JP2013148626A - Optical antenna for optical radio communication apparatus using infrared laser beam - Google Patents

Abstract

PROBLEM TO BE SOLVED: To inexpensively provide an optical system for optical radio communication that can provide an imaging performance of diffraction limit in an infrared wavelength region of wavelengths 1.0 μm to 1.6 μm in four times wider or more viewing field than the conventional art, by combination of commercial lenses.SOLUTION: An optical antenna for a Kepler-type optical radio communication apparatus has an object lens facing toward the exterior space and an eyepiece facing toward the inside of the apparatus, and includes an object lens B1 and an eyepiece B3 which are lenses selected from commercial lenses whose optical aberration is corrected for general imaging. The ratio of curvature radiuses of a front face and a back face of the eyepiece is 0.8 to 1.2. A plano-concave lens serving as a correction lens B2 is arranged between the object lens B1 and the eyepiece B3 so as to direct the concave surface of the plano-concave lens toward an object lens side. The position of the correction lens B2 is in a range of 0.25 to 0.9 times the focal distance of the object lens B1, and corrects the optical aberrations of the object lens B1 and the eyepiece B3 simultaneously.

Description

  In optical wireless communication, in order to put into practical use a new optical wireless communication system having a transmission capacity exceeding 10 Gbps / s, it is assumed that it is connected to an existing single mode fiber (hereinafter also referred to as “SMF”). An optical wireless communication system has been developed in which light transmitted through free space is directly coupled to an optical fiber.

  The main optical system of the optical wireless communication device utilizing the optical fiber technology is that the spatial light outside the device and the space inside the device with respect to laser light having a wavelength of 1.31 μm or 1.55 μm used in optical fiber communication A beam expander for converting the beam diameter of light and an imaging lens for a fiber coupler for efficiently introducing spatial light in the apparatus into an optical fiber. A beam expander, which is an optical antenna, includes an objective lens that collects spatial light from the outside of the apparatus and an eyepiece that converts the collected light beam into a small-diameter parallel beam. At this time, in a generally used beam expander, a convex lens having a long focal length is used as an objective lens, and a concave lens having a short focal length is used as an eyepiece. This configuration is the same as a Galileo telescope.

  However, in an optical antenna (beam expander) for optical wireless communication, it is necessary to place a mirror drive mechanism at the position of the exit pupil to reduce the displacement of the beam inside the apparatus. For this purpose, the same configuration as a Kepler-type telescope using a convex lens as an eyepiece is required. A beam expander with a Kepler configuration is not available on the market, so it has a diffraction-limited optical characteristic necessary for image formation of laser light in the above wavelength band and a good transmittance with low optical power loss. There is a need for technology to design and inexpensively manufacture pandas.

  By the way, in the objective lens of a microscope objective or a telescope, ray tracing is performed from the incident surface toward the reference surface of the focal point as an aberration-free standard, and an optical path difference (OPD) is obtained for a plurality of incident rays, and the maximum value is obtained. Is used, and the square root (rms value) of the optical path difference is obtained for the entire incident light beam and the square root (rms value) thereof is λ / 13 or less. In the following description, as a diffraction-limited imaging condition for laser light, the root mean square of the optical path difference over the entire incident light beam (this value is referred to as total wavefront aberration) is λ / 10 (rms) or less. The condition of the wavefront aberration is assumed.

  The beam expander's objective lens and eyepiece used in optical wireless communication devices have a wide range from 0.2 degrees to 4.0 degrees in order to allow fluctuations in the incident angle of the laser beam due to vibration or thermal expansion of the apparatus. It is necessary to maintain diffraction limited performance with respect to the incident angle.

  In other words, the beam expander used in the ultrahigh-speed optical fiber direct-coupled optical wireless communication device has an appropriate achromatic characteristic corresponding to wavelength multiplexing and a large field of view that can cope with fluctuations in the incident angle of a laser beam flying in space. A corner is required.

  Even in optical wireless communication, downsizing of the optical system is demanded from the request of easy installation and reduction of manufacturing cost. A beam expander used in optical wireless communication is a so-called afocal imaging optical system. However, in order to shorten the axial length and reduce the size, a configuration in which a concave lens is inserted between an objective lens and an eyepiece (see Patent Document 1). (Refer to Patent Document 2) in which an optical path is folded using a reflecting mirror.

  However, in the configuration disclosed in Patent Document 1, only the imaging condition related to the focal length is considered, and the optical condition for achieving the diffraction-limited imaging performance over the wide field of view is considered. Not. In addition, in the configuration disclosed in Patent Document 2, since the field of view is as narrow as 1 mrad (0.058 degrees), the entire optical communication device is installed on some drive mechanism to change the incident angle of the laser beam. Must be within this field of view. In addition to these documents, it has been considered that conventional laser beam expanders are problematic only in the imaging performance of on-axis rays, and it is impossible to obtain good off-axis imaging performance.

Japanese Patent Laid-Open No. 8-68936 Japanese Patent Laid-Open No. 5-72477

  Unlike camera lenses in the visible light region, optical systems with diffraction-limited imaging performance in the infrared wavelength band are not common, and there are few commercial products. It was necessary to assemble the device together with a high-precision optical system holding mechanism using the manufactured lens and reflecting mirror. For this reason, mass production effects cannot be expected, and the apparatus is often very expensive (a million or more per unit). Further, in the conventional condensing optical system, the field of view that can guarantee the diffraction-limited imaging performance is relatively narrow at about 0.2 degrees when the magnification is 10 times, and there are many cases where installation adjustment of the apparatus is difficult.

  Therefore, in the present invention, two optical systems that can provide diffraction-limited imaging performance in the infrared wavelength range of 1.0 μm to 1.6 μm in a wider field of view that is four times or more that of the prior art for optical wireless communication. It is an object of the present invention to provide a method for manufacturing at low cost by combining three commercially available lenses.

  An optical antenna according to a first aspect of the present invention is an optical antenna for a Kepler-type optical wireless communication apparatus having an objective lens directed to an external space and an eyepiece lens directed to the inside of the apparatus. The lens and the eyepiece lens are selected from commercially available lenses whose aberrations are corrected for general imaging, and the ratio of the curvature radius of the front surface of the eyepiece lens to the curvature radius of the rear surface is 0.8 to 1.2. FIG. 3 shows an optical antenna according to claim 1.

  An optical antenna according to a second aspect of the present invention is the optical antenna according to the first aspect, wherein one plano-concave lens has a concave surface on the objective lens side as a correction lens between the objective lens and the eyepiece. The position of the correction lens is within a range of 0.25 to 0.9 times the focal length of the objective lens, and is a position for simultaneously correcting optical aberrations of the objective lens and the eyepiece. Features. FIG. 4 shows an optical antenna according to claim 2.

  An optical antenna according to a third aspect of the present invention is the optical antenna according to the second aspect, wherein the objective lens is a convex assembled lens composed of a plurality of two or more lenses. FIG. 5 shows an optical antenna according to a third aspect.

  An optical antenna according to a fourth aspect of the present invention is the optical antenna according to the second aspect, wherein the eyepiece is a combined lens composed of a plurality of two or more lenses. FIG. 6 shows an optical antenna according to a fourth aspect.

  An optical antenna according to a fifth aspect of the present invention is the optical antenna according to the second aspect, wherein the objective lens is a combined lens including a convex doublet lens and another lens, and the eyepiece is a convex doublet. It is a combined lens composed of a lens and another lens.

  By using the optical antenna according to the present invention, a laser beam transmitted through a space of a plurality of wavelengths (for example, 1.31 μm and 1.55 μm) and a single mode fiber while the mirror driving mechanism is operated. In the future, even if ultra-high-capacity optical wireless communication exceeding 100 Tbps using all of this wavelength band becomes necessary, the optical system can be provided at low cost. In addition, since the optical antenna according to the present invention has a wide viewing angle, it is possible to omit mechanical parts such as a motor and an angle sensor that have been conventionally used for controlling the direction of the entire optical wireless communication apparatus, and the size is small. Thus, a more practical optical wireless communication device can be realized. Furthermore, the application range of the present invention is not limited to optical wireless communication, but can be applied to the optical measurement field such as a laser radar (rider) or a laser distance measuring device that requires a diffraction-limited optical system.

It is an optical system block diagram of an optical wireless communication apparatus. 1 is an overall configuration diagram of an optical wireless communication apparatus. It is a lens shape and optical path diagram of the optical antenna according to claim 1. It is a lens shape and optical path diagram of the optical antenna according to claim 2. It is a lens shape and optical path diagram of the optical antenna according to claim 3. It is a lens shape and optical path diagram of the optical antenna according to claim 4.

  FIG. 1 shows an optical system configuration diagram of an optical wireless communication apparatus. The entire optical system has three parts, that is, a beam expander 5 that is an optical antenna that receives light from the external space and relays it into the device and radiates light from the device to the external space. The mirror driving mechanism 4 and a fiber coupler unit incorporating a tracking sensor. In FIG. 1, a light ray incident along the optical axis of the beam expander indicated by a one-dot chain line is indicated by a solid line, and a light ray incident at a large arrival angle is indicated by a dotted line.

  In order to drive the mirror by the error signal of the tracking sensor and make the incident condition to the fiber coupler unit constant, it is necessary to make the exit pupil of the optical antenna coincide with the position of the mirror driving mechanism. Therefore, it is necessary to adopt the Kepler type shown in FIG. 1 instead of the Galileo type as the configuration of the beam expander.

  In order to reduce the degradation of the coupling efficiency between the spatial light and the SMF to 1 dB or less, the aberration of the entire optical system is designed to be 1/13 (rms) or less of the wavelength. In order to satisfy such a diffraction-limited imaging condition, conventionally, an increase in aberration is suppressed by narrowing the aperture of an imaging system such as a lens (for example, NA is 0.06 or less for a single lens). I have been. Alternatively, if only the imaging performance on the axis is a problem as in the imaging lens of the fiber coupler portion shown in FIG. 1, the performance of the diffraction limit can be realized by using an aspheric lens. However, in order to reduce the size and weight of the entire optical system, there is a problem that a long-focus optical system like the former cannot be adopted, and the latter aspherical lens cannot cope with a large change in incident angle.

  If the magnification of the beam expander is M, the diameter of the light beam from the partner station is reduced to 1 / M times and the deflection angle of the beam is increased M times. Is required. Looking at the off-axis light beam indicated by the dotted line in FIG. 1, the principal ray and the optical axis are almost parallel on the objective lens side, and the principal ray is not parallel to the optical axis on the eyepiece side. It is thought that object-side telecentric imaging is performed.

  In terms of specific numerical values, in the case of the optical wireless communication device developed this time, the internal beam diameter is 2 mm, and a beam diameter of about 20 mm is necessary for use in a transmission distance of about 1 km. The magnification is about 10. Further, the NA of the objective lens and the eyepiece lens needs to be about 0.1.

  On the other hand, if an arrival angle variation of about 0.4 degrees is allowed from the installation conditions of the apparatus, when using a beam expander with a magnification of 10 times, it is necessary to compensate for a beam deflection angle of about 4 degrees. Therefore, diffraction-limited imaging characteristics are required even when the deflection angle of a beam of the same size is changed.

  As described above, the SMF coupling optical system having a mirror driving mechanism requires a diffraction-limited imaging optical system having a wide field of view and a relatively large NA (˜0.1). In the past, eyepieces that fulfilled these conditions often consisted of a large number of assembled lenses, but by using commercially available lenses and devising their arrangement, an optical system that meets the required performance can be manufactured. Became.

  FIG. 2 shows a specific example of an optical system having a tracking function using a mirror drive mechanism. In addition to the optical system of FIG. 1, the tracking sensor 11 by the 4-partition Si photodetector and the light source 9 of the beacon light of a wavelength 980nm band are increasing. In FIG. 2, beacon light is indicated by a dotted line, and signal light having a wavelength of 1.55 μm is indicated by a solid line.

  In order to minimize the components of the optical system, the tracking sensor 11 is integrated with the fiber coupler unit. A transparent glass ferrule holding the SMF is used to couple the spatial light with the fiber light, and the beacon light leaking outside the SMF is guided to the tracking sensor 11 by an obliquely cut mirror. The transmission beacon light is multiplexed with the signal light by the wavelength multiplexing coupler 7 and supplied to the fiber coupler unit.

  By multiplexing the beacon light and the signal light by the wavelength multiplexing coupler 7, the coaxiality is always maintained for the light beams having two different wavelengths.

  In order to spatially separate transmitted / received beacon light at the end face of the SMF, it is necessary to place the focal position of the fiber imaging lens 3 in the fiber coupler portion at least 1 mm forward from the end face of the SMF. On the other hand, for transmission beacon light, in order to maintain stable beacon tracking on the receiving side, it is necessary to have an appropriate beam spread of about 2 milliradians when transmitting from the optical antenna.

  In order to satisfy the above two conditions for the beacon light, the lens has a certain chromatic aberration, the 1.5 μm band signal light is parallel light, and the 0.98 μm beacon light is sufficiently large at the reception point. Convergent / divergent light that has

  In general, optical glass has a strong refractive power as the wavelength is shorter. Therefore, as shown by the dotted line in FIG. 2, the optical glass is focused once immediately after transmission from the optical antenna, and then has the necessary beam spread. The beam reaches the receiving point.

  In FIG. 2, the mirror drive mechanism has a biaxial degree of freedom, and is driven based on an error signal from the tracking sensor 11 by a 2 mm square four-divided Si photodetector to realize the tracking function of the optical wireless communication apparatus. .

  The internal loss of the entire apparatus with respect to signal light having a wavelength of 1.5 μm is 2.2 dB. This numerical value includes a theoretical coupling loss of 0.9 dB between the spatial light and the SMF, so that the reduction in loss as an optical fiber direct-coupled optical wireless device has almost reached its limit.

  Conventionally, it has been common knowledge in such high-precision optical systems to assemble specially designed and manufactured optical components together with a fine adjustment mechanism and an expensive measuring device. Despite achieving an accuracy of 1 μm or less (1 μrad for an external beam angle), no fine adjustment mechanism is used other than the focus adjustment mechanism and mirror drive mechanism of the beam expander.

  The reason why the above can be realized is as follows. (1) The SMF is an ideal point light source, and the request for axis alignment is eased by transmitting and receiving light of a plurality of wavelengths from this aperture. (2) Since the wavelength of the laser beam to be used is longer than that of visible light, even a commercially available lens can obtain good wavefront aberration characteristics. (3) The effect of wavefront aberration is reduced by reducing the spatial light beam diameter as much as possible. There is.

[Example 1] Optical antenna for beam diameter 20 mm An example in which a beam expander having an effective diameter of 20 mm and a magnification of 10 times is realized with the configuration of FIG. The first lens A1 is a doublet of Edmund Optics 45806-L with a focal length f = 100 mm, and the second lens A2 is a Thorlabs AC060-010-C with a doublet of focal length f = 10 mm. The lens interval is 103.5 mm. The curvature radius of the front surface of the second lens A2 is 9.20 mm, and the curvature radius of the rear surface is 10.35 mm, which satisfies the condition regarding the curvature radius of the eyepiece lens according to claim 1.

  With this configuration, it was found that a diffraction-limited performance was exhibited at a full angle of 0.8 ° with respect to an incident beam having a diameter of 20 mm. Table 1 shows optical aberrations of the entire optical system including the optical antenna and the fiber imaging lens. The first row shows the entire optical aberration, the rows A1 and A2 show the optical aberrations of the lenses A1 and A2 constituting the optical antenna, and the row A3 shows the aberration of the fiber imaging lens. The vertical columns indicate spherical aberration, coma, astigmatism, field curvature, field distortion, and chromatic aberration in order. From Table 1, it can be seen that the main component of the aberration is the spherical aberration of the optical antenna first lens.

  Table 1 shows the optical aberration characteristics of the two-pack 10 × optical antenna of Example 1.

  For these lenses, an optimal combination was selected from commercially available doublet lenses having anti-reflection coating in the wavelength band from the wavelength of beacon light (982/972 nm) to the wavelength of signal light (1550 nm). As this candidate, NIRII coated product of doublet for infrared communication manufactured by Edmund can be used. In addition, by specifying the C coat for the ThorLab lens, it is possible to select a product having good antireflection coating characteristics in the wavelength band of 1050 nm to 1550 nm.

  Next, Table 2 shows a list of optical antennas designed and manufactured by using the configuration of FIG. 4 among commercially available infrared coated lenses of Edomum Optics, Thorlabs, and CVI. By combining the lenses shown in Table 2, a wide-field, diffraction-limited beam expander with a magnification of 4 to 20 times was realized.

  In this table, the curvature radius of the front surface of AC064-013-C used as an eyepiece in this table is 12.42 mm, and the curvature radius of the rear surface is 13.15 mm. This lens is also a doublet for infrared laser imaging satisfying the condition relating to the radius of curvature of the eyepiece of claim 1, and good imaging performance can be obtained in a wide field of view. Further, the position of the second lens B2, which is a correction lens added in FIG. 4, is 0.25 to 0.9 times the focal length of the first lens B1, and good aberration correction characteristics are obtained. The configuration of two typical examples, e and a in Table 2, will be described below.

  Table 2 shows the relationship among the lens, antenna aperture, magnification, and effective viewing angle used in the prototype.

[Example 2] Optical antenna for beam aperture of 13 mm (row e in Table 2)
Designing and manufacturing a short-distance optical antenna with an effective aperture of 15 mm using the lens shown in row e of Table 2 with the three-lens configuration shown in FIG. 4, assuming that it is used for short-distance transmission with a distance of about 10 m. did. The magnification of the beam expander is 6.5 times. The first lens B1 is a doublet with a focal length f = 45 mm from Edmund Optics 45802-L, the second lens B2 is a correction lens, a plano-concave lens with a focal length f = -50 mm from LC1439-C from Thorlabs. The three lenses B3 are Thorlabs AC060-010-C and a doublet with a focal length of f = 10 mm.

  The distance between the first lens B1 and the second lens B2 is 22.9 mm, and the distance between the second lens B2 and the third lens B3 is 28.0 mm. The position of the correction lens B2 is determined so that the optical aberrations of the objective lens and the eyepiece lens can be corrected simultaneously, and is 0.51 times the focal length of 45 mm of the objective lens B1.

  With this configuration, the accuracy of diffraction limit was obtained over a full angle field of 2.0 ° with respect to an incident beam having a diameter of 12.9 mm. Table 3 shows optical aberrations including the optical antenna and the fiber imaging lens according to the configuration of row e in Table 2 described above. The first row shows the aberration of the entire optical system, the rows B1 to B3 show the aberration of each lens constituting the optical antenna, and the row B4 shows the aberration of the fiber imaging lens. As compared with the optical antenna having the two-lens configuration shown in Table 1, it can be seen that the second lens B2 can correct most of the spherical aberration and coma of the first lens.

  Table 3 shows the optical aberration characteristics of the triplex 6.5 × optical antenna of Example 2 (optical antenna in row e of Table 2).

[Example 3] Optical antenna for a beam diameter of 42 mm (line a in Table 2)
FIG. 5 shows the result of a beam expander with a magnification of 21 times designed using the large-diameter bonded doublet of 50 mm in diameter manufactured by Edmund Optics with the configuration of FIG. 4. The first lens B1 is 47318-L of Edmund Optics with a focal length f = 150 mm, and the second lens B2 is PLCC-15.0- of CVI. 38.6-C-1050-1600 is a plano-concave lens having a diameter of 15 mm and a focal length f = -75 mm, and the third lens B3 is a doublet of Thorlabs AC060-010-C having a focal length f = 10 mm.

  The distance between the first lens B1 and the second lens B2 is 119.4 mm, and the distance between the second lens B2 and the third lens B3 is 36.4 mm. The position of the correction lens B2 is determined so that the optical aberrations of the objective lens and the eyepiece lens are corrected simultaneously, and is 0.796 times the focal length of 45 mm of the objective lens B1.

  With this configuration, diffraction limit accuracy was obtained over an incident angle of 42 mm over a full-angle field of 0.6 °. Table 4 shows optical aberrations including the optical antenna and the fiber imaging lens according to the configuration of row a in Table 2. The first row shows the aberration of the entire optical system, the rows B1 to B3 show the aberration of each lens constituting the optical antenna, and the row B4 shows the aberration of the fiber imaging lens. Although most of the coma and astigmatism of the first lens can be corrected by the second lens B2 (plano-concave lens), the effect of correcting spherical aberration is less than that of Table 3.

  Table 4 shows the optical aberration characteristics of the optical antenna of the triplex 21 times optical antenna of Example 3 (line a in Table 2).

[Example 4] Optical antenna for a beam aperture of 46 mm FIG. 5 shows the result of designing a beam expander with a magnification of 23 times in the configuration of FIG. 5 using a plurality of combination lenses as an objective lens. The objective lenses (first lens C1 and second lens C2) are Thorlabs AC508-250-C and LE1985-C. The third lens C3 is a plano-concave lens LC1054-C having a focal length f = −25 mm from Thorlabs. For the fourth lens C4, the same Thorlabs AC060-010-C as in FIG. 4 is used. The distance between the first lens C1 and the second lens C2 is 1 mm, the distance between the second lens C2 and the third lens C3 is 118.6 mm, and the distance between the third lens C3 and the fourth lens C4 is 22.5 mm.

  With this configuration, diffraction-limited accuracy was obtained over an incident angle of 45.9 mm over a full-angle field of view of 0.4 °. Table 5 shows optical aberrations of the optical system of Example 4 including the optical antenna and the fiber imaging lens. The first row shows the aberration of the entire optical system, the rows C1 to C4 show the aberration of each lens constituting the optical antenna, and the row C5 shows the aberration of the fiber imaging lens.

  The third lens C3 (plano-concave lens) corrects most of the spherical aberration, coma and astigmatism of the first lens C1. The reason that the field of view is smaller than that of Example 3 is that the ray height of the fourth lens C4 (eyepiece lens) is increased and the aberration is rapidly increased. However, the beam expander which shows the performance of the diffraction limit at the beam aperture and the incident angle cannot be realized by the conventional technology. The total length of the beam expander was 168 mm, and the beam expander was reduced in size to 23 times.

  Table 5 shows the optical aberration characteristics of the four-element set 23-fold optical antenna of Example 4.

[Example 5] Optical antenna with a wide field of view Since it was found that the factor limiting the field of view with the optical antenna of Example 4 was the optical performance of the eyepiece lens. The configuration shown in FIG. 6 is changed from a doublet (two-disc set) used for the third lens to a three-triplet triplet configuration. Among the commercially available lenses, this third lens D3 (eyepiece lens) is a symmetrical laminated three-lens lens, and the only lens with such a configuration is a Steinheile triplet lens manufactured by Edmund Optics. It does not support up to outside wavelengths.

  Therefore, the imaging performance was evaluated using a commercially available Steinheile triplet 47673-L having a focal length of 12.5 mm. The remaining lenses are the same as in Example 3. With this configuration, the magnification was 17.5 times, and a diffraction-limited optical performance up to 0.8 ° in full angle was obtained for an incident beam having a diameter of 35 mm. Table 6 shows the calculation results of the optical aberration of Example 5. In such a symmetrical triplet, the curvature radius of the front surface and the curvature radius of the rear surface are equal, and the condition of claim 1 is satisfied.

  Table 6 shows optical aberration characteristics of the 17.5 × optical antenna using the triplet eyepiece of Example 5.

[Example 6] Optical antenna having a large aperture and a wide field of view Further, by combining the objective lens group (C1, C2) used in Example 4 with the three-piece eyepiece D3 bonded in Example 5 An optical antenna with a large aperture and a wide field of view could be realized. This configuration corresponds to claim 5. As the number of lenses that make up an optical antenna increases, the effects of reflection loss and ghosting on each surface increase, and the weight and cost also increase, so the optimal lens configuration depends on the required field of view and beam aperture. However, by referring to this example, it is understood that an optical antenna (beam expander) with optical performance that has not been conceived in the past can be realized by selecting an optimal combination from commercially available lenses. It was.

DESCRIPTION OF SYMBOLS 1 Objective lens 2 Eyepiece 3 Fiber imaging lens 4 Mirror drive mechanism 5 Optical antenna (beam expander)
6 Glass Ferrule 7 Wavelength Multiplexing Coupler 8 Fiber Coupler 9 Beacon Light Source 10 Interference Filter 11 Four-Division Si-PD

Claims (5)

An optical antenna for a Kepler-type optical wireless communication device having an objective lens directed to an external space and an eyepiece directed to the inside of the device,
The objective lens and the eyepiece lens are lenses selected from commercially available lenses with optical aberrations corrected for general imaging,
An optical antenna, wherein a ratio of a curvature radius of a front surface of the eyepiece and a curvature radius of a rear surface is 0.8 to 1.2. Between the objective lens and the eyepiece lens, one plano-concave lens is arranged as a correction lens with the concave surface facing the objective lens side,
The position of the correction lens is a position that simultaneously corrects optical aberrations of the objective lens and the eyepiece in a range of 0.25 to 0.9 times the focal length of the objective lens. The optical antenna according to 1.   The optical antenna according to claim 2, wherein the objective lens is a convex lens assembly composed of a plurality of two or more lenses.   The optical antenna according to claim 2, wherein the eyepiece is a combined lens composed of a plurality of two or more lenses. The objective lens is a combined lens composed of a convex doublet lens and another lens,
The optical antenna according to claim 2, wherein the eyepiece is a combined lens including a convex doublet lens and another lens.

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