Reception apparatus

Abstract

This reception apparatus is provided with: an optical device that modulates the wavefront of first beam light on the basis of a control signal; a splitter that splits the first beam light with the modulated wavefront into second beam light and third beam light; a sensor that acquires wavefront data of the third beam light; an analysis unit that analyzes distortion of the wavefront on the basis of the wavefront data; and a control unit that generates a control signal on the basis of the time average of the distortion. The control unit may also generate a control signal on the basis of the time average of Zernike coefficients of the distortion for each Zernike mode. The control unit may also generate a control signal so as to improve the fiber coupling efficiency of the second beam light.

Description

Receiving device

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

[0002] A transmitter installed on the ground transmits an optical signal (light beam) to a receiver installed on the ground. The optical signal propagates through the atmosphere and is coupled to an optical fiber at the receiver. Such an optical wireless communication system has been studied (see Non-Patent Document 1).

[0003] E. Ciaramella et al., "1.28 terabit / s (32x40 Gbit / s) wdm transmission system for free space optical communications," in IEEE Journal on Selected Areas in Communications, vol. 27, no. 9, pp. 1639-1645, December 2009.

[0004] When optical wireless communication is performed between a transmitter and a receiver using an optical signal (beam light) propagated through the atmosphere, the wavefront of the optical signal is distorted by the influence of the atmosphere. This causes spatial variations in intensity (speckle) in the optical signal arriving at the receiver. This wavefront distortion fluctuates rapidly over time in response to atmospheric turbulence. This rapid time variation of the wavefront distortion is a major obstacle to achieving stable optical wireless communication.

[0005] Furthermore, optical systems are subject to considerable imperfections. Examples of imperfections include optical axis misalignment and manufacturing accuracy of spatial optical elements. To maximize the efficiency of fiber coupling, wavefront distortions caused by optical system imperfections must be compensated for using a wavefront control device. To compensate for the wavefront distortion of an optical signal, the wavefront distortion must be estimated. Furthermore, outdoors, sunlight causes thermal expansion and contraction in a specific area of ​​a housing (e.g., a metal frame) supporting the optical system. Temperature changes over the course of morning, noon, and night cause thermal expansion and contraction of the housing, as well as thermal expansion and changes in the refractive index of the lens. As a result, the wavefront distortion caused by the optical system changes from moment to moment.

[0006] The receiving device observes the combined wavefront distortion caused by atmospheric turbulence and the wavefront distortion caused by optical system imperfections as wavefront distortion. Compensating for this observed wavefront distortion requires a high-speed wavefront control device and control unit capable of tracking the wavefront distortion caused by atmospheric turbulence, which varies rapidly over time, resulting in a high-cost receiving device. In contrast, if a receiving device using an inexpensive wavefront control device and control unit that are not capable of high-speed operation estimates and compensates only for the wavefront distortion caused by the optical system, the wavefront distortion caused by atmospheric turbulence will degrade the accuracy of estimating the wavefront distortion caused by the optical system. Thus, there is a problem in that it is not possible to improve the accuracy of estimating the wavefront distortion of an optical signal caused by optical system imperfections in an outdoor atmospheric turbulence environment.

[0007] In view of the above circumstances, the present invention aims to provide a receiving device that can improve the accuracy of estimating the wavefront distortion of an optical signal caused by imperfections in the optical system in an outdoor atmospheric turbulence environment.

[0008] One aspect of the present invention is a receiving apparatus comprising an optical device that modulates the wavefront of a first beam of light based on a control signal, a splitter that splits the first beam of light with the modulated wavefront into a second beam of light and a third beam of light, a sensor that acquires wavefront data of the third beam of light, an analysis unit that analyzes distortion of the wavefront from the wavefront data, and a control unit that generates the control signal based on a time average of the distortion.

[0009] According to the present invention, it is possible to improve the accuracy of estimating distortion of the wavefront of an optical signal caused by imperfections in an optical system in an outdoor atmospheric turbulence environment.

[0010] Fig. 1 is a diagram showing an example of the configuration of a communication system in an embodiment. Fig. 2 is a diagram showing a wavefront pattern image at a first time and component amounts of Zernike modes (Zernike coefficients) in an embodiment. Fig. 3 is a diagram showing a wavefront pattern image at a second time and component amounts of Zernike modes (Zernike coefficients) in an embodiment. Fig. 4 is a diagram showing a wavefront pattern image at a third time and component amounts of Zernike modes (Zernike coefficients) in an embodiment. Fig. 5 is a flowchart showing an example of the operation of a communication device in an embodiment. Fig. 6 is a diagram showing an example of the hardware configuration of a communication device in an embodiment.

[0011] An embodiment of the present invention will be described in detail with reference to the drawings. Fig. 1 is a diagram showing an example of the configuration of a communication system 1 according to the embodiment. The communication system 1 is a system (optical wireless communication system) that performs wireless communication using optical signals (light beams). The communication system 1 includes one or more transmitting devices 2 (first communication devices) and one or more receiving devices 3 (second communication devices).

[0012] The receiving device 3 includes an optical antenna 31, a first optical device 32, a second optical device 33, a splitter 34, a third optical device 35, a receiving unit 36, a sensor 37, an analyzing unit 38, a storage unit 39, and a control unit 40. The receiving unit 36 ​​includes an optical fiber (not shown).

[0013] In the communication system 1, an optical signal (light beam) propagates through the atmosphere between a transmitting device 2 and a receiving device 3. The receiving device 3 estimates distortion (aberration) that occurs in the wavefront of the optical signal that has propagated through the atmosphere. Based on the estimation result, the receiving device 3 compensates for the distortion using adaptive optics. In the communication system 1, a wavefront control device is used to compensate for the distortion that occurs in the wavefront. The wavefront control device is, for example, a deformable mirror. The wavefront control device may also be, for example, a spatial light modulator (LCOS-SLM: Liquid Crystal on Silicon - Spatial Light Modulator).

[0014] The transmitter 2 generates an electrical signal in accordance with data. The transmitter 2 converts the electrical signal into an optical signal. The optical signal (light beam) propagates through the atmosphere and arrives at (incident on) the optical antenna 31. The light beam is, for example, a laser light beam.

[0015] The optical antenna 31 receives an optical signal arriving (incident) from the transmitter 2. The first optical device 32 includes a fast steering mirror (FSM), a beam splitter, and a quadrant detector (QD) sensor. The first optical device 32 corrects the angle of the optical signal incident from the optical antenna 31. Depending on the optical system, the first optical device 32 may further include optical elements such as an optical filter for wavelength separation and a lens.

[0016] The optical signal whose angle has been corrected by the first optical device 32 is incident from the first optical device 32 to the second optical device 33. The second optical device 33 is a wavefront control device. The second optical device 33 modulates the wavefront of the optical signal incident on the second optical device 33 based on the control of the control unit 40. That is, the second optical device 33 modulates the wavefront of the optical signal incident from the first optical device 32 based on the control of the control unit 40. The optical signal whose wavefront has been modulated is incident on the splitter 34.

[0017] The splitter 34 splits the optical signal whose wavefront has been modulated by the second optical device 33 to a third optical device 35 and a sensor 37. The third optical device 35 is, for example, a condenser lens. The third optical device 35 couples the optical signal (one of the split optical signals) to an optical fiber of the receiving unit 36.

[0018] The receiver 36 converts the optical signal coupled to the optical fiber into an electrical signal. The receiver 36 performs predetermined signal processing (e.g., demodulation processing) on ​​the converted electrical signal. The receiver 36 obtains data transmitted from the transmitter 2 using the optical signal from the electrical signal by performing the predetermined signal processing.

[0019] The sensor 37 is a wave-front sensor. The sensor 37 observes the optical signal split by the splitter 34 (the other split optical signal). The sensor 37 outputs wavefront data of this split optical signal, which serves as the basis for wavefront analysis, to the analysis unit 38. The wavefront data is position data of the focused spots of the lenses of the microlens array provided in the sensor 37.

[0020] The analysis unit 38 acquires wavefront data from the sensor 37. The analysis unit 38 estimates (reconstructs) the wavefront (spatial phase distribution) of the optical signal based on the acquired wavefront data. The analysis unit 38 also analyzes each component of the estimated wavefront, from higher-order components to lower-order components. For example, the analysis unit 38 analyzes the breakdown (Zernike coefficients) of the multiple components (Zernike modes) that make up the wavefront by expanding the wavefront into Zernike polynomials.

[0021] That is, the analyzer 38 estimates one or more Zernike modes (phase patterns) that constitute the wavefront. The wavefront "W(r, θ)" is expressed as in Equation (1).

[0022]

[0023] Here, "r" represents the distance from the origin in the polar coordinate system. "θ" represents the angle of deviation in the polar coordinate system. ｉ " represents the Zernike coefficient. "i" represents the number of the Zernike mode. ｉ " represents a Zernike mode. Zernike mode "Z ｉ " is expressed as in equation (2).

[0024]

[0025] Here, "n" and "m" respectively represent the order. The order "n" is a non-negative integer. The order "m" is an integer that satisfies "n≧|m|". "n" and "m" are determined according to the number "i" of the Zernike mode. The column vector "W" of the wavefront "W(r, θ)" is expressed as in equation (3).

[0026]

[0027] Here, "Z" on the right side of equation (3) is the Zernike mode "Z ｉ " represents a column vector of the Zernike mode "Z ｉ The column vector of " is expressed as in equation (4).

[0028]

[0029] Furthermore, the column vector "A" of the Zernike coefficients is expressed as in equation (5).

[0030]

[0031] The analysis unit 38 records Zernike coefficients representing each analyzed Zernike mode component for the optical signal arriving at the optical antenna 31 during a predetermined period including, for example, the first time to the third time, in the storage unit 39. The analysis unit 38 may also record wavefront data acquired by the sensor 37 and the wavefront reconstruction result (spatial phase distribution) for the optical signal arriving at the optical antenna 31 during a predetermined period including, for example, the first time to the third time.

[0032] Each of Figures 2 to 4 illustrates a wavefront pattern image (an image showing a reconstruction result of a wavefront), Zernike modes, and Zernike coefficients (component amounts of Zernike modes). Figure 2 is a diagram showing a wavefront pattern image and Zernike coefficients at a first time in an embodiment. Figure 3 is a diagram showing a wavefront pattern image and Zernike coefficients at a second time in an embodiment. Figure 4 is a diagram showing a wavefront pattern image and Zernike coefficients at a third time in an embodiment.

[0033] The difference between the first time and the second time is, for example, several tens of milliseconds. Similarly, the difference between the second time and the third time is, for example, several tens of milliseconds. Here, the speed of time fluctuation of the wavefront distortion caused by atmospheric turbulence is approximately 100 Hz to 1 kHz.

[0034] The control unit 40 acquires Zernike coefficients representing each analyzed Zernike mode component for an optical signal arriving at the optical antenna 31 during a predetermined period from the storage unit 39 (buffer). The control unit 40 may acquire all components of the Zernike polynomial (e.g., Zernike coefficients of 12 types of Zernike modes), or may acquire components with a predetermined amount or more (components with Zernike coefficients equal to or greater than a threshold) from among the predetermined components. In other words, the control unit 40 may acquire the main components (e.g., Zernike coefficients of 10 types of Zernike modes out of the 12 types).

[0035] The control unit 40 generates a control signal based on the time average of the distortion of the wavefront. The control unit 40 generates a control signal based on the time average of the Zernike coefficients for each Zernike mode of the wavefront. The control unit 40 may also generate a control signal based on the time average of the wavefront pattern reconstructed as in each of Figures 2 to 4.

[0036] The control unit 40 generates a control signal to improve the fiber coupling efficiency of the second beam light. Here, the control unit 40 may generate the control signal to compensate (minimize) an error between a wavefront obtained based on the time average of the Zernike coefficients of each Zernike mode and an optimal wavefront that maximizes the fiber coupling efficiency of the second beam light. The optimal wavefront that maximizes the fiber coupling efficiency is measured in advance, for example.

[0037] The control unit 40 determines the phase conjugate pattern of the acquired component as the compensation pattern for the component. The control unit 40 controls the phase modulation operation by the second optical device 33 according to the determined compensation pattern. That is, the control unit 40 controls the wavefront of the optical signal arriving at the second optical device 33 by generating a control signal according to the determined compensation pattern.

[0038] Next, an example of operation of the communication system 1 will be described. FIG. 5 is a flowchart showing an example of operation of the communication device according to the embodiment. The second optical device 33 modulates the wavefront (spatial phase distribution) of the first light beam incident from the first optical device 32 as a wavefront compensation process based on the control signal (step S101). The splitter 34 splits the first light beam into a second light beam and a third light beam (step S102). The sensor 37 acquires wavefront data of the third light beam (step S103). The analyzer 38 analyzes the wavefront distortion of the third light beam (step S104). The controller 40 generates a control signal based on the time average of the wavefront distortion (step S105).

[0039] 5 is performed at least once, thereby compensating for wavefront distortion caused by the optical system in an outdoor atmospheric turbulence environment. By performing the operation illustrated in Fig. 5 at a predetermined cycle (e.g., a cycle of several tens of minutes to several hours), the operation can follow the time fluctuation of wavefront distortion caused by the optical system due to temperature changes over time (e.g., morning, noon, and night).

[0040] As described above, the second optical device 33 (wavefront control device) modulates the wavefront (spatial phase distribution) of the first beam of light incident from the first optical device 32 based on the control signal. The splitter 34 splits the first beam of light, whose phase distribution has been modulated, into the second beam of light and the third beam of light. The third optical device 35 couples the second beam of light to an optical fiber. The splitter 34 causes the third beam of light to be incident on the sensor 37. The sensor 37 (wavefront sensor) acquires wavefront data of the third beam of light. The analyzer 38 analyzes the distortion of the wavefront of the third beam of light. For example, the analyzer 38 may analyze each Zernike mode component constituting the distortion of the wavefront of the third beam of light by expanding the wavefront into Zernike polynomials. The controller 40 generates a control signal based on the time average of the wavefront distortion.

[0041] This time averaging process removes the wavefront distortion of the optical signal caused by atmospheric turbulence from the wavefront analysis results, thereby improving the accuracy of estimating the wavefront distortion of the optical signal caused by imperfections in the optical system in an outdoor atmospheric turbulence environment.

[0042] The rate of time fluctuation of wavefront distortion caused by atmospheric turbulence is approximately 100 Hz to 1 kHz. In contrast, the period of time fluctuation of wavefront distortion caused by the optical system is slow, such as several tens of minutes to several hours. Therefore, it is possible to suppress only the time fluctuation of distortion caused by atmospheric turbulence by averaging over a time period of several seconds to several tens of seconds.

[0043] The control unit 40 may generate a control signal based on a time average of the Zernike coefficients for each Zernike mode of the wavefront. The control unit 40 may generate a control signal based on a time average of the wavefront pattern reconstructed as in each of the diagrams of Figures 2 to 4. The control unit 40 may generate a control signal so as to improve the fiber coupling efficiency of the second beam light.

[0044] The analysis unit 38 and control unit 40 included in the device of the present invention can also be realized by a computer and a program, and the program can be recorded on a recording medium or provided via a network.

[0045] (Hardware Configuration Example) FIG. 8 is a diagram illustrating an example of the hardware configuration of the communication device 10 according to an embodiment. The communication device 10 corresponds to at least the receiving device of a transmitting device (first communication device) and a receiving device (second communication device). Some or all of the functional units of the communication device 10 are implemented as software by a processor 101, such as a central processing unit (CPU), executing a program stored in a storage device 102 having a non-volatile recording medium (non-transitory recording medium) and a memory 103. The program may be recorded on a computer-readable non-transitory recording medium. Examples of computer-readable non-transitory recording media include portable media such as a flexible disk, a magneto-optical disk, a read-only memory (ROM), and a compact disc read-only memory (CD-ROM), and storage devices such as a hard disk built into a computer system. The communication unit 104 executes a predetermined communication process. The communication unit 104 may acquire data and programs.

[0046] Some or all of the functional units of the communication device 10 may be realized using hardware including electronic circuits (electronic circuits or circuitry) using, for example, an LSI (Large Scale Integrated circuit), an ASIC (Application Specific Integrated Circuit), a PLD (Programmable Logic Device), or an FPGA (Field Programmable Gate Array).

[0047] Although an embodiment of the present invention has been described above in detail with reference to the drawings, the specific configuration is not limited to this embodiment, and includes designs within the scope of the gist of the present invention.

[0048] The present invention is applicable to optical communication systems that perform wireless communication using optical signals.

[0049] REFERENCE SIGNS LIST 1...communication system, 2...transmitting device, 3...receiving device, 31...optical antenna, 32...first optical device, 33...second optical device, 34...splitter, 35...third optical device, 36...receiving section, 37...sensor, 38...analyzing section, 39...storing section, 40...control section

Claims

1. A receiving apparatus comprising: an optical device that modulates the wavefront of a first beam of light based on a control signal; a splitter that splits the first beam of light with the modulated wavefront into a second beam of light and a third beam of light; a sensor that acquires wavefront data of the third beam of light; an analysis unit that analyzes distortion of the wavefront from the wavefront data; and a control unit that generates the control signal based on the time average of the distortion.

2. The receiving device according to claim 1, wherein the control unit generates the control signal based on a time average of Zernike coefficients for each Zernike mode of the wavefront.

3. The receiving device described in claim 1, wherein the control unit generates the control signal so as to improve the fiber coupling efficiency of the second beam light based on a wavefront obtained based on the time average of the Zernike coefficients for each Zernike mode of the wavefront and an optimal wavefront that maximizes the fiber coupling efficiency of the second beam light.

4. The receiving device of claim 1, wherein the sensor acquires wavefront data of the third light beam at a predetermined period, the analysis unit analyzes the wavefront distortion from the wavefront data at the predetermined period, and the control unit generates the control signal based on the time average of the distortion at the predetermined period.

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