Optical communication equipment

Abstract

In a free-space optical (FSO) communication system, coherent reception is performed using light generated by the receiving optical communication device. [Solution] The optical communication device includes a rotor HWP that outputs a second local beam by rotating the polarization plane of a first local beam input through a lens 11, a polarization multiplexer PBS that outputs a first composite beam obtained by polarization multiplexing a first-direction component of the second local beam and a second-direction component of the first optical beam, and a second composite beam obtained by polarization multiplexing a second-direction component of the second local beam and a first-direction component of the first optical beam, based on the first and second local beams, a polarizer POL that allows polarization in directions different from the first and second directions to pass through, an imaging unit that acquires brightness distribution information showing the brightness distribution in the cross-section of the first composite beam that has passed through the polarizer, and an adjustment unit that adjusts the position or orientation of the light source or lens and the amount of rotation of the polarization plane of the first local beam in the rotating means based on the brightness distribution information.

Description

Technical Field

[0001] The present disclosure relates to an optical communication device used in a free space optics (FSO) communication system.

Background Art

[0002] FSO communication is a communication method that transmits an optical signal through free space instead of using a fixed medium such as an optical fiber. In the following description, the light transmitted into free space for FSO communication is referred to as an "optical beam". Further, in the following description, an optical communication device that transmits an optical beam is also referred to as an "optical transmitter", and an optical communication device that receives an optical beam is also referred to as an "optical receiver". Note that an optical communication device may include an optical transmitter and an optical receiver for two-way communication.

[0003] Non-Patent Document 1 discloses a configuration in which an optical beam for coherent reception (hereinafter, referred to as a second optical beam) is transmitted from an optical transmitter to an optical receiver together with an optical beam (hereinafter, referred to as a first optical beam) that carries a signal. By transmitting both the first optical beam and the second optical beam from the optical transmitter, the deviation of the wavefronts of the first optical beam and the second optical beam can be reduced. Therefore, it is possible to suppress a decrease in the power of the beat component between the first optical beam and the second optical beam obtained in coherent reception due to the influence of the deviation of the wavefronts.

[0004] Non-Patent Document 2 discloses an optical receiver using a Stokes vector detection method. The optical receiver receives signal light obtained by polarization multiplexing continuous light (unmodulated light) with an X polarization from an optical transmitter and modulated light with a Y polarization orthogonal to the X polarization. The optical receiver obtains Stokes parameters S2 and S3 of the received signal light, and demodulates information transmitted by the optical transmitter from the values of S2 and S3.

Prior Art Documents

Non-Patent Documents

[0005]

Non-Patent Document 1

[0006] In the configuration described in Non-Patent Document 1, the power of both the first and second light beams received by the optical receiver is reduced due to propagation loss in free space. Therefore, even if the decrease in the power of the beat component due to the wavefront shift between the first and second light beams is suppressed, the power of the beat component is also small because the power of both light beams is weak, and thus the demodulation accuracy may deteriorate.

[0007] This disclosure provides a technology for performing coherent reception in FSO communication using light generated by the receiving optical communication device. [Means for solving the problem]

[0008] According to one aspect of the present disclosure, an optical communication device that receives a light beam through free space includes an output means for outputting a first local beam, a lens, and a first rotating means for outputting a second local beam by rotating the polarization plane of the first local beam input through the lens, wherein the direction of the polarization plane of the second local beam is different from a first direction and a second direction orthogonal to the first direction, respectively, and based on the first rotating means, a first light beam having polarization planes in directions different from the first direction and the second direction, and the second local beam, the component of the second local beam in the first direction and the component of the first light beam in the first direction The system includes a polarization multiplexing means that outputs a first composite beam obtained by polarization multiplexing components in two directions, and a second composite beam obtained by polarization multiplexing the component of the second local beam in the second direction and the component of the first optical beam in the first direction; a polarization member that allows polarization in directions different from the first and second directions to pass through; an acquisition means that acquires luminance distribution information showing the luminance distribution in a cross section that intersects with the propagation direction of the first composite beam that has passed through the polarization member; and an adjustment means that adjusts the position or orientation of at least one of the light source and the lens, and the first amount of rotation of the polarization plane of the first local beam in the first rotation means, based on the luminance distribution information. [Effects of the Invention]

[0009] According to this disclosure, in FSO communication, coherent reception can be performed using light generated by the receiving optical communication device. [Brief explanation of the drawing]

[0010] [Figure 1] A diagram illustrating the configuration of an optical communication device according to one embodiment. [Figure 2] A flowchart of the process performed by the adjustment unit according to one embodiment. [Figure 3] A diagram showing an example of the configuration of the demodulation unit. [Figure 4] A diagram showing an example of the configuration of the demodulation unit. [Modes for carrying out the invention]

[0011] Hereinafter, embodiments will be described in detail with reference to the accompanying drawings. Note that the following embodiments do not limit the invention according to the claims, and not all combinations of the features described in the embodiments are essential for the invention. Two or more of the plurality of features described in the embodiments may be arbitrarily combined. Also, the same or similar configurations are given the same reference numerals, and duplicate descriptions are omitted.

[0012] <First Embodiment> FIG. 1 is a configuration diagram of the receiving side of the optical communication device according to the present embodiment, that is, the optical receiving device. The optical receiving device receives an optical beam that conveys information through free space from the transmitting side of the optical communication device that is the communication partner, that is, the optical transmitting device. Note that the optical receiving device has a configuration for receiving the optical beam via an optical antenna or the like and tracking the received optical beam, but this is not relevant to the description of the present disclosure and is thus omitted in FIG. 1. In the following description, the optical beam received by the optical receiving device from the optical transmitting device is referred to as the "received beam". The received beam 50 received by the optical receiving device is input to a half-wave plate (HWP) 16. In the following description, the direction of the polarization plane of this received beam 50 is taken as the X direction, and the direction orthogonal to the X direction is taken as the Y direction. Let the electric field component of the received beam 50 be E , , Then, the Jones vector of the received beam 50 is

[0013] [Equation] is.

[0014] The HWP 16 is arranged so as to rotate the polarization plane of the received beam 50 by an angle θ1. Therefore, the Jones vector of the received beam 51 after passing through the HWP 16 is

[0015] [Equation] becomes. Thus, the HWP 16 functions as a rotator that rotates the polarization plane of the received beam 50.

[0016] The light source 10 functions as an output device that generates and outputs an X - polarized optical beam 60 for coherent reception. In the following description, the optical beam generated by the light source 10 is referred to as the "local beam". The local beam 60 generated by the light source 10 is incident on the HWP 12 via the lens 11. The lens 11 is an optical member that collimates the local beam 60 from the light source 10 into parallel light, for example, a collimator lens. The HWP 12 is arranged so as to rotate the polarization plane of the local beam 60 by an angle θ2. Therefore, the Jones vector of the local beam 61 after passing through the HWP 12 is such that if the electric field component of the local beam 60 generated by the light source 10 is E LO then

[0017]

Number

[0018] The polarization beam splitter (PBS) 13 allows the X - polarized component of the input optical beam to pass straight through and deflects the Y - polarized component by 90 degrees. Therefore, the PBS 13 outputs a combined beam 71 in which the X - polarized component of the local beam 61 and the Y - polarized component of the received beam 51 are polarization - multiplexed, and a combined beam 72 in which the Y - polarized component of the local beam 61 and the X - polarized component of the received beam 51 are polarization - multiplexed. Thus, the PBS 13 functions as a polarization multiplexer. The Jones vector of the combined beam 71 is

[0019]

Number

[0020]

Number

[0021] In the combined beam 71, the received beam component is contained only in the Y polarization, and the local beam component is contained only in the X polarization, so the received beam and the local beam do not interfere with each other. The polarizer (POL) 14 is a polarizer provided to interfere the received beam component and the local beam component contained in the combined beam 71, and is provided to allow only components in directions different from the X and Y directions to pass through. For example, the POL 14 may be provided to allow only components in the direction of 45 degrees to the X and Y directions to pass through. Note that the direction of the polarization plane through which the POL 14 passes does not need to be an integer multiple of 90 degrees to the X direction, and is not limited to a configuration that allows only components in the direction of 45 degrees to the X direction to pass through. When the POL 14 is arranged to allow only components in the direction of 45 degrees to the X direction to pass through, the Jones vector of the combined beam 73 after passing through the POL 14 is:

[0022]

number

[0023] The composite beam 73 is incident on the imaging surface of the imaging unit 15. The imaging unit 15 is positioned such that the direction parallel to the imaging surface intersects with the direction of propagation of the composite beam 73. For example, the imaging unit 15 may be positioned such that the direction parallel to the imaging surface is perpendicular to the direction of propagation of the composite beam 73. The imaging unit 15 has an imaging device such as a charge-coupled device (CCD) sensor or a photodiode (PD) array, and acquires luminance distribution information showing the spatial distribution of luminance on the imaging surface and outputs it to the adjustment unit 17. The luminance distribution information shows the spatial distribution of luminance in a cross-section of the composite beam 73 that intersects with the direction of propagation of the composite beam 73. When the direction parallel to the imaging surface is perpendicular to the direction of propagation of the composite beam 73, the luminance distribution information shows the spatial distribution of luminance in a cross-section of the composite beam 73 that is perpendicular to the direction of propagation of the composite beam 73.

[0024] The adjustment unit 17 adjusts the direction of the lagging axis of the HWP 12, that is, the amount of rotation θ2 of the polarization plane of the local beam 60 at the HWP 12, the position of the lens 11, and the angle (attitude) of the optical axis of the lens 11 with respect to a predetermined direction, based on the brightness distribution information. Note that the configuration may adjust only one of the position and attitude of the lens 11, rather than both.

[0025] If the wavefronts of the local beam 61 component and the receiving beam 51 component in the composite beam 73 incident on the imaging unit 15 do not match, interference fringes will occur due to interference between the local beam 61 component and the receiving beam 51 component. If the wavefronts match, no interference fringes will occur. Therefore, the adjustment unit 17 controls the position and orientation of the lens 11 so that the interference fringes disappear, thereby reducing the difference in wavefronts between the local beam 61 component and the receiving beam 51 component in the composite beam 73. When interference fringes are present, multiple maxima occur in the luminance distribution shown in the luminance distribution information. However, when no interference fringes are present, the luminance distribution approaches a Gaussian distribution, and there is only one maxima. Therefore, the adjustment unit 17 controls the position and orientation of the lens 11 so that there is only one maxima in the luminance distribution shown in the luminance distribution information.

[0026] Furthermore, because the power of the receiving beam 50 is small, the component of the receiving beam 51 included in the composite beam 73 is "sinθ1E sig The power of "cosθ2E" is also very small. On the other hand, the local beam 60 is generated within the optical receiver, and its power is strong, therefore, depending on the value of the angle θ2, the component of the local beam 61 included in the composite beam 73, "cosθ2E" LO The power of "sinθ1E" is a component of the received beam 51 contained in the composite beam 73. sig It becomes significantly larger than the power of "

[0027] Here, if the power of the local beam 61 component included in the composite beam 73 becomes significantly larger than the power of the receiving beam 51 component included in the composite beam 73, the luminance distribution shown by the luminance distribution information will represent the wavefront of the local beam 61 included in the composite beam 73. Even if the wavefronts of the local beam 61 component and the receiving beam 51 component in the composite beam 73 do not match, interference fringes will not be observed. Therefore, the adjustment unit 17 first adjusts the direction of the lagging axis of the HWP 12 to reduce the power of the local beam 61 component included in the composite beam 73 to a level where interference fringes can be observed, and then controls the lens 11 so that the interference fringes disappear after they are observed.

[0028] Figure 2 is a flowchart of the process performed by the adjustment unit 17 to reduce the wavefront misalignment between the local beam 61 component and the receiving beam 51 component included in the composite beam 73. In S10, the adjustment unit 17 adjusts the direction of the lagging axis of HWP 12, that is, adjusts the value of the rotation amount θ2, until interference fringes are generated. As mentioned above, since the power of the receiving beam 50 is small, the component of the receiving beam 51 included in the composite beam 73 is "sinθ1E sig The power of "cosθ2E" is also very small. Therefore, the component of the local beam 61 contained in the composite beam 73 is "cosθ2E LO The value of the rotation amount θ2, which is a parameter for adjusting the rotation, does not need to be changed from 0 to 90 degrees. It is sufficient to adjust it within a predetermined range (adjustment range) that significantly reduces the power of the local beam 61 component, for example, within a range of 85 degrees or more and less than 90 degrees.

[0029] For example, if the initial position and orientation of the lens 11 are such that the wavefronts of the local beam 61 component and the receiving beam 51 component in the composite beam 73 incident on the imaging unit 15 match, then no interference fringes will occur even if the rotation amount θ2 is adjusted within a predetermined range. Therefore, if no interference fringes occur even when the rotation amount θ2 is adjusted within a predetermined range, the adjustment unit 17 adjusts the position and orientation of the lens 11 in S12 and repeats the process from S10 until interference fringes occur.

[0030] When interference fringes are observed, the adjustment unit 17 adjusts the position and orientation of the lens 11 in S13 and determines in S14 whether the interference fringes have disappeared. If the interference fringes have not disappeared, the adjustment unit 17 repeats the process from S13. On the other hand, if the interference fringes disappear in S14, the adjustment unit 17 determines that the wavefronts of the local beam 61 component and the received beam 51 component in the composite beam 73 match and terminates the process shown in Figure 2.

[0031] When the wavefronts of the local beam 61 component and the received beam 51 component in the combined beam 73 coincide, the wavefronts of the local beam 61 component and the received beam 51 component in the combined beam 72 also coincide accordingly. This combined beam 72 is demodulated in the demodulation unit 18. The configuration of the demodulation unit 18 can be based on the configurations described in Non-Patent Document 1 and Non-Patent Document 2. Below, an example of the configuration of the demodulation unit 18 will be briefly described.

[0032] Figure 3 shows an example configuration of the demodulator 18 when heterodyne reception is performed, that is, when the frequencies of the local beam 60 and the received beam 50 are different. The PBS 21 separates the input light into a first polarization and a second polarization, outputs the first polarization light to the photodetector 22, and outputs the second polarization light, which is orthogonal to the first polarization, to the photodetector 23. Hereafter, the direction of the polarization plane of the first polarization will be referred to as the first direction. The first direction is unrelated to the X direction. The lagging axis of the HWP 20 is tilted by 22.5 degrees with respect to the first direction.

[0033] The composite beam 72 is converted to composite beam 73 by HWP20, and separated into a first-polarized composite beam 74 and a second-polarized composite beam 75 by PBS21. The composite beam 74 is converted to a first electrical signal by photoelectric conversion at photodetector 22, and the composite beam 75 is converted to a second electrical signal by photoelectric conversion at photodetector 23. The calculation unit 24 generates an intermediate frequency (IF) signal, which is the beat component of the received beam 50 and the local beam 60, based on the difference between the first electrical signal and the second electrical signal, and performs demodulation.

[0034] Figure 4 shows an example configuration of the demodulation unit 18 when homodyne reception is performed, that is, when the frequencies of the local beam 60 and the received beam 50 are the same. The beam splitter (BS) 25 splits the combined beam 72 into two and outputs them to the HWP 20 and the quarter-wave plate (QWP) 26. The processing of the combined beam 72 output to the HWP 20 is the same as in Figure 3.

[0035] PBS27 separates the incoming light into a first polarization and a second polarization, outputs the first polarization light to photodetector 28, and outputs the second polarization light to photodetector 29. The lagging axis of QWP20 is tilted by 45 degrees with respect to the first direction.

[0036] The composite beam 72 input to the QWP26 is converted into a composite beam 76 by the QWP26, and then separated into a first-polarized composite beam 77 and a second-polarized composite beam 78 by the PBS27. The composite beam 77 is converted into a third electrical signal by photoelectric conversion at the photodetector 28, and the composite beam 78 is converted into a fourth electrical signal by photoelectric conversion at the photodetector 29. The calculation unit 24 performs demodulation by determining the in-phase (I) component of the baseband signal that was the basis of the received beam 50 based on the difference between the first electrical signal and the second electrical signal, and by determining the quadrature-phase (Q) component of the baseband signal based on the difference between the third electrical signal and the fourth electrical signal.

[0037] In the flowchart of Figure 2, the adjustment unit 17 adjusted the lens 11 so that there was one maximum point in the luminance distribution shown by the luminance distribution information. However, the lens 11 may also be configured to be adjusted so that the error between the luminance distribution shown by the luminance distribution information and the Gaussian distribution is minimized. Alternatively, the adjustment unit 17 may first adjust the lens 11 so that there is one maximum point in the luminance distribution shown by the luminance distribution information, and if there is not one maximum point, adjust the lens 11 so that the error between the luminance distribution shown by the luminance distribution information and the Gaussian distribution is minimized. With this configuration, even if there is not one maximum point, the wavefront misalignment between the local beam and the received beam can be minimized.

[0038] Furthermore, in the above explanation, the position and / or orientation of the lens 11 was adjusted to reduce the wavefront misalignment between the local beam and the received beam included in the composite beam 73. However, instead of adjusting the position and / or orientation of the lens 11, or in addition to adjusting it, the position and / or orientation of the light source 10 can also be adjusted. Moreover, the light source 10 and the lens 11 can be integrated as an output unit, that is, the positional relationship between the light source 10 and the lens 11 can be fixed, and the position and / or orientation of the output unit can be adjusted.

[0039] Furthermore, in Figure 1, the HWP 16 converted the received beam 50 into an optical beam 51 having both X-polarization and Y-polarization components. However, if the polarization direction of the received beam 50 is different from the X-direction and Y-direction, the HWP 16 can be omitted.

[0040] Furthermore, in the flowchart of Figure 2, the adjustment unit 17 controlled only the rotation amount θ2 by HWP12 in order to reduce the difference between the power of the received beam and the power of the local beam included in the composite beam 73. However, it is also possible to configure it to control both the rotation amount θ2 by HWP12 and the rotation amount θ1 by HWP16. Note that the component of the received beam included in the composite beam 72 used for demodulation is "cosθ1E sig To prevent the power from becoming too low, the rotation amount θ1 by HWP16 can also be configured to have an adjustment range. In this case, the adjustment unit 17 changes the combination of rotation amount θ2 and rotation amount θ1 values ​​within their respective adjustment ranges until interference fringes are generated.

[0041] Furthermore, in order to reduce the difference between the power of the received beam and the power of the local beam included in the composite beam 73, a configuration can be adopted in which only the rotation amount θ1 by the HWP 16 is controlled. In this case, by generating a local beam 60 having X-polarization and Y-polarization components in the light source 10, the HWP 12 can be omitted and the local beam 60 can be input to the PBS 13.

[0042] With the above configuration, coherent reception can be performed in FSO communications using light generated by the receiving optical communication device. Therefore, it becomes possible to contribute to Goal 9 of the United Nations-led Sustainable Development Goals (SDGs), "Build resilient infrastructure, promote sustainable industrialization and foster innovation." [Explanation of symbols]

[0043] 10: Light source, 11: Lens, 12: HWP, 13: PBS, 15: Imaging unit, 14: POOL, 17: Adjustment unit

Claims

1. An optical communication device that receives a light beam through free space, A light source that outputs a first local beam, Lens and, A first rotating means that outputs a second local beam by rotating the polarization plane of the first local beam input through the lens, wherein the direction of the polarization plane of the second local beam is different from the first direction and the second direction which is orthogonal to the first direction, and the first rotating means A polarization multiplexing means outputs a first composite beam obtained by polarization multiplexing the component of the second local beam in the first direction and the component of the first light beam in the second direction, based on a first light beam having polarization planes in directions different from the first and second directions, respectively, and a second local beam, and a second composite beam obtained by polarization multiplexing the component of the second local beam in the second direction and the component of the first light beam in the first direction, A polarizing member that allows polarization in directions different from the first and second directions to pass through, A means for acquiring brightness distribution information that shows the brightness distribution in a cross-section intersecting the propagation direction of the first composite beam that has passed through the polarizing member, Based on the luminance distribution information, an adjustment means adjusts the position or orientation of at least one of the light source and the lens, and the first amount of rotation of the polarization plane of the first local beam in the first rotating means. An optical communication device equipped with the following features.

2. The optical communication device according to claim 1, wherein the adjusting means adjusts the first rotation amount so that multiple maximum points occur in the luminance distribution, and then adjusts the position or orientation of at least one of the light source and the lens so that there is one maximum point in the luminance distribution.

3. The optical communication device according to claim 2, wherein the adjusting means adjusts the position or orientation of at least one of the light source and the lens such that the error between the luminance distribution and the Gaussian distribution is minimized when there is not a single maximum point in the luminance distribution.

4. The optical communication device according to claim 2, wherein if the adjustment means does not produce multiple maximum points in the luminance distribution even when the first rotation amount is adjusted within a predetermined range, the adjustment means repeats adjusting the position or orientation of at least one of the light source and the lens to adjust the first rotation amount within the predetermined range until multiple maximum points occur in the luminance distribution.

5. The system further comprises a second rotating means for outputting the first light beam by rotating the polarization plane of the light beam received through free space, The optical communication device according to any one of claims 1 to 4, wherein the adjusting means further adjusts the amount of second rotation of the polarization plane of the light beam in the second rotating means based on the brightness distribution information.

6. The optical communication device according to claim 5, wherein the first rotating means and the second rotating means are half-wave plates.

7. The optical communication device according to claim 5, wherein the adjusting means adjusts the first rotation amount and the second rotation amount so that multiple maximum points occur in the luminance distribution, and then adjusts the position or orientation of at least one of the light source and the lens so that there is one maximum point in the luminance distribution.

8. The optical communication device according to any one of claims 1 to 4, wherein the first optical beam is obtained by rotating the polarization plane of the optical beam received through free space by a predetermined amount.

9. The optical communication device according to any one of claims 1 to 4, wherein the polarization multiplexing means is a polarization beam splitter.

10. The optical communication apparatus according to any one of claims 1 to 4, wherein the acquisition means comprises an imaging device.

11. The optical communication apparatus according to any one of claims 1 to 4, further comprising demodulation means for demodulating an optical signal based on the optical beam based on the second composite beam.

12. An optical communication device that receives a light beam through free space, A light source that outputs a first local beam, Lens and, A first rotating means that outputs a second local beam by rotating the polarization plane of the first local beam input through the lens by a predetermined amount, wherein the direction of the polarization plane of the second local beam is different from the first direction and the second direction which is perpendicular to the first direction, and the first rotating means A second rotating means for outputting a first light beam by rotating the polarization plane of the light beam, wherein the direction of the polarization plane of the first light beam is different from the first direction and the second direction, respectively, and A polarization multiplexing means that outputs a first combined beam obtained by polarization multiplexing the first direction component of the second local beam and the second direction component of the first light beam, based on the first light beam and the second local beam, and a second combined beam obtained by polarization multiplexing the second direction component of the second local beam and the first direction component of the first light beam, A polarizing member that allows polarization in directions different from the first and second directions to pass through, A means for acquiring brightness distribution information that shows the brightness distribution in a cross-section intersecting the propagation direction of the first composite beam that has passed through the polarizing member, Based on the luminance distribution information, the position or orientation of at least one of the light source and the lens, and the amount of rotation of the polarization plane of the light beam in the second rotating means, An optical communication device equipped with the following features.

13. An optical communication device that receives a light beam through free space, A light source that outputs a first local beam having polarization planes in directions different from the first direction and the second direction orthogonal to the first direction, Lens and, A rotating means that outputs a first light beam by rotating the polarization plane of the light beam, wherein the direction of the polarization plane of the first light beam is different from the first direction and the second direction, respectively, and the rotating means A combining means that outputs a first combined beam obtained by polarization-multiplexing the first local beam's component in the first direction and the first optical beam's component in the second direction, based on the first local beam input through the lens and the first optical beam, and a second combined beam obtained by polarization-multiplexing the first local beam's component in the second direction and the first optical beam's component in the first direction. A polarizing member that allows polarization in directions different from the first and second directions to pass through, A means for acquiring brightness distribution information that shows the brightness distribution in a cross-section intersecting the propagation direction of the first composite beam that has passed through the polarizing member, Based on the luminance distribution information, the position or orientation of at least one of the light source and the lens, and the amount of rotation of the polarization plane of the light beam in the rotating means, An optical communication device equipped with the following features.

14. The optical communication device according to claim 12 or 13, wherein the adjusting means adjusts the amount of rotation so that multiple maximum points occur in the luminance distribution, and then adjusts the position or orientation of at least one of the light source and the lens so that there is one maximum point in the luminance distribution.

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