Photoelectric conversion device

Abstract

This photoelectric conversion device comprises: a first light source that outputs a plurality of lights for generating a first electrical signal; a second light source that outputs a plurality of lights for generating a second electrical signal; a plurality of lights that are obtained on the basis of the second light source and that have a plane of polarization or a polarization rotational direction different from the plurality of lights output from the first light source; and one or more photo mixers that generate one or more radio frequency signals on the basis of the plurality of lights output from the first light source.　

Description

Photoelectric conversion device

[0001] The present invention relates to a photoelectric conversion device.

[0002] The use of high frequency bands, above the millimeter wave band, is progressing as a means of achieving faster and larger capacity wireless communications. The spatial propagation loss of radio waves increases as the frequency increases. For example, free space propagation loss increases in proportion to the square of the frequency. For this reason, antennas with high gain are often used in high frequency bands. High gain antennas always have high directivity, so it is necessary to align the direction of their beam with the other station in the wireless communication. When the direction of the other station changes dynamically, a means of dynamically controlling the beam direction, i.e., the application of beam steering, becomes essential.

[0003] Beam steering in antennas is required not only for wireless communications but also for applications such as radar, imaging, and wireless power transmission. Various beam steering methods have been devised and used, including mechanically controlling the direction of the antenna and controlling the radio waves emitted from the antenna by refracting or reflecting them with a movable lens or reflector. Phased array antennas are widely used because they have high durability and movement tracking capabilities as they do not use mechanical moving parts, and are therefore suitable for miniaturization and weight reduction.

[0004] A phased array antenna performs electronic beam steering by controlling the phase and amplitude of an RF (Radio Frequency) signal fed to each antenna element using means such as a variable delay circuit, a variable attenuator circuit, or digital signal processing connected to multiple antenna elements arranged on a line or a surface. Hereinafter, controlling the phase and amplitude will be referred to as weighting.

[0005] Phased array antennas that use analog circuitry for weighting are widely used in fifth-generation mobile communication systems and millimeter-wave wireless LAN (Local Area Network) systems that use millimeter-wave bands. In many wireless communication systems, the range in which wireless communication partners exist varies not within a two-dimensional plane but within three-dimensional space, requiring beam steering along two axes, such as azimuth and elevation. Therefore, phased array antennas require weighting for two-dimensional beam steering using a two-dimensional array antenna in which multiple antenna elements are arranged in a plane.

[0006] Non-Patent Document 1 discloses a 256-element phased array antenna for use in a fifth-generation mobile communication base station in the 28 GHz band. For example, if the radio frequency increases by approximately 10 times, such as to 300 GHz, the propagation loss in free space increases by 100 times, and tens of thousands of antenna elements would be required. At a radio frequency of 300 GHz, the free-space wavelength is 1 mm, so the spacing between multiple antenna elements is typically half the wavelength, or 0.5 mm. In this case, it is difficult to install phase shifter circuits near the antenna elements at spacings equivalent to the spacing between the multiple antenna elements. Furthermore, to configure a circuit that forms multiple beams (a multi-beam forming circuit), it is necessary to arrange the same number of phase shifters in parallel as the number of beams, which is expected to be even more difficult.

[0007] Instead of implementing phase-shift circuits according to the number of antenna elements, there is a method of using a passive circuit with a fixed phase shift amount and switching the input terminal of the passive circuit. For example, Non-Patent Document 2 discloses a method of performing two-dimensional beam steering using a passive circuit. However, the circuit needs to be assembled three-dimensionally, and implementation in high-frequency bands requires a waveguide configuration, making mass production difficult and making it difficult to accommodate multiple elements.

[0008] Non-Patent Document 3 discloses a method of converting a signal into light and weighting it using an optical circuit. As a method of weighting it using an optical circuit, Patent Document 1 discloses a three-dimensional optical circuit that performs two-dimensional beam steering using a wavelength dispersion line. Non-Patent Document 4 discloses a method that uses a loop configuration in which a phase shifter is repeatedly reused while converting the optical wavelength. Non-Patent Document 5 discloses a means for performing two-dimensional beam steering by combining a planar phase shift circuit with an FBG (Fiber Bragg Grating) reflection line whose delay time varies depending on the optical wavelength, thereby enabling beam steering in a plane perpendicular to the plane performed by the FBG reflection line in addition to one-dimensional (within one plane) beam steering performed by the planar phase shift circuit.

[0009] However, with the above-disclosed devices and methods, as the frequency increases, it becomes difficult to manufacture a three-dimensional structure and the number of components becomes enormous. To realize a high-speed wireless transmission system utilizing high-frequency bands, a large number of antenna elements are required to obtain high antenna gain in order to compensate for the large spatial propagation loss. This results in high directivity and narrow beam widths, so two-dimensional fine beam scanning is required to accurately direct the beam toward the communicating wireless device. For a single base station to accommodate many terminal stations and to achieve higher frequency utilization efficiency in wireless communication, multi-beam communication, which simultaneously uses multiple beams each transmitting a different information stream, is effective. However, when realizing a circuit for controlling the multi-beam, it is also important to simplify and miniaturize the configuration.

[0010] Non-Patent Document 6 discloses a method for configuring a multi-beam control circuit that performs two-dimensional scanning using two phase-shift circuits for one-dimensional scanning, without using a complex and large-scale phase-shift circuit for two-dimensional scanning, in order to easily realize a phase-shift circuit for simultaneous scanning of multiple beams.

[0011] Japanese Patent Application Laid-Open No. 2004-023400

[0012] H. Watanabe, S. Uga, H. Nakamizo, T. Tsutsumi, S. Shinjo, and Y. Kuriyama, “Millimeter-Wave Antenna and RF Front-End Technology for 5th Generation Mobile Communication Base Stations,” IEICE Communications Society Magazine, 2020, Vol. 14, No. 3, pp. 222-231. Dong-Hun KIM, Jiro HIROKAWA, Makoto ANDO, “One-Body 2-D Beam-Switching Butler Matrix with Waveguide Short-Slot 2-Plane Couplers,” IEICE TRANSACTIONS on Electronics, Vol. E100-C, No. 10, pp. 884-892C. Tsokos et al., “Analysis of a Multibeam Optical Beamforming Network Based on Blass Matrix Architecture,” JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 36, NO. 16, AUGUST 15, 2018. Y. Liu and J. Klamkin, “Scalable Integrated Photonics Beamforming Circuits,” Asia Communications and Photonics Conference 2020, pp. 1-3. B. Ortega, J. Mora and R. Chulia, “Optical Beamformer for 2-D Phased Array Antenna With Subarray Partitioning Capability,” in IEEE Photonics Journal, vol. 8, no. 3, pp. 1-9, June 2016. Honoka Ito, Ken Hiraga, and Riichi Kudo, “Study on 2-D Beamforming Using Optical Phase Control,” IEICE Technical Report, vol. 123, pp. 13-17, May 11, 2023.

[0013] In the method described in Non-Patent Document 6, to simultaneously control multiple beams, the RF signals of each beam must be converted into optical signals with different frequencies. In this circuit, photoelectric conversion is performed to generate a radio signal using a "photomixing method," which generates an RF signal using the beat of two waves of light. To generate multiple RF signals, multiple pairs of two-wave light, each arranged in a different optical wavelength band, are input into a single photomixer, and multiple RF signals can be output from the photomixer. However, unnecessary RF components other than the desired frequency are also generated at the same time, which can cause problems such as interference with other wireless communications. The simplest way to prevent the generation of such unwanted RF components when generating RF signals from optical signals is to provide an RF filter circuit that passes only the desired frequency band.

[0014] However, when the wavelength multiplexing interval is small, unwanted RF components occur in frequency bands close to the desired frequency band. Therefore, a filter with a steep cutoff characteristic is required, which makes it difficult to configure such a filter circuit or increases the scale and size of the circuit. When configuring a circuit compatible with multiple RF bands, the passband of the RF filter circuit must be dynamically changed each time the RF band used is changed, which can greatly increase the likelihood of configuring and controlling such a circuit. Other than Non-Patent Document 6, there are few methods disclosed for inputting multiple optical wavelength-multiplexed signals into a photomixer to generate RF signals for multiple radio beams, and there are almost no documents disclosing methods for suppressing unwanted RF components in this process. Thus, conventional techniques have had the problem of being unable to suppress the generation of unwanted RF components with a simple circuit configuration.

[0015] In view of the above circumstances, an object of the present invention is to provide a technique capable of suppressing the generation of unnecessary RF components with a simple circuit configuration.

[0016] One aspect of the present invention is a photoelectric conversion device comprising: a first light source that outputs a plurality of light beams for generating a first electrical signal; a second light source that outputs a plurality of light beams for generating a second electrical signal; a plurality of light beams having a polarization plane or polarization rotation direction different from that of the plurality of light beams output from the first light source, which are obtained based on the second light source; and one or more photomixers that generate one or more radio frequency signals based on the plurality of light beams output from the first light source.

[0017] According to the present invention, it is possible to suppress the generation of unnecessary RF components with a simple circuit configuration.

[0018] FIG. 1 is a diagram illustrating an example of the configuration of a conventional photoelectric conversion device; FIG. 2 is a diagram illustrating an example of light generated by an optical signal source; FIG. 3 is a diagram illustrating an example of an RF signal output by a photomixer; FIG. 4 is a diagram illustrating an example of the configuration of a conventional photoelectric conversion device; FIG. 5 is a diagram illustrating an example of the frequency of a pair of optical signal sources; FIG. 6 is a diagram illustrating an example of an RF signal output by a photomixer; FIG. 7 is a diagram illustrating an example of the configuration of a conventional photoelectric conversion device; ｗｄｍ 9 is a diagram showing an example of the frequency of a pair of optical signal sources when the wavelength multiplexing interval (f ｗｄｍ ) is the RF frequency f ＲＦ1 is a diagram illustrating an example of the frequencies of a pair of optical signal sources when the spacing between the optical frequencies of the optical signal pairs of each radio beam is smaller than f. FIG. 2 is a diagram illustrating an example of the configuration of a photoelectric conversion device according to the first embodiment. FIG. 3 is a conceptual diagram of the frequency spectrum of each optical signal source. FIG. 4 is a diagram illustrating an example of an RF signal output from a photomixer in the configuration according to the first embodiment. FIG. 5 is a diagram illustrating an example of a plurality of optical signals input to a photomixer in the second embodiment. FIG. 6 is a diagram illustrating an example of an RF signal output from a photomixer in the second embodiment. FIG. 7 is a diagram illustrating an effect of the photoelectric conversion device according to the second embodiment. FIG. 8 is a diagram illustrating an example of the configuration of a photoelectric conversion device according to the third embodiment. FIG. 9 is a diagram illustrating another example of the photoelectric conversion device according to the third embodiment. FIG. 10 is a diagram illustrating an example of the configuration of a photoelectric conversion device according to the fourth embodiment. FIG. 11 is a diagram illustrating an overview of processing performed by the photoelectric conversion device according to the fourth embodiment. FIG. 12 is a diagram illustrating another example of the photoelectric conversion device according to the fourth embodiment. FIG. 13 is a diagram illustrating an example of an RF signal output from a photomixer in the fourth embodiment. ｗｄｍ FIG. 25 is a diagram illustrating an example when the optical wavelength division multiplexing ratio is smaller than 1 / 2. FIG. 26 is a diagram illustrating a problem in a conventional photoelectric conversion device under the conditions shown in FIG. 24. FIG. 27 is a diagram illustrating the effect of the photoelectric conversion device in the fourth embodiment under the conditions shown in FIG. 24. FIG. 28 is a diagram illustrating an outline of processing performed by the photoelectric conversion device in the fifth embodiment. FIG. 29 is a diagram illustrating an outline of processing performed by the photoelectric conversion device in the fifth embodiment. FIG. 30 is a diagram illustrating an outline of processing performed by the photoelectric conversion device in the fifth embodiment. FIG. 31 is a diagram illustrating optical wavelength multiplexing of three optical signal pairs under different conditions to generate three radio beams. FIG. 32 is a diagram illustrating the effect of the photoelectric conversion device in the fifth embodiment. FIG. 33 is a diagram illustrating an example of the configuration of a photoelectric conversion device in the sixth embodiment. FIG. 34 is a diagram illustrating an example of the configuration of a photoelectric conversion device in the seventh embodiment.

[0019] Hereinafter, an embodiment of the present invention will be described with reference to the drawings.

[0020] (Problems with the Related Art) Before describing the configuration of the present invention, the problems with the related art will be described in detail. As an orthodox method for inputting a plurality of optical wavelength-multiplexed signals into a photomixer to generate RF signals of a plurality of radio beams, there is a technique of suppressing unnecessary RF components using an RF filter.

[0021] Fig. 1 is a diagram showing an example of the configuration of a conventional photoelectric conversion device 1. The photoelectric conversion device 1 includes an information signal source 10, multiple optical signal sources 11, and a photomixer 13. The photoelectric conversion device 1 includes two optical signal sources 11-1 and 11-2 as the multiple optical signal sources 11. The photoelectric conversion device 1 shown in Fig. 1 is the simplest form in which an RF signal is generated by photomixing.

[0022] The information signal source 10 outputs data to be transmitted to the optical signal source 11-2. The optical signal sources 11-1 and 11-2 each have a frequency f ＲＦ An example of the light generated by the optical signal sources 11-1 and 11-2 is shown in FIG. 2. The light from the optical signal source 11-2 is modulated by the information signal source 10, and the amplitude or phase, or both, of the light fluctuate over time in accordance with the information signal. The light output from the optical signal source 11-1 and the light output from the optical signal source 11-2 are multiplexed by a multiplexer (not shown) and input to the photomixer 13. The photomixer 13 outputs an RF signal, which is an electrical signal, based on the multiple input lights. An example of the RF signal output by the photomixer 13 is shown in FIG. 3. As shown in FIG. 3, the frequency of the RF signal output by the photomixer 13 is f ＲＦ In this specification, this f ＲＦ is called the desired RF component. When this photoelectric conversion device 1 is used in a wireless transmitter, the output of the photomixer 13 is radiated into space from a wireless transmission antenna via an RF amplifier or the like.

[0023] Next, a configuration in which a plurality of information signals are input to a single photomixer to generate a plurality of RF signals will be described. Fig. 4 is a diagram showing an example configuration of a conventional photoelectric conversion device 1a. The photoelectric conversion device 1a includes a plurality of information signal sources 10, a plurality of optical signal sources 11, and a photomixer 13. The photoelectric conversion device 1a includes two information signal sources 10-1 and 10-2 as the plurality of information signal sources 10, and four optical signal sources 11-1-1, 11-1-2, 11-2-1, and 11-2-2 as the plurality of optical signal sources 11.

[0024] 4, the light output from the pair of optical signal sources 11-1-1 and 11-1-2 and the light output from the pair of optical signal sources 11-2-1 and 11-2-2 are multiplexed by a multiplexer (not shown) and input to a photomixer 13. The pair of optical signal sources 11-1-1 and 11-1-2 is a light source that outputs multiple light waves for generating a first radio beam. In the following description, the pair of optical signal sources 11-1-1 and 11-1-2 is a light source that outputs two waves of light. The pair of optical signal sources 11-2-1 and 11-2-2 is a light source that outputs multiple (two or more) waves of light for generating a second radio beam. In the following description, the pair of optical signal sources 11-2-1 and 11-2-2 is a light source that outputs two waves of light. The photomixer 13 outputs an RF signal, which is an electrical signal, based on the input light. The frequencies of the pair of optical signal sources 11-1-1 and 11-1-2 are f ＲＦ The frequencies of the pair of optical signal sources 11-2-1 and 11-2-2 are f ＲＦ Far away.

[0025] The frequencies of the optical signal sources 11-1-1 and 11-2-1 are f ｗｄｍ That is, the two optical signal pairs are separated by f ｗｄｍ In this specification, this optical frequency interval is called an optical wavelength multiplexing interval. An example of an RF signal output by the photomixer 13 is shown in FIG. 6. As shown in FIG. 6, in the same way as in FIG. 3, the frequency f ＲＦIn addition to generating an RF signal at frequency f ｗｄｍ -f ＲＦ , f ｗｄｍ , f ｗｄｍ +f ＲＦ ) are also generated. These RF components at other frequencies are undesired RF components and are referred to herein as undesired RF components.

[0026] The generation of unwanted RF components causes radio wave interference with other wireless communications, adversely affecting the speed, capacity, etc. of wireless communications. One example of a configuration for suppressing unwanted RF components is a photoelectric conversion device 1b that includes an RF filter 14 on the output side of a photomixer 13, as shown in FIG. 7. FIG. 7 is a diagram showing an example configuration of a conventional photoelectric conversion device 1b. The photoelectric conversion device 1b includes a plurality of information signal sources 10, a plurality of optical signal sources 11, a photomixer 13, and an RF filter 14. The photoelectric conversion device 1b has the same configuration as the photoelectric conversion device 1a, except for the addition of the RF filter 14.

[0027] The RF filter 14 is provided on the output side of the photomixer 13. The RF filter 14 passes only the frequency band of the desired RF component, or suppresses only the frequency band of the unwanted RF component. By providing the RF filter 14, the photoelectric conversion device 1b can suppress the unwanted RF component (frequency f ｗｄｍ -f ＲＦ , f ｗｄｍ , f ｗｄｍ +f ＲＦ ) output can be suppressed.

[0028] As shown in FIG. 9, the wavelength multiplexing interval f ｗｄｍ In FIG. 9, the wavelength multiplexing interval f ｗｄｍ is the RF frequency f ＲＦ 10 shows the RF signal output in this case. In the example shown in FIG. 10, unwanted RF components are generated in a frequency band closer to the desired RF component than in the case of FIG. 6. In particular, unwanted RF components (frequency f ｗｄｍ -f ＲＦ ) occurs, and it is assumed that it may be difficult to remove it with the RF filter 14.

[0029] Furthermore, as shown in FIG. 11, the wavelength multiplexing interval (f ｗｄｍ ) is the RF frequency f ＲＦ It is also assumed that the wavelength multiplexing interval f is smaller than the desired RF component. In many cases, it can be expected that unwanted RF components with frequencies higher than the desired RF component are unlikely to be generated due to the operating frequency limit of the photomixer 13. On the other hand, unwanted RF components with frequencies lower than the desired RF component cannot be expected to be suppressed due to the operating frequency limit characteristic of the photomixer 13, so an RF filter 14 with high suppression characteristics must be inserted. In this way, when the wavelength multiplexing interval f ｗｄｍ If is small, the frequency bands of the desired RF component and the unwanted RF component are close to each other, so an RF filter with steep frequency characteristics is required, which may make it difficult to configure such a filter circuit or may result in a large circuit scale or size.In addition, it may be possible that unwanted RF components are generated in all or part of the band of the RF signal, making it impossible to remove only the unwanted RF components using the RF filter 14.

[0030] Furthermore, when configuring a circuit compatible with multiple RF frequency bands by utilizing the ultra-wideband nature of RF generation in photomixing, it is necessary to dynamically change the passband of the RF filter circuit every time the RF band being used is changed, and it is highly anticipated that configuring and controlling such a circuit will be difficult.In addition, when configuring an array antenna in which many antenna elements are arranged at a narrow antenna pitch, such as in the millimeter wave band or terahertz band, it is desirable to place a photomixer directly below the antenna elements, but it is highly anticipated that placing an RF filter for each antenna element will be difficult from the perspective of mounting area size.

[0031] Therefore, a method is desired for suppressing unnecessary RF components without using the RF filter 14 or by using only an RF filter with a simple configuration (such an RF filter often does not have a steep frequency characteristic).

[0032] Therefore, in the present invention, the photoelectric conversion device prevents the polarization of a pair of optical signals that generate unwanted RF components from matching (or, in some cases, orthogonalizing) in the optical circuit, thereby suppressing the generation of "swells" caused by the pair of optical signals that generate unwanted RF components. This suppresses the generation of unwanted RF components. Specific configurations for realizing such operations will be described below using several embodiments as examples.

[0033] (First Embodiment) Fig. 12 is a diagram showing an example of the configuration of a photoelectric conversion device 100 according to the first embodiment. The photoelectric conversion device 100 includes a plurality of information signal sources 10, a plurality of optical signal sources 11, a photomixer 13, and a polarization rotator 15. The photoelectric conversion device 100 includes two information signal sources 10-1 and 10-2 as the plurality of information signal sources 10, and four optical signal sources 11-1-1, 11-1-2, 11-2-1, and 11-2-2 as the plurality of optical signal sources 11. The configuration of the photoelectric conversion device 100 is similar to that of the photoelectric conversion device 1a shown in Fig. 4, except for the polarization rotator 15. The following description will focus on the differences from the photoelectric conversion device 1a.

[0034] The photoelectric conversion device 100 generates an RF signal of one radio beam by photoelectrically converting the optical signals of the two radio beams using the photomixer 13. Each of the optical signal sources 11-1-1, 11-1-2, 11-2-1, and 11-2-2 outputs light of the same polarization state (for example, linearly polarized or circularly polarized). In the following description, it is assumed, as an example, that each of the optical signal sources 11-1-1, 11-1-2, 11-2-1, and 11-2-2 outputs light of the same polarization state (for example, vertically linearly polarized).

[0035] The RF signal of the first radio beam is generated at the difference frequency between the optical signal source 11-1-1 and the optical signal source 11-1-2, and the RF signal of the second radio beam is generated at the difference frequency between the optical signal source 11-2-1 and the optical signal source 11-2-2. An information signal to be transmitted in the radio beam may be superimposed on the optical signal. In this case, in FIG. 12, the light output from the optical signal source 11-1-2 is modulated by the electrical signal output from the information signal source 10-1, and the light output from the optical signal source 11-2-2 is modulated by the electrical signal output from the information signal source 10-2. The pair of the optical signal source 11-1-1 and the optical signal source 11-1-2 is one aspect of a first light source, and the pair of the optical signal source 11-2-1 and the optical signal source 11-2-2 is one aspect of a second light source.

[0036] The polarization rotator 15 changes the polarization state of the light output from each of the optical signal sources 11-2-1 and 11-2-2. For example, the polarization rotator 15 rotates the polarization of the light output from each of the optical signal sources 11-2-1 and 11-2-2 by 90 degrees. As a result, the polarization state of the light output from each of the optical signal sources 11-2-1 and 11-2-2 differs from the polarization state of the light output from each of the optical signal sources 11-1-1 and 11-1-2. In this way, the polarization rotator 15 adjusts the plane of polarization of the light output from each of the optical signal sources 11-2-1 and 11-2-2 so that it differs from the plane of polarization of the light output from each of the optical signal sources 11-1-1 and 11-1-2.

[0037] A conceptual diagram of the frequency spectrum of each of the optical signal sources 11-1-1, 11-1-2, 11-2-1, and 11-2-2 is shown in Fig. 13. As shown in Fig. 13, the difference frequency between the optical signal pair of the first radio beam (the optical signals of the optical signal source 11-1-1 and the optical signal source 11-1-2) and the optical signal pair of the second radio beam (the optical signals of the optical signal source 11-2-1 and the optical signal source 11-2-2) is f ＲＦ The optical frequency interval between the optical signal pair of the first radio beam and the optical signal pair of the second radio beam is f ｗｄｍ is.

[0038] The optical signal pair of the first radio beam is vertically polarized, but the optical signal pair of the second radio beam passes through the polarization rotator 15, and the polarization is rotated by the polarization rotator 15. As a result, the optical signal pair of the second radio beam becomes horizontally polarized. Although the polarization rotator 15 is explicitly provided in FIG. 12 , the photoelectric conversion device 100 does not include the polarization rotator 15, and it is sufficient that a predetermined polarization plane is formed in the optical signal source 11. Note that, although the polarization of the second radio beam is rotated in this embodiment, the polarization of the first radio beam may also be rotated. In this case, the polarization rotator 15 is connected to the optical signal sources 11-1-1 and 11-1-2. This also applies to the subsequent embodiments.

[0039] As shown in Figure 13, four optical signals are input to the photomixer 13. Of these, the desired RF component is generated by the swell of the optical signal source 11-1-1 and the optical signal source 11-1-2, and the desired RF component is generated by the swell of the optical signal source 11-2-1 and the optical signal source 11-2-2. Combinatorially, there are four combinations of optical signals that generate unwanted RF, and there are three frequency bands (f ｗｄｍ , f ｗｄｍ -f ＲＦ , f ｗｄｍ +f ＲＦ ) generates an unwanted RF component. However, in the combination of optical signals that generates unwanted RF, the polarization planes of the two optical signals are orthogonal to each other, so undulations are unlikely to occur and the unwanted RF component is suppressed to a very small value. Therefore, the RF signal output from the photomixer 13 is as shown in FIG.

[0040] The photoelectric conversion device 100 configured as described above makes it possible to suppress the generation of unnecessary RF components with a simple circuit configuration. Specifically, the photoelectric conversion device 100 includes a plurality of optical signal sources 11-1-1 and 11-1-2 that output a plurality of light beams for generating a first radio beam, a plurality of optical signal sources 11-2-1 and 11-2-2 that output a plurality of light beams for generating a second radio beam, a polarization rotator 15 that adjusts the polarization planes of the light output from the plurality of optical signal sources 11-2-1 and 11-2-2 so that they are different from the polarization planes of the light output from the plurality of optical signal sources 11-1-1 and 11-1-2, and a photomixer 13 that generates an RF signal based on the light output from the plurality of optical signal sources 11-1-1 and 11-1-2 and the light whose polarization planes have been adjusted by the polarization rotator 15.

[0041] This makes it possible to eliminate the need for an RF filter circuit on the photomixer output side to suppress the transmission of unwanted RF components in wireless communication devices that use photoelectric conversion to generate RF signals using the "photomixing method," which generates RF signals by beating two waves of light. Furthermore, a simpler circuit can be achieved, and the generation of unwanted RF components can be suppressed. Because the generation of unwanted RF components can be suppressed, interference with other wireless communications can be suppressed, and performance such as the speed and capacity of wireless communications can be improved.

[0042] Second Embodiment In the first embodiment, the difference frequency between the optical signal pair of the first radio beam and the optical signal pair of the second radio beam is the same. In the second embodiment, a difference frequency between the optical signal pair of the first radio beam and the optical signal pair of the second radio beam is different.

[0043] The configuration of the photoelectric conversion device 100 in the second embodiment is the same as that in the first embodiment. The following mainly describes the differences from the first embodiment. When configuring a wireless transmitter using the photoelectric conversion device 100, it is not often expected that multiple signals will be transmitted at the same frequency from a single antenna, but it is quite possible that two wireless signals to be transmitted on adjacent RF channels will be generated by separate pairs of optical signals, as shown in the second embodiment.

[0044] The photoelectric conversion device 100 in the second embodiment generates RF signals of two radio beams by photoelectrically converting the optical signals of the two radio beams in the photomixer 13. The RF signal of the first radio beam has a difference frequency f between the optical signal source 11-1-1 and the optical signal source 11-1-2. ＲＦ The RF signal of the second radio beam is generated by the difference frequency f between the optical signal source 11-2-1 and the optical signal source 11-2-2. ＲＦ +Δf. This causes the frequency of the pair of optical signal sources 11-1-1 and 11-1-2 to be f ＲＦ The frequencies of the pair of optical signal sources 11-2-1 and 11-2-2 are f ＲＦ +Δf away.

[0045] Therefore, a plurality of optical signals as shown in Fig. 15 are input to the photomixer 13. In the case without the present invention, two desired RF components (f ＲＦ , f ＲＦ +Δf) and four unnecessary RF components (f ｗｄｍ , f ｗｄｍ -f ＲＦ , f ｗｄｍ +Δf, f ｗｄｍ +f ＲＦ +Δf) (four components) are output, but in the case of the present invention, the four unnecessary RF components are suppressed as shown in FIG.

[0046] According to the photoelectric conversion device 100 of the second embodiment configured as described above, the same effect as in the first embodiment can be obtained even when the difference frequency of the pair of optical signal sources 11-1-1 and 11-1-2 is different from the difference frequency of the pair of optical signal sources 11-2-1 and 11-2-2.

[0047] Third Embodiment In a third embodiment, a case where the present invention is used in a wireless transmitter having a multi-beam array antenna will be described.

[0048] FIG. 18 is a diagram illustrating an example configuration of a photoelectric conversion device 100a according to the third embodiment. The photoelectric conversion device 100a includes a plurality of information signal sources 10, a plurality of optical signal sources 11, a plurality of photomixers 13, a polarization rotator 15, and a multi-beam weighting circuit 16. The photoelectric conversion device 100a includes two information signal sources 10-1 and 10-2 as the plurality of information signal sources 10, four optical signal sources 11-1-1, 11-1-2, 11-2-1, and 11-2-2 as the plurality of optical signal sources 11, and four photomixers 13-1 to 13-4 as the plurality of photomixers 13. The configuration of the photoelectric conversion device 100a differs from that of the photoelectric conversion device 100 according to the first embodiment in that it includes a plurality of photomixers 13 and a new multi-beam weighting circuit 16. The following description will focus on the differences from the photoelectric conversion device 100 according to the first embodiment.

[0049] The photoelectric conversion device 100a is provided in a radio transmitter having a multi-beam array antenna connected to each of a plurality of photomixers 13. The radio transmitter forms a plurality of radio beams at the same RF frequency and transmits the beams via different multi-beam array antennas.

[0050] In the photoelectric conversion device 100a, the light sources for the first radio beam (optical signal sources 11-1-1 and 11-1-2) and the light sources for the second radio beam (optical signal sources 11-2-1 and 11-2-2) are each connected to a multi-beam weighting circuit 16. The multi-beam weighting circuit 16 applies a time delay to the input light, thereby weighting each radio beam with a different RF phase.

[0051] The multi-beam weighting circuit 16 is composed of a plurality of dividers 17, a plurality of phase shifters 18, and a plurality of combiners 19. The multi-beam weighting circuit 16 includes dividers 17-1 and 17-2 as the plurality of dividers 17, eight phase shifters 18-1-1 to 18-1-4 and 18-2-1 to 18-2-4 as the plurality of phase shifters 18, and four combiners 19-1 to 19-4 as the plurality of combiners 19. Hereinafter, when there is no need to distinguish between the phase shifters 18-1-1 to 18-1-4, they will be referred to as phase shifter 18-1, and when there is no need to distinguish between the phase shifters 18-2-1 to 18-2-4, they will be referred to as phase shifter 18-2.

[0052] The distributors 17 distribute the input light. Each distributor 17 distributes light in the same number as the number of antenna elements. For example, distributor 17-1 multiplexes the light output from optical signal sources 11-1-1 and 11-1-2 and then distributes the resulting light to each phase shifter 18-1. For example, distributor 17-2 multiplexes the light output from optical signal sources 11-2-1 and 11-2-2 after polarization rotation output from polarization rotator 15 and then distributes the resulting light to each phase shifter 18-2.

[0053] Phase shifter 18 applies a time delay, a so-called true-time delay, to the input light, corresponding to the phase shift of the radio signal. Phase shifter 18-1 applies a time delay corresponding to the phase shift of the radio signal to the light distributed by, for example, distributor 17-1. Phase shifter 18-2 applies a time delay corresponding to the phase shift of the radio signal to the light distributed by, for example, distributor 17-2.

[0054] The multiplexer 19 multiplexes multiple light beams that have been time delayed by the phase shifters 18. The multiplexer 19-1 multiplexes the light beams that have been time delayed by the phase shifters 18-1-1 and 18-2-1, and outputs the result to the photomixer 13-1. The multiplexer 19-2 multiplexes the light beams that have been time delayed by the phase shifters 18-1-2 and 18-2-2, and outputs the result to the photomixer 13-2. The multiplexer 19-3 multiplexes the light beams that have been time delayed by the phase shifters 18-1-3 and 18-2-3, and outputs the result to the photomixer 13-3. The multiplexer 19-4 multiplexes the light to which a time delay has been applied by the phase shifter 18-1-4 and the light to which a time delay has been applied by the phase shifter 18-2-4, and outputs the multiplexed light to the photomixer 13-4.

[0055] The photomixer 13 photoelectrically converts the light multiplexed by the multiplexer 19 to generate an RF signal for a radio beam. The photomixer 13-1 photoelectrically converts the light multiplexed by the multiplexer 19-1 to generate an RF signal for a radio beam. The photomixer 13-2 photoelectrically converts the light multiplexed by the multiplexer 19-2 to generate an RF signal for a radio beam. The photomixer 13-3 photoelectrically converts the light multiplexed by the multiplexer 19-3 to generate an RF signal for a radio beam. The photomixer 13-4 photoelectrically converts the light multiplexed by the multiplexer 19-4 to generate an RF signal for a radio beam.

[0056] As described above, in the photoelectric conversion device 100a, the signal for the first radio beam and the signal for the second radio beam are multiplexed by the multiplexer 19 and then input to the photomixers 13 connected to each antenna element ANT. The optical signals input to each photomixer 13 are, for example, as shown in Fig. 13, and the output of each photomixer 13 is, for example, as shown in Fig. 14. Each radio beam is weighted differently by the multi-beam weighting circuit 16, so that radio waves can be emitted in different directions.

[0057] The configuration shown in Fig. 18 may also be a method of applying a phase shift in the optical domain in a phase shifter 18 as shown in Fig. 19. Fig. 19 is a diagram showing another example of a photoelectric conversion device 100a according to the third embodiment. The photoelectric conversion device 100a shown in Fig. 19 performs phase weighting in the optical domain in a multi-beam weighting circuit 16 only on the light output from optical signal sources 11-1-2 and 11-2-2. Note that the polarization of the light output from optical signal source 11-2-2 is rotated by a polarization rotator 15-1 before being input to the multi-beam weighting circuit 16.

[0058] The photoelectric conversion device 100a then combines the weighted optical signals output from the multi-beam weighting circuit 16 with the light output from the optical signal sources 11-1-1 and 11-2-1 at the same phase shift, and inputs the combined signals to each photomixer 13. The polarization of the light output from the optical signal source 11-2-1 is rotated by the polarization rotator 15-2 before being combined. This allows a predetermined weighting to be applied to the RF signal output from the photomixer 13. Even with this configuration, the optical signals input to each photomixer 13 are, for example, as shown in FIG. 13, and the output of each photomixer 13 is, for example, as shown in FIG. 14.

[0059] According to the photoelectric conversion device 100a configured as described above, it is possible to suppress unnecessary RF components in both a multi-beam array antenna that performs true-time delay type weighting as shown in FIG. 18 and a multi-beam array antenna that performs phase weighting in the optical domain as shown in FIG. 19.

[0060] (Fourth Embodiment) In the above embodiments, the configuration in which the number of radio beams is two has been described. In contrast, the technique of the present invention can also be applied to cases in which the number of radio beams is three or more. Therefore, in the fourth embodiment, a case in which the number of multiplexed signals is three or more (three or more beams) will be described.

[0061] 20 is a diagram showing an example of the configuration of a photoelectric conversion device 100b according to the fourth embodiment. The photoelectric conversion device 100b includes a plurality of information signal sources 10, a plurality of optical signal sources 11, a photomixer 13, a plurality of polarization rotators 15, and a polarization plane control circuit 20. The photoelectric conversion device 100b includes N (N is an integer of 3 or greater) information signal sources 10-1 to 10-N as the plurality of information signal sources 10, 2N optical signal sources 11 as the plurality of optical signal sources 11, and N photomixers 13-1 to 13-N as the plurality of polarization rotators 15.

[0062] As shown in FIG. 20 , the photoelectric conversion device 100 b includes N polarization rotators 15 for rotating the plane of polarization of the optical signal of each radio beam. The polarization rotators 15 set the polarization plane for changing the polarization state of the input light in accordance with a user instruction or an instruction from a polarization plane control circuit 20. The polarization plane control circuit 20 controls the amount of rotation by which the polarization of the light input to each polarization rotator 15 is rotated. For example, if the allocation of optical signals for each radio beam is arranged on the frequency axis in order of beam number as shown in FIG. 21 , the polarization plane control circuit 20 may control the amount of rotation of each polarization rotator 15 so that the polarization plane alternates between vertical and horizontal directions from the lowest frequency. The polarization plane control circuit 20 is one aspect of a control circuit.

[0063] In the configuration of the photoelectric conversion device 100b, for example, when the allocation of optical signals for each radio beam is arranged on the frequency axis in the order of beam numbers as shown in FIG. 21, a polarization rotator 15 may be inserted as shown in FIG. 22, which tilts the polarization plane of light for the radio beam corresponding to the even-numbered information signal source 10 by 90 degrees, so that the polarization plane alternates between vertical and horizontal directions from the lowest frequency.

[0064] The optical frequency interval of each optical signal pair of each radio beam is f ｗｄｍ The difference frequency (= RF frequency) of each optical signal pair is f ＲＦ In this case, the RF signal output from the photomixer 13 is as shown in FIG. 23, and the unnecessary RF components are suppressed. ＲＦ For each radio beam, the optical frequency intervals of the optical signal pairs are all f ｗｄｍ In such a case, an RF signal output from a photoelectric conversion device not equipped with the functions of the present invention (for example, a conventional photoelectric conversion device) will have a large number of unwanted RF components (10 frequency components in this case) in a frequency band that is lower than the desired RF component and difficult to suppress by the RF filter 14, as shown in FIG.

[0065] However, when the photoelectric conversion device 100b is used, although some unwanted RF components cannot be suppressed as shown in Fig. 26, many (six in this case) unwanted RF components can be suppressed. In particular, unwanted RF components are suppressed at frequencies close to the desired RF component, and unwanted RF components occur in relatively distant frequency bands, so these can be expected to be suppressed by inserting an RF filter, by the characteristics of the output matching circuit of the photomixer 13, by the frequency-dependent characteristics of the antenna, etc. Here, a case is shown in which optical signal pairs of many radio beams are arranged at equal intervals on the frequency axis, but the intervals do not necessarily have to be equal.

[0066] The photoelectric conversion device 100b configured as described above can achieve the same effects as the first embodiment even when generating three or more beams. In particular, the photoelectric conversion device 100b controls the polarization planes of adjacent optical signals to be different from each other. This makes it possible to suppress unnecessary RF components in the RF signal output from the photomixer 13.

[0067] Fifth Embodiment In the fifth embodiment, a method will be described in which optical wavelength-multiplexed (M+1) signal pairs are grouped to set the tilt of the polarization plane to 180 degrees / (M+1).

[0068] As shown in FIGS. 27 and 28, ＲＦ <f ｗｄｍ In this case, it is preferable that the inclinations of the planes of polarization between adjacent optical signal pairs differ by 90° as described above. ＲＦ <f ｗｄｍ In this case, the unwanted RF component is ＲＦ ) occurs only on the high frequency side. Therefore, even if the photoelectric conversion device 100 according to the present invention is not used, it is difficult to occur in the first place due to the characteristics of the photomixer 13, or it is easy to suppress with an RF low-pass filter. However, as shown in FIG. ＲＦ <f ｗｄｍ <2f ＲＦ In the case of (1), unwanted RF components are generated at frequencies lower than the desired RF components, and therefore the photoelectric conversion device according to the present invention is highly effective.

[0069] As shown in FIG. 29, when the wavelength multiplexing interval is smaller than the RF frequency, for example, f ｗｄｍ <f ＲＦ <2f ｗｄｍ In this case, as shown in the fourth embodiment, there are unnecessary RF components that can be suppressed and unnecessary RF components that cannot be suppressed at all by simply making the polarization planes orthogonal between adjacent optical signal pairs. ｗｄｍ -f ＲＦ " cannot be suppressed. While there is a method of suppressing the unsuppressable unnecessary RF components using an RF filter, there is also a method of uniformly suppressing the unnecessary RF components to some extent. In the fifth embodiment, such a method will be described.

[0070] Figures 30A to 30C show how three optical signal pairs are optically wavelength-multiplexed to generate three radio beams. Figures 31A to 31C show RF signals output when the optical signals shown in Figures 30A to 30C are input to the photomixer 13. Figure 30A shows an example where all signals have the same plane of polarization (no suppression), Figure 30B shows an example where adjacent signals have different planes of polarization (for example, the planes of polarization differ by 90°), and Figure 30C shows an example where adjacent signals have different planes of polarization (for example, the planes of polarization differ by 60°).

[0071] When three optical signal pairs are input to the photomixer 13 with the same polarization plane as in Fig. 30A, three unwanted RF components are generated combinatorially as shown in Fig. 31A. Here, the explanation is given excluding unwanted RF components with higher frequencies than the desired RF component (because they can be easily suppressed by the characteristics of an RF filter, etc.). If the polarization planes between adjacent optical signal pairs are made orthogonal (as explained in the previous embodiments) as in Fig. 30B, the two unwanted RF components are significantly suppressed, leaving one unwanted RF component, as shown in Fig. 31B.

[0072] In Figure 30(C), the polarization plane of the optical signal pair with the lowest frequency is set to 0°, the polarization plane of the optical signal pair with the next lowest frequency is set to 60°, and the polarization plane of the optical signal pair with the highest frequency is set to 120°. In this case, the output RF signal is as shown in Figure 31(C). Because the polarization planes of the three optical signal pairs are offset by 60° from each other, the amplitude of the undulations caused by the interference of the two waves of light is multiplied by cos 60°, or halved. Therefore, the three unwanted RF components can be uniformly suppressed by 6 dB. In some cases, it is better to suppress all unwanted RF components to a certain extent uniformly using the frequency-dependent characteristics of the RF filter and antenna provided at the photomixer output, rather than having some unwanted RF components that cannot be suppressed as in Figure 31(B). In such cases, this method is effective.

[0073] 30 and 31 show the case where there are three optical signal pairs, but in general, (M+1) optical signal pairs that are wavelength-multiplexed are grouped, where M is a natural number, and the tilt of the polarization plane is set to 180° / (M+1). When more than M+1 signal pairs are wavelength-multiplexed, the tilt of the polarization plane is set cyclically for each of the (M+1) signal pairs. M is determined as a natural number that satisfies the following equation (1).

[0074] (M-1)f ｗｄｍ <f ＲＦ <Mfwdm...Formula (1)

[0075] As explained above, f ｗｄｍ represents the optical wavelength multiplexing interval, and f ＲＦ represents a desired RF frequency. The device configuration for realizing the above processing may be the photoelectric conversion device 100b shown in FIG.

[0076] The photoelectric conversion device 100b according to the fifth embodiment configured as described above can achieve the same effects as those of the first embodiment. In particular, the photoelectric conversion device 100b can uniformly suppress the three unwanted components by 6 dB.

[0077] Sixth Embodiment In the sixth embodiment, dynamic control of the polarization plane according to the optical signal frequency allocation will be described.

[0078] The optical wavelengths of the optical signal pairs corresponding to each radio beam are not necessarily assigned in a fixed manner, and the optical wavelengths assigned to the optical signal pairs for a certain radio beam may be dynamically changed. For example, the following cases may be assumed.

[0079] (When the optical wavelength is dynamically changed) (1) When the optical wavelength is changed due to the need for optical wavelength management in a radio on fiber (RoF) section outside the wireless transmitter device. (2) When the multi-beam weighting circuit 16 has optical wavelength dependency, and in order to minimize the impact on the quality of the wireless beam formed due to this dependency, the optical wavelength to be assigned is changed depending on the direction and communication distance of the wireless beam.

[0080] Therefore, in the sixth embodiment, this problem is solved by a photoelectric conversion device 100c as shown in Fig. 32. Fig. 32 is a diagram showing an example of the configuration of the photoelectric conversion device 100c in the sixth embodiment. The photoelectric conversion device 100c includes a plurality of information signal sources 10, a plurality of optical signal sources 11, a photomixer 13, a plurality of polarization rotators 15, and a polarization plane control circuit 20. The photoelectric conversion device 100c includes N information signal sources 10-1 to 10-N as the plurality of information signal sources 10, 2N optical signal sources 11 as the plurality of optical signal sources 11, and N photomixers 13-1 to 13-N as the plurality of polarization rotators 15.

[0081] 32, the 2N optical signal sources 11 included in the photoelectric conversion device 100c are included in an optical wavelength information collection circuit 21. The optical wavelength information collection circuit 21 collects information indicating the frequency of the optical signal output from each optical signal source 11. The optical wavelength information collection circuit 21 outputs information indicating the collected information indicating the frequency of the optical signal to the polarization plane control circuit 20 as "optical wavelength information."

[0082] 32 , the 2N optical signal sources 11 do not have to be included in the optical wavelength information collection circuit 21. In this case, the photoelectric conversion device 100c may be configured to include an optical wavelength detection circuit having a function of detecting the wavelength of the optical signal being used on the path through which light from the 2N optical signal sources 11 passes, and the optical wavelength information collection circuit 21 may collect information indicating the frequency of the optical signal from the optical wavelength detection circuit, or the optical wavelength information collection circuit 21 may be installed outside the photoelectric conversion device 100c and collect information indicating the frequency of the optical signal from an input from a user or from another optical wavelength management device.

[0083] The polarization plane control circuit 20 controls the polarization rotation characteristics (e.g., the amount of rotation or the direction of polarization rotation) of each of the N polarization rotators 15 based on the optical wavelength information output from the optical wavelength information collection circuit 21, the number of radio beams (= the number of optical signal pairs = the number of wavelengths multiplexed), the wavelength multiplexing interval, etc. Specifically, the polarization plane control circuit 20 performs the following control. Note that the specific control is the same as in the fifth embodiment.

[0084] (Control 1) The planes of polarization are made orthogonal between pairs of optical signals with adjacent frequencies. (Control 2) The planes of polarization are made 180° / (M+1), where M is an integer of 2 or greater.

[0085] The polarization plane control circuit 20 outputs information for controlling the polarization rotation characteristics of the polarization rotator 15-1 as first polarization setting information to the polarization rotator 15-1. Similarly, the polarization plane control circuit 20 outputs information for controlling the phase rotation characteristics of the polarization rotators 15-2 to 15-N as second polarization setting information to Nth polarization setting information to the polarization rotators 15-2 to 15-N, respectively.

[0086] Each polarization rotator 15 rotates the plane of polarization of the input light based on the value indicated by the polarization setting information output from the polarization plane control circuit 20 .

[0087] According to the photoelectric conversion device 100c configured as above, even when the optical wavelength is dynamically changed, it is possible to suppress the generation of unnecessary RF components with a simple circuit configuration.

[0088] Seventh Embodiment In the seventh embodiment, dynamic control of the polarization plane according to the generation state of the unwanted RF component will be described.

[0089] 33 is a diagram showing an example of the configuration of a photoelectric conversion device 100d according to the seventh embodiment. The photoelectric conversion device 100d includes a plurality of information signal sources 10, a plurality of optical signal sources 11, a photomixer 13, a plurality of polarization rotators 15, a polarization plane control circuit 20, and an unwanted RF component monitoring circuit 22. The photoelectric conversion device 100d includes N information signal sources 10-1 to 10-N as the plurality of information signal sources 10, 2N optical signal sources 11 as the plurality of optical signal sources 11, and N photomixers 13-1 to 13-N as the plurality of polarization rotators 15.

[0090] The unwanted RF component monitoring circuit 22 monitors the level of unwanted RF components in the signal output from the output terminal of the photomixer 13. The unwanted RF component monitoring circuit 22 outputs the monitoring result to the polarization plane control circuit 20 as "unwanted RF component information." The polarization plane control circuit 20 controls the polarization rotation characteristics of the N polarization rotators 15 based on the unwanted RF component information output from the unwanted RF component monitoring circuit 22. In this case, the polarization plane control circuit 20 may perform control such that unwanted RF components that are difficult to suppress by an RF filter or antenna installed on the output side of the polarization rotator 15 are preferentially suppressed so as to minimize the level of the unwanted RF components. This control may be combined with the control in the fifth embodiment.

[0091] The polarization plane control circuit 20 outputs information for controlling the polarization rotation characteristics of the polarization rotator 15-1 based on the unwanted RF component information as first polarization setting information to the polarization rotator 15-1. Similarly, the polarization plane control circuit 20 outputs information for controlling the phase rotation characteristics of each of the polarization rotators 15-2 to 15-N based on the unwanted RF component information to the polarization rotators 15-2 to 15-N as second polarization setting information to Nth polarization setting information.

[0092] The photoelectric conversion device 100d configured as described above further includes an unwanted RF component monitoring circuit 22 that monitors the level of unwanted RF components in the signal output from the output terminal of the photomixer 13, and the polarization plane control circuit 20 controls the polarization rotation characteristics of the N polarization rotators 15 based on the unwanted RF component information output from the unwanted RF component monitoring circuit 22. This makes it possible to control the polarization rotators 15 after grasping the level of unwanted RF components based on the actually generated RF signal. Therefore, it is possible to efficiently suppress unwanted RF components.

[0093] (Variation 1 related to the first to seventh embodiments) In each of the above-described embodiments, the case where the multiple optical signal sources 11 are each independent light sources has been described. However, all or some of the multiple optical signal sources 11 may be configured as light sources that emit light of a desired optical frequency using a circuit that converts the optical frequency (specifically, an optical modulator) based on light generated by a common seed light source.

[0094] (Variation 2 Related to the First to Seventh Embodiments) In the configurations of the above-described embodiments that use two orthogonal linearly polarized light (for example, vertically polarized light and horizontally polarized light), two orthogonal circularly polarized light (right-handed circularly polarized light and left-handed circularly polarized light) may also be used. In this case, each optical signal source 11 outputs light with the same circularly polarized light rotation direction. As an example, in the first embodiment, the polarization rotator 15 adjusts the rotation direction of the circularly polarized light output by each of the optical signal sources 11-2-1 and 11-2-2 so that it differs from the rotation direction of the circularly polarized light output by each of the optical signal sources 11-2-1 and 11-2-2. Note that the polarization rotator 15 may also adjust the rotation direction of the circularly polarized light output by each of the optical signal sources 11-2-1 and 11-2-2 so that it differs from the rotation direction of the circularly polarized light output by each of the optical signal sources 11-2-1 and 11-2-2. A specific means for adjusting the rotation direction of the circularly polarized light to be different is to use a circular polarizer as the polarization rotator 15 to convert right-handed polarized light to left-handed polarized light, or left-handed polarized light to right-handed polarized light. Here, the first embodiment has been described as an example, but the same applies to other embodiments using two orthogonal linearly polarized lights (e.g., vertically polarized light and horizontally polarized light). In an embodiment including a polarization plane control circuit 20, the polarization plane control circuit 20 controls the polarization rotation direction of light input to each polarization rotator 15. For example, in the fourth embodiment, when the beams are arranged on the frequency axis in order of beam number as shown in FIG. 21 , the polarization plane control circuit 20 may control the rotation direction of the circularly polarized light for the radio beams corresponding to even-numbered information signal sources 10, so that the rotation direction alternates from low frequency to right-handed polarized light, left-handed polarized light, or left-handed polarized light, right-handed polarized light.

[0095] (Variation 3 Related to the First to Seventh Embodiments) In each of the above-described embodiments, the multiple optical signal sources 11 are used as light sources for forming radio beams, but the multiple optical signal sources 11 may be used for purposes other than forming radio beams. Therefore, the multiple optical signal sources 11 are light sources that output multiple light beams for generating at least electrical signals.

[0096] (Variation 4 Related to the First to Seventh Embodiments) In each of the above-described embodiments, a configuration has been described in which the polarization rotator 15 is used to adjust the polarization plane or polarization rotation direction of the plurality of light beams output from the optical signal source 11. Alternatively, a configuration may be adopted in which the polarization plane or polarization rotation direction of the plurality of light beams output from the optical signal source 11 is adjusted without using the polarization rotator 15 by devising an arrangement of the optical signal source 11. Taking the first embodiment as an example, at the start of processing, the optical signal sources 11-2-1 and 11-2-2 may be arranged differently from the optical signal sources 11-1-1 and 11-1-2 (for example, the optical signal sources 11-2-1 and 11-2-2 may be arranged at an angle of 90 degrees), thereby making the polarization plane of the plurality of light beams output from the optical signal sources 11-2-1 and 11-2-2 different from the polarization plane of the plurality of light beams output from the optical signal sources 11-1-1 and 11-1-2. This also applies to the above-described circularly polarized light. In the above example, optical signal sources 11-2-1 and 11-2-2 are arranged at an angle of 90°, but they may be arranged at any angle as long as the polarization planes of the multiple light beams output from optical signal sources 11-2-1 and 11-2-2 are different in polarization plane or polarization rotation direction from the polarization planes of the multiple light beams output from optical signal sources 11-1-1 and 11-1-2.

[0097] Other embodiments may be configured similarly, and for example, when there are N optical signal sources 11 as in the fourth embodiment, the planes of polarization or the directions of polarization rotation of the multiple light beams may be adjusted by arranging the even-numbered optical signal sources 11 differently from the odd-numbered optical signal sources 11. For example, in the fifth embodiment, the planes of polarization or the directions of polarization rotation of the multiple light beams may be adjusted by adjusting the arrangement of each of the N optical signal sources 11.

[0098] Although an embodiment of the present invention has been described in detail above 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.

[0099] The present invention can be applied to a photoelectric conversion device that generates an RF signal by photomixing.

[0100] 10, 10-1 to 10-N... Information signal source, 11, 11-1-1, 11-1-2, 11-2-1, 11-2-2, 11-3-1, 11-3-2, 11-N-1, 11-N-2... Optical signal source, 13, 13-1 to 13-4... Photomixer, 15, 15-1 to 15-N... Polarization rotator, 16... Multi-beam weighting circuit, 17, 17-1 to 17-2... Distributor, 18, 18-1-1 to 18-1-4, 18-2-1 to 18-2-4... Phase shifter, 19, 19-1 to 19-4... Multiplexer, 20... Polarization plane control circuit, 21... Optical wavelength information collection circuit, 22... Unwanted RF component monitoring circuit, 100, 100a, 100b, 100c, 100d...photoelectric conversion device

Claims

1. A photoelectric conversion device comprising: a first light source that outputs a plurality of lights for generating a first electrical signal; a second light source that outputs a plurality of lights for generating a second electrical signal; a plurality of lights having a polarization plane or a polarization rotation direction different from that of the plurality of lights output from the first light source based on the second light source; and one or more photomixers that generate one or more radio frequency signals based on the plurality of lights output from the first light source.

2. The photoelectric conversion device according to claim 1, further comprising: one or more polarization rotors provided inside the second light source to output the plurality of lights whose polarization plane or polarization rotation direction is adjusted to be different from that of the plurality of lights output from the first light source from the second light source, or provided outside the second light source to adjust at least the polarization plane or polarization rotation direction of the plurality of lights output from the second light source to be different from that of the plurality of lights output from the first light source; and the one or more photomixers generate one or more radio frequency signals based on the plurality of lights output from the first light source and the plurality of lights whose polarization plane or polarization rotation direction is adjusted by the one or more polarization rotors.

3. The photoelectric conversion device according to claim 1 or 2, wherein the first light source outputs a plurality of lights whose frequencies of the plurality of lights are separated by a first frequency; the second light source outputs lights whose frequencies of the plurality of lights are separated by the first frequency; and the photomixer generates one radio frequency signal based on the plurality of lights output from the first light source and the plurality of lights having a polarization plane or a polarization rotation direction different from that of the plurality of lights output from the first light source.

4. The photoelectric conversion device according to claim 1 or 2, wherein the first light source outputs a plurality of lights whose frequencies of the plurality of lights are separated by a first frequency; the second light source outputs lights whose frequencies of the plurality of lights are separated by a second frequency different from the first frequency; and the photomixer generates a plurality of radio frequency signals based on the plurality of lights output from the first light source and the plurality of lights having a polarization plane or a polarization rotation direction different from that of the plurality of lights output from the first light source.

5. The photoelectric conversion device according to claim 1 or 2, further comprising a weighting circuit that provides a time delay to at least one of the plurality of lights output from the first light source and at least one of the plurality of lights having a polarization plane or a polarization rotation direction different from that of the plurality of lights output from the first light source; The one or more photomixers are a plurality of photomixers; The plurality of photomixers generate the one or more radio frequency signals based on the plurality of lights provided with a time delay by the weighting circuit.

6. The photoelectric conversion device according to claim 1, further comprising a plurality of Nth light sources (N is an integer of 3 or more) that output a plurality of lights for generating an Nth (N is an integer of 3 or more) electrical signal; Each of the plurality of Nth light sources, or the polarization plane or the polarization rotation direction of the plurality of lights is adjusted by adjusting the arrangement of the even-numbered light sources; Or The polarization plane or the polarization rotation direction of the plurality of lights output from each of the plurality of Nth light sources or the even-numbered light sources is adjusted by a plurality of polarization rotators that adjust the polarization plane or the polarization rotation direction of the light.

7. The photoelectric conversion device according to claim 6, further comprising a control circuit that controls the polarization state of the light input to the plurality of polarization rotators.

8. The control circuit of the photoelectric conversion device according to claim 7 controls the polarization rotation characteristics of the plurality of polarization rotators based on the optical wavelength information collected by an optical wavelength information collection circuit that collects the optical wavelength information indicating the frequency of the optical signal output from each optical signal source.

9. The photoelectric conversion device according to claim 7, further comprising a monitoring circuit that monitors the level of unnecessary radio frequency components in the radio frequency signal generated by the one or more photomixers; The control circuit controls the polarization rotation characteristics of the plurality of polarization rotators based on the monitoring result of the monitoring circuit.

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