.0000 -296.7708 -1.00000 3 -616.6667 358.3334 1.00000 4 0.0000 0.0000 1.00000

[0059] [Table 8] F 1000.00000 Fno 6.12 W 2.00000 Dm 185.00 Ds 86.94 L 369.90 f’’ -67.6

[0060] From Table 8, the numerical values of relational expressions (1) to (3) in Example 4 are calculated as follows. That is, relational expression (1): Ds / Dm = 0.47, relational expression (2): L / f = 0.370, relational expression (3): |f’’ / f| = 0.07.

[0061] FIG. 11 is a diagram showing the longitudinal aberration at infinity focus of the receiving optical system of Example 4. The meanings of the symbols in the figure are as described in FIG. 2.

[0062] [Example 5] Next, Example 5 of the present invention will be described below. FIG. 12 is a lens cross-sectional view of the receiving optical system according to Example 5. The receiving optical system of Example 5 has, in order from the object side, a secondary mirror (reflector) L2, a primary mirror (reflector) L3, and an optical axis adjustment lens group L4. An aperture stop S is disposed between the primary mirror L3 and the optical axis adjustment lens group L4. Table 9 is a table of surface data of the receiving optical system of Example 5. Also, Table 10 shows the specification table of the receiving optical system of Example 5. The meanings of the symbols in Tables 9 and 10 are as described in Tables 1 and 2.

[0063] [Table 9] No. R D Nd ABV 1 0.0000 300.0000 1.00000 2 -650.0000 -210.0000 -1.00000 3STOP -450.0000 220.0000 1.00000 4 -100.0000 4.0000 1.51680 64.17 5 -140.0000 13.2133 1.00000 6 0.00000 0.0000 1.00000

[0064] [Table 10] F 686.3607 Fno 5.71 (effective FNO) W 2.0000 Dm 172.36 Ds 53.33 L 327.21 f’’ -701.1

[0065] From Table 10, the numerical values of relational expressions (1) to (3) in Example 5 are calculated as follows. That is, relational expression (1): Ds / Dm = 0.31, relational expression (2): L / f = 0.477, relational expression (3): |f’’ / f| = 1.02

[0066] FIG. 13 is a diagram showing the longitudinal aberration at infinity focus of the receiving optical system of Example 5. The meanings of the symbols in the figure are as described in FIG. 2

[0067] FIG. 14 is a diagram showing the lateral aberration diagrams of the receiving optical system of Example 5 before and after optical axis adjustment using the optical axis adjustment lens group. The meanings of the symbols in the figure are as described in FIG. 3

[0068] [Transmitting Optical System of Wireless Optical Transmitter] Next, the transmitting optical system included in the wireless optical transmitter according to an embodiment of the present invention will be described. Note that the transmitting optical system described below is one aspect of the transmitting optical system according to the present invention, and the transmitting optical system according to the present invention is not limited to the following aspects

[0069] 1. Optical Configuration The transmission optical system according to this embodiment is a transmission optical system that transmits signal light used for wireless optical communication to a reception optical system. The transmission optical system includes a light source that generates signal light and an optical element. The optical element is disposed within the transmission optical system and converts the intensity profile in a direction orthogonal to the optical axis in the signal light. Specifically, the optical element converts the intensity profile of the signal light in the direction orthogonal to the optical axis into a ring-shaped or top-hat-shaped intensity profile.

[0070] As shown in FIG. 15, the intensity profile of the signal light emitted from the light source has a so-called Gaussian distribution in the direction orthogonal to the optical axis L, where the intensity is strongest at the portion of the optical axis L and gradually decreases toward the outer peripheral direction. In FIG. 15, the horizontal axis represents the direction orthogonal to the optical axis L, and the vertical axis represents the relative intensity of the signal light. The same applies to the subsequent figures. The optical element converts such an intensity profile of the signal light into, for example, a ring-shaped intensity profile as shown in FIG. 16, where the intensity is stronger on the outer peripheral side than at the central portion of the signal light. Or the optical element converts it into a top-hat-shaped intensity profile with a generally smooth intensity distribution as shown in FIG. 17.

[0071] In other words, it is preferable that the optical element converts the intensity profile in the direction orthogonal to the optical axis L in the signal light, for example, into an intensity profile in which the maximum value of the intensity exists on the outer peripheral side rather than on the optical axis L. Furthermore, it is preferably a ring shape in which the intensity of the signal light near the optical axis L is 10% or less compared to the intensity of the signal light on the outer peripheral side.

[0072] Alternatively, it is preferable that the optical element converts it into, for example, a top-hat-shaped intensity profile with a generally smooth (substantially uniform) intensity distribution. Note that the intensity distribution may not be flat as shown in FIG. 17 and may have height differences. For example, as shown in FIG. 18, it is preferable that the intensity of the signal light on the outer peripheral side is 10% or more greater than the intensity of the signal light on the optical axis L. Conversely, the intensity of the signal light on the outer peripheral side may be about 10% smaller than the intensity of the signal light on the optical axis L (not shown).

[0073] 2. Optical characteristics The transmitting optical system according to this embodiment includes an optical element that converts the intensity profile in a direction orthogonal to the optical axis of the signal light. By transmitting the signal light with the converted intensity profile, compared to the intensity profile of the Gaussian distribution, in the receiving optical system that receives the signal light transmitted from the transmitting optical system, the loss energy amount of the signal light blocked and lost by the secondary mirror can be reduced, and the signal light can be received with higher efficiency.

[0074] Examples of the optical element having the function of converting the intensity profile of the signal light include a diffractive element (DOE, Diffractive Optical Element), a diffuser, an aspherical lens, an axicon lens, etc. As for their arrangements, in the transmitting optical system, it is preferable to have a diffuser and an axicon lens on the light source side of the primary mirror. Also, such an optical element may be used in the receiving optical system. In the receiving optical system, it is preferable to have a diffuser and an axicon lens on the receiving side of the primary mirror. Also, it is preferable that the DOE and the aspherical lens are inside the optical system.

[0075] As described above, the receiving optical system according to the above embodiment can reduce the increase in the size of the wireless optical communication device while suppressing the decrease in sensitivity while increasing the aperture. Also, the transmitting optical system according to the above embodiment can transmit the signal light that can be received with high efficiency in the receiving optical system. A wireless optical communication system including such a wireless optical receiving device and a wireless optical transmitting device makes it easy to carry in and install the wireless optical receiving device and the wireless optical transmitting device at a desired location and align the optical axes.

[0076] 〔Summary〕 The receiving optical system according to this embodiment is a receiving optical system that receives signal light, includes a primary mirror and a secondary mirror, the primary mirror reflects the received signal light to the object side of the receiving optical system, the secondary mirror reflects the signal light reflected by the primary mirror to the image side of the receiving optical system, and satisfies the following relational expression. 0.15 < Ds / Dm < 0.80 ··· (1) However, Ds: Optical effective diameter of the secondary mirror Dm: Effective optical diameter of the primary mirror

[0077] With such a configuration, it is possible to suppress the decrease in the sensitivity of the receiving optical system as much as possible, and to obtain the effect of minimizing the increase in the size of the receiving optical system and thus the wireless optical communication device.

[0078] In addition, the receiving optical system according to this embodiment satisfies the following relational expression. 0.10 < L / f < 0.80 ··· (2) However, L: Overall optical length of the receiving optical system when focused at infinity f: Focal length of the receiving optical system

[0079] With such a configuration, it is possible to suppress the decrease in the sensitivity of the receiving optical system as much as possible, and to obtain the effect of minimizing the increase in the size of the receiving optical system and thus the wireless optical communication device.

[0080] In addition, the receiving optical system according to this embodiment includes an optical axis adjustment lens group having a negative refractive power that is movable in a direction perpendicular to the optical axis of the signal light on the image plane side of the primary mirror and on the object side of the condenser lens for the signal light, and satisfies the following relational expression. 0.01 < |f’’ / f| < 2.00 ··· (3) However, f’’: Focal length of the optical axis adjustment lens group f: Focal length of the receiving optical system

[0081] With such a configuration, it is possible to minimize the stroke required for optical axis adjustment, and to obtain the effect of minimizing the increase in the size of the receiving optical system and thus the wireless optical communication device.

[0082] In addition, the receiving optical system according to this embodiment includes a lens having a positive refractive power with a larger effective diameter than the primary mirror on the object side of the primary mirror.

[0083] With such a configuration, it is possible to increase the aperture of the telephoto lens without increasing the size of the main mirror.

[0084] The wireless optical reception device according to this embodiment has the above-described reception optical system.

[0085] With such a configuration, it is possible to reduce the increase in the size of the wireless optical reception device while increasing the aperture of the reception optical system.

[0086] In addition, the transmission optical system according to this embodiment is a transmission optical system that transmits signal light, and includes an optical element that converts the intensity profile of the signal light in a direction orthogonal to the optical axis into a ring-shaped or top-hat-shaped intensity profile.

[0087] With such a configuration, in the reception optical system that receives the signal light transmitted from the transmission optical system, compared with the intensity profile of the Gaussian distribution, it is possible to reduce the amount of loss energy of the signal light blocked and lost by the secondary mirror, and it is possible to receive the signal light with higher efficiency.

[0088] In addition, the optical element of the transmission optical system according to this embodiment is at least one of a diffraction element, a diffuser, an aspherical lens, and an axicon lens.

[0089] By using such an optical element, it is possible to convert the intensity profile of the signal light in a direction orthogonal to the optical axis into a ring-shaped or top-hat-shaped intensity profile.

[0090] The wireless optical transmission device according to this embodiment has the above-described transmission optical system.

[0091] With such a configuration, it is possible to reduce the increase in the size of the wireless optical transmission device while increasing the aperture of the transmission optical system.

[0092] The wireless optical communication system according to this embodiment has the above-described wireless optical reception device and the above-described wireless optical transmission device.

[0093] With such a configuration, it is possible to realize a wireless optical communication system that reduces the increase in the size of the wireless optical communication device while increasing the aperture of the receiving optical system. In addition, such a wireless optical communication system can be easily carried into a desired location, installed, and the optical axes can be aligned.

[0094] 〔Supplementary Notes〕 The present invention is not limited to the above-described embodiments, and various modifications are possible within the scope shown in the claims. Other embodiments obtained by appropriately combining the technical means disclosed in the above-described embodiments are also included in the technical scope of the present invention.

Explanation of Reference Numerals

[0095] L1 Lens having a positive refractive power L2 Secondary mirror (reflecting mirror) L3 Primary mirror (reflecting mirror) L4 Optical axis adjustment lens group L5 Condensing lens S Aperture stop CG Cover glass

Claims

1. A receiving optical system for receiving signal light, comprising: a primary mirror and a secondary mirror; the primary mirror reflects the received signal light to the object side of the receiving optical system; the secondary mirror reflects the signal light reflected by the primary mirror to the image plane side of the receiving optical system; the primary mirror and the secondary mirror satisfy the following relational expression (1): 0.15 < Ds / Dm < 0.80... (1) a group of optical axis adjustment lenses having a negative refractive power that can be moved in a direction perpendicular to the optical axis of the signal light, on the image plane side of the primary mirror and on the object side of the condenser lens for the signal light; the group of optical axis adjustment lenses satisfies the following relational expression (3): a receiving optical system. 0.01 < |f'' / f| < 2.00... (3) However, Ds: the effective optical diameter of the secondary mirror Dm: the effective optical diameter of the primary mirror f'': the focal length of the group of optical axis adjustment lenses f: the focal length of the receiving optical system

2. A receiving optical system for receiving signal light, comprising: a primary mirror and a secondary mirror; the primary mirror reflects the received signal light to the object side of the receiving optical system; the secondary mirror reflects the signal light reflected by the primary mirror to the image plane side of the receiving optical system; the primary mirror and the secondary mirror satisfy the following relational expression (1): 0.15 < Ds / Dm < 0.80... (1) a lens having a positive refractive power with an effective diameter larger than that of the primary mirror, on the object side of the primary mirror; a receiving optical system.

3. A wireless optical receiver having the receiving optical system according to Claim 1 or 2.

4. A wireless optical communication system having the wireless optical receiver according to Claim 3 and a wireless optical transmitter.

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