Photoelectric conversion device

Abstract

This photoelectric conversion device comprises: a plurality of information signal sources that output data to be transmitted; a plurality of optical signal sources that output a plurality of modulated light beams generated on the basis of the data output from the plurality of optical signal sources, and a plurality of unmodulated light beams; a photo-mixer that generates a plurality of wireless signals by photo-mixing the plurality of modulated light beams and the plurality of non-modulated light beams output from the plurality of optical signal sources; and a setting unit that determines a wavelength multiplexing interval or a maximum allowable signal bandwidth for a desired wireless signal so as to satisfy a non-interference condition, which is that the frequency band of the desired wireless signal and the frequency band of an unnecessary frequency component do not overlap and which is derived from the relationship between the wireless signal frequency, the signal bandwidth and the wavelength multiplexing interval, and the setting unit then sets the determined wavelength multiplexing interval in the plurality of optical signal sources, or sets the determined maximum allowable signal bandwidth in the plurality of information signal sources.　

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] JP 2004-023400 A JP 2007-165956 A

[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,” in Journal of Lightwave Technology, vol. 36, no. 16, pp. 3354-3372, 15 Aug.15, 2018.Y. Liu and J. Klamkin, “Scalable Integrated Photonics Beamforming Circuits”, 2020 Asia Communications and Photonics Conference (ACP) and International Conference on Information Photonics and Optical Communications (IPOC), 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. Ito, Hiraga, and Kudo, "Study on 2D Beamforming Using Optical Phase Control," IEICE Technical Report, Volume: 123, Issue: 33 (CQ2023 1-7), Pages: 13-17 (WEB ONLY), Published: 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 methods have had the problem of being unable to reduce the interference effect on the desired RF signal caused by unwanted RF components generated when multiple radio signals are simultaneously generated by photomixing.

[0015] In view of the above circumstances, the present invention aims to provide a technology that can reduce the influence of interference on a desired RF signal caused by unnecessary RF components that are generated when multiple radio signals are generated simultaneously by photomixing.

[0016] One aspect of the present invention is an optoelectronic conversion device comprising: a plurality of information signal sources that output data to be transmitted; a plurality of optical signal sources that output a plurality of modulated light beams and a plurality of unmodulated light beams that are generated based on the data output from the plurality of information signal sources; a photomixer that generates a plurality of radio signals by photomixing the plurality of unmodulated light beams output from the plurality of optical signal sources with the plurality of modulated light beams; and a setting unit that determines a wavelength multiplexing interval or a maximum allowable signal bandwidth for the desired radio signal so as to satisfy a non-interference condition that the frequency band of the desired radio signal does not overlap with the frequency band of unnecessary frequency components, the non-interference condition being derived from the relationship between the frequency, signal bandwidth, and wavelength multiplexing interval of the radio signal, and sets the determined wavelength multiplexing interval to the plurality of optical signal sources or sets the maximum allowable signal bandwidth to the plurality of information signal sources.

[0017] According to the present invention, it is possible to reduce the influence of interference on a desired RF signal caused by unwanted RF components that are generated when multiple radio signals are generated collectively by photomixing.

[0018] 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 frequencies 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 frequencies of a pair of optical signal sources when the wavelength multiplexing interval W is small; FIG. 8 is a diagram illustrating an example of an RF signal output by a photomixer; FIG. 9 is a diagram illustrating an example of the configuration of a conventional photoelectric conversion device; FIG. 10 is a diagram illustrating an example of a plurality of light beams input to a photomixer; FIG. 11 is a diagram illustrating the relationship between the center frequency and frequency bandwidth of spurious emissions that may be generated by photomixing; FIG. 12 is a diagram illustrating an example of a plurality of light beams input to a photomixer; FIG. 13 is a diagram illustrating an example of an RF signal output by a photomixer; FIG. 14 is a diagram illustrating an example of a plurality of light beams input to a photomixer; 1 is a diagram illustrating an example of an RF signal output by a photomixer. FIG. 2 is a diagram illustrating an example of a plurality of light beams input to a photomixer. 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 a calculation of the relationship of the maximum bandwidth that satisfies the spurious non-interference condition. FIG. 5 is a diagram illustrating an example of the configuration of a photoelectric conversion device in a first embodiment. FIG. 6 is a flowchart illustrating the flow of processing by a photoelectric conversion device in a modified example of the first embodiment. FIG. 7 is a diagram illustrating an example of the configuration of a photoelectric conversion device in a second embodiment. FIG. 8 is a flowchart illustrating the flow of processing by a photoelectric conversion device in the second embodiment. FIG. 9 is a diagram illustrating an example of the configuration of a photoelectric conversion device in a third embodiment. FIG. 10 is a diagram illustrating the basic configuration of a photoelectric conversion device used in a fourth embodiment. FIG. 11 is a diagram for explaining an overview of processing performed by a photoelectric conversion device in the fourth embodiment. FIG. 12 is a diagram illustrating another example of the basic configuration of a photoelectric conversion device used in the fourth embodiment. FIG. 13 is a diagram illustrating an example of a plurality of light beams input to a photomixer in the fourth embodiment. FIG. 14 is a diagram illustrating an example of an RF signal output by a photomixer in the fourth embodiment.FIG. 10 is a diagram showing an example of an RF signal output by a photomixer in the fourth embodiment. FIG. 11 is a diagram showing an example of a plurality of light beams input to a photomixer in the fourth embodiment. FIG. 12 is a diagram showing an example of an RF signal output by a photomixer in the fourth embodiment. FIG. 13 is a diagram showing an example of an RF signal output by a photomixer in the fourth embodiment. FIG. 14 is a diagram showing an example of a calculation of the relationship of the maximum bandwidth that satisfies the spurious non-interference condition. FIG. 15 is a diagram showing a configuration example (part 1) of a photoelectric conversion device in the fourth embodiment. FIG. 16 is a diagram showing a configuration example (part 2) of a photoelectric conversion device in the fourth embodiment.

[0019] An embodiment of the present invention will be described below with reference to the drawings. (Problems with the Related Art) First, before describing the configuration of the present invention, problems with the related art will be described in detail. FIG. 1 is a diagram showing an example 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 of device that generates an RF signal by photomixing.

[0020] The information signal source 10 outputs data to be transmitted (hereinafter referred to as "transmission data") 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 optical signal sources 11-1 and 11-2 is shown in FIG. 2. The light from optical signal source 11-2 is modulated based on the transmission data output from information signal source 10, and the amplitude and / or phase of the light varies over time in accordance with the information signal. The light output from optical signal source 11-1 and the light output from optical signal source 11-2 are multiplexed by a multiplexer (not shown) and input to photomixer 13. For example, multiplexed light obtained by multiplexing the unmodulated light output from optical signal source 11-1 and the modulated light output from optical signal source 11-2 is input to photomixer 13.

[0021] The photomixer 13 outputs an RF signal, which is an electrical signal, based on the input multiplexed light. 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.

[0022] (Issues When the Number of Radio Beams is Two) 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.

[0023] As shown in FIG. 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 the photomixer 13.

[0024] The pair of optical signal source 11-1-1 and optical signal source 11-1-2 is a light source that outputs a plurality of light waves (two or more waves) for generating a first radio beam. In the following explanation, the pair of optical signal source 11-1-1 and optical signal source 11-1-2 will be described as a light source that outputs two light waves.

[0025] The pair of optical signal source 11-2-1 and optical signal source 11-2-2 is a light source that outputs a plurality of light waves (two or more waves) for generating a second radio beam. In the following description, the pair of optical signal source 11-2-1 and optical signal source 11-2-2 will be described as a light source that outputs two light waves.

[0026] 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.

[0027] As shown in Fig. 5, the frequencies of the optical signal source 11-1-1 and the optical signal source 11-2-1 are separated from each other by the wavelength multiplexing interval W. In other words, the two pairs of optical signals are separated from each other by the wavelength multiplexing interval W. Fig. 6 shows an example of the RF signal output by the photomixer 13. As shown in Fig. 6, the frequency f ＲＦ In addition to generating an RF signal of the frequency W−f ＲＦ , W, W+f ＲＦ ) are also generated. These RF components at other frequencies are undesired RF components and are referred to herein as undesired RF components.

[0028] 4 to 6, it is assumed that an RF signal is generated by the beat of unmodulated light and upper sideband light, but the same applies to the case where an RF signal is generated by the beat of unmodulated light and lower sideband light. In the configuration shown in FIG. 4, four frequency components are photomixed by the photomixer 13, so the optical beat is ４ C ２ = 6. Therefore, when RF signals of two radio beams are generated by photomixing, four spurious components are also generated.

[0029] The spurious components are generated by dividing the wavelength multiplexing interval W by the RF frequency f ＲＦ It is assumed that spurious components will not be generated due to the characteristics of the photomixer 13 by setting this frequency sufficiently large relative to twice the operating frequency of the photodiode (it is not output because it is higher than the operating frequency of the photodiode). In addition, it is possible to suppress spurious components by providing an RF band filter (RF filter) on the output side of the photomixer 13, or by having the operating frequency band of the antenna have filter characteristics.

[0030] (Problems when wavelength multiplexing interval W is small) When wavelength multiplexing optical signals of many radio beams in an optical circuit or an optical fiber section, many optical signals must be wavelength multiplexed within a limited optical wavelength band. ＲＦ As shown in FIG. 7, it is possible that the wavelength multiplexing interval W cannot be made sufficiently large compared to twice the RF frequency f ＲＦ , the RF signal output from the photomixer 13 is as shown in FIG. 8. As shown in FIG. 8, the unwanted RF component (W−f ＲＦ ) is the RF frequency f ＲＦ In such cases, it may be difficult to remove the signal using an RF filter.

[0031] In addition, the wavelength multiplexing interval W is the RF frequency f ＲＦ It is also possible that the frequency is smaller than . Unwanted low-frequency RF components can be removed by an RF filter. However, when forming a radio beam in multiple RF bands, the problem of a complex configuration, such as dynamically changing the pass characteristics of the RF filter, can be expected.

[0032] (Issues When the Number of Radio Beams is Three or More) In the above example, for simplicity, the case where the number of radio beams is two is described. However, the following describes the case where the number of radio beams is three or more. Note that, again, the case where an RF signal is generated by the beat of unmodulated light and upper sideband light is described as an example. FIG. 9 is a diagram showing an example of the configuration of a conventional photoelectric conversion device 1b. The photoelectric conversion device 1b includes multiple information signal sources 10, multiple optical signal sources 11, and a photomixer 13. The photoelectric conversion device 1b includes M (M is an integer equal to or greater than 3) information signal sources 10-1, 10-2, ..., 10-M as the multiple information signal sources 10, and 2M optical signal sources 11-1-1, 11-1-2, 11-2-1, 11-2-2, ..., 11-M-1, 11-M-2 as the multiple optical signal sources 11.

[0033] 9, the pair of optical signal source 11-m-1 (1≦m≦M) and optical signal source 11-m-2 is a light source that outputs a plurality of light waves (two or more waves) for generating the m-th radio beam. Note that in the following explanation, the pair of optical signal source 11-m-1 and optical signal source 11-m-2 will be described as a light source that outputs two light waves.

[0034] 10 shows a plurality of light beams input to the photomixer 13 in the configuration shown in FIG. 9. In the example shown in FIG. 10, N=f ＲＦ / W quotient, R = f ＲＦ / W remainder, that is, f ＲＦ = NW + R (N is a non-negative integer). Note that in this example, N = 1. FIG. 11 shows the relationship between the center frequency and frequency bandwidth of spurious signals that can be generated by photomixing. FIG. 11 shows four optical beat patterns, (1) to (4). As shown in FIG. 11, the center frequency of spurious signals can be formulated for each optical beat combination pattern using a non-negative integer n (depending on the number of beams) and wavelength multiplexing intervals W and R. Note that in optical beat pattern (1), when n = N, this is excluded because it represents the desired RF signal. Furthermore, even when the number of beams is large, all spurious components generated by two-wave optical beats are expressed by the equation shown in FIG. 11. Spurious signals whose frequency bands may overlap with those of the RF signal shown in FIG. 11 will be described in detail below.

[0035] (When an RF signal is generated using the beat of unmodulated light and upper sideband light, and (i) R<W / 2) We will organize the positional relationship between the RF signal and spurious signals on the radio frequency axis. The positional relationship between the RF signal and spurious signals on the radio frequency axis differs depending on the magnitude relationship between R and W / 2. Therefore, we will first explain the case of (i) R<W / 2 in optical beat pattern (3), and then explain the case of (ii) R>W / 2.

[0036] (i) A plurality of lights input to the photomixer 13 when R<W / 2 is shown in FIG. 12. The output of the photomixer 13 when R<W / 2 is shown in FIG. 13. In FIG. 13, when the frequency band occupied by the RF signal (f ＲＦThere are two spurious components that may overlap with the frequency (N+1)W−R: a component at frequency NW and a component at frequency (N+1)W−R.

[0037] (When an RF signal is generated by the beat of unmodulated light and upper sideband light, and (ii) R>W / 2) (ii) A plurality of lights input to the photomixer 13 in the case of R>W / 2 is shown in FIG. 14. Also, the output of the photomixer 13 in the case of R>W / 2 is shown in FIG. 15. In FIG. 15, when the occupied frequency band is the RF signal (f ＲＦ There are two spurious components that may overlap with the frequency (N+1)W: a component at frequency (N+1)W−R.

[0038] ((Generalized to Three or More Radio Beams) When an RF Signal is Generated by Beats of Unmodulated Light and Lower Sideband Light) Next, we will explain the case where an RF signal is generated by beats of unmodulated light and lower sideband light. FIG. 16 shows the multiple light beams input to the photomixer 13 in this case. Note that in this example, N = 1. FIG. 17 shows the relationship between the center frequency and frequency bandwidth of spurious signals that can be generated by photomixing. FIG. 17 shows four optical beat patterns, (1) to (4). As shown in FIG. 17, the center frequency of spurious signals can be formulated for each optical beat combination pattern using a non-negative integer n (which depends on the number of beams) and wavelength multiplexing intervals W and R. Note that in optical beat pattern (2), when n = N, this is excluded because it is the desired RF signal. Below, we will explain in detail the spurious signals whose frequency bands may overlap with those of the RF signal shown in FIG. 17.

[0039] (When an RF signal is generated by the beat of unmodulated light and lower sideband light, and (i) R<W / 2) FIG. 18 shows a plurality of lights input to the photomixer 13 in the case of (i) R<W / 2. FIG. 19 shows the output of the photomixer 13 in the case of R<W / 2. In FIG. 19, when the occupied frequency band is the RF signal (f ＲＦThe spurious components that may overlap with the frequency (N+1)W-R are the component at frequency NW and the component at frequency (N+1)W-R. The center frequency of the spurious components is mathematically the same as when an RF signal is generated by beating unmodulated light and upper sideband light.

[0040] (When an RF signal is generated by the beat of unmodulated light and lower sideband light, and (ii) when R>W / 2) FIG. 20 shows a plurality of lights input to the photomixer 13 in the case of (ii) R>W / 2. FIG. 21 shows the output of the photomixer 13 in the case of R>W / 2. In FIG. 21, when the occupied frequency band is the RF signal (f ＲＦ There are two spurious components that may overlap with the frequency (N+1)W: one at frequency (N+1)W and one at frequency (N+1)W-R. The center frequency of the spurious components is mathematically the same as when an RF signal is generated by beating unmodulated light and upper sideband light.

[0041] (Conditions for No Spurious Interference (i) When R<W / 2) Based on the above explanation, the conditions for no spurious interference will now be explained. Based on the positional relationship between the RF signal and the spurious on the radio frequency axis formulated as above, the conditions for no overlap between the frequency bands of the RF signal and the spurious (hereinafter referred to as "spurious non-interference conditions") are determined. As mentioned above, the center frequency of the spurious component that can interfere with the RF signal is the same when the RF signal is generated by the beat of unmodulated light and upper sideband light and when the RF signal is generated by the beat of unmodulated light and lower sideband light. Therefore, in the following explanation, these two cases will not be distinguished and will be described together.

[0042] Based on Figures 13 and 19, in order to prevent interference between the RF signal and two adjacent spurious components on the frequency axis (the component at frequency NW and the component at frequency (N+1)W-R), the wavelength multiplexing interval W and the signal bandwidths B and R need only satisfy the two equations shown in equation (1).

[0043]

[0044] Transforming equation (1) yields equation (2): As a result, it is sufficient that the wavelength multiplexing interval W and the signal bandwidths B and R satisfy the two equations shown in equation (2).

[0045]

[0046] (Condition for No Spurious Interference (ii) When R>W / 2) As described above, the center frequency of the spurious components that can interfere with the RF signal is the same when the RF signal is generated by the beat of unmodulated light and upper sideband light and when the RF signal is generated by the beat of unmodulated light and lower sideband light. Therefore, in the following explanation, these two cases will be described together without distinguishing between them.

[0047] Based on Figures 15 and 21, in order to prevent interference between the RF signal and two adjacent spurious components on the frequency axis (the component at frequency NW and the component at frequency (N+1)W-R), the wavelength multiplexing interval W and the signal bandwidths B and R need only satisfy the two equations shown in equation (3).

[0048]

[0049] Transforming equation (3) yields equation (4). As a result, it is sufficient that the wavelength multiplexing interval W and the signal bandwidths B and R satisfy the two equations shown in equation (4).

[0050]

[0051] As described above, in order to satisfy the spurious non-interference condition, it is sufficient to satisfy the four equations shown in the above-mentioned equations (2) and (4). As shown in the above-mentioned equations (2) and (4), the spurious non-interference condition is satisfied when the frequency f ＲＦ , is derived from the relationship between the signal bandwidth B and the wavelength multiplexing interval W. Based on the above equations (2) and (4), the maximum bandwidth (hereinafter referred to as the "maximum bandwidth") b that satisfies the condition that spurious interference does not occur with the wavelength multiplexing interval W is ｍａｘ An example of the calculation of the relationship is shown in Figure 22. In Figure 22, the dotted line L11 indicates the maximum bandwidth b ｍａｘ In FIG. 22, the RF frequency is f ＲＦIt should be noted that R is also a function of the wavelength multiplexing interval W. Also, N is a function of f ＲＦ / W quotient, R is f ＲＦ / W is the remainder.

[0052] As shown in FIG. ＲＦ / 3), the maximum bandwidth b ｍａｘ is a constant value (2f ＲＦ As shown in FIG. 22, W<(8f ＲＦ In the range of R = 0 or R = W / 2, the maximum bandwidth b ｍａｘ =0.

[0053] As described above, the spurious non-interference condition is satisfied when the center frequency of the RF signal is f ＲＦ Based on the relationship between the bandwidth B of the RF signal and the wavelength multiplexing interval W, the center frequency f of the RF signal can be calculated as shown in equations (2) and (4). ＲＦ and the bandwidth B of the RF signal are given, it is possible to calculate the range of wavelength multiplexing spacing W that satisfies the spurious non-interference condition. Below, a specific example of a configuration that suppresses unwanted RF components by calculating the range of wavelength multiplexing spacing W that satisfies the spurious non-interference condition will be described.

[0054] First Embodiment Fig. 23 is a diagram illustrating an example of the configuration of a photoelectric conversion device 100 according to a first embodiment. The photoelectric conversion device 100 generates multiple RF signals simultaneously by photomixing multiple unmodulated light beams and multiple sideband light beams that are wavelength-multiplexed at a predetermined wavelength multiplexing interval. The photoelectric conversion device 100 includes multiple information signal sources 10, multiple optical signal sources 11, a photomixer 13, and a wavelength spacing selection circuit 15. The photoelectric conversion device 100 includes the wavelength spacing selection circuit 15 in addition to the configuration shown in Fig. 9. Note that while Fig. 23 illustrates a configuration in which the photoelectric conversion device 100 generates three or more radio beams, the photoelectric conversion device 100 can also be applied to a configuration in which two radio beams are generated.

[0055] The wavelength interval selection circuit 15 selects the center frequency f of the RF signal from each information signal source 10. ＲＦ and the bandwidth B of the RF signal. The wavelength spacing selection circuit 15 then selects the center frequency f ＲＦ and information indicating the bandwidth B of the RF signal, the range of the wavelength multiplexing spacing W that satisfies the spurious non-interference condition is calculated based on the above equations (2) and (4). The wavelength spacing selection circuit 15 determines the wavelength multiplexing spacing W for each radio beam based on the calculation result, and sets the optical wavelength of each optical signal source 11 to the determined wavelength multiplexing spacing W. The wavelength spacing selection circuit 15 is one aspect of the setting unit.

[0056] When determining the wavelength multiplexing interval W in the wavelength interval selection circuit 15, for example, any of the following methods may be used. Note that the following method is an example, and other methods may also be used.

[0057] (Methods for Determining the Wavelength Multiplexing Interval W) (Method 1) Select the minimum value that satisfies the spurious non-interference condition. In this case, the optical wavelength band used can be minimized. (Method 2) Select a value that matches the grid of the optical wavelength multiplexing transmission (e.g., Dense Wavelength Division Multiplexing (DWDM) specified in ITU-T G.694.1). In this case, the device can be constructed economically by utilizing commercially available optical circuit components for optical wavelength multiplexing. (Method 3) When connecting each optical signal source 11 to the photomixer 13 via a wired or wireless optical transmission section, it is assumed that the optical wavelength assignment for each signal in that optical transmission section is predetermined. Therefore, select a value that matches the wavelength interval of that optical transmission section. In this case, optical wavelength conversion processing that causes signal degradation between the optical transmission section and the photomixer 13 is minimized, thereby minimizing degradation of wireless transmission signal quality and reducing power consumption.

[0058] Each optical signal source 11 generates light that satisfies the wavelength multiplexing interval W set by the wavelength interval selection circuit 15. The frequencies of the paired optical signal sources 11 are frequency f ＲＦ Far away.

[0059] 24 is a flowchart showing the flow of processing in the photoelectric conversion device 100 according to the first embodiment. The wavelength spacing selection circuit 15 selects the center frequency f of the RF signal obtained from each information signal source 10. ＲＦ and information indicating the bandwidth B of the RF signal (step S101). ＲＦ and the bandwidth B of the RF signal, and based on the above formulas (2) and (4), calculates the range of the wavelength multiplexing spacing W that satisfies the spurious non-interference condition (step S102). The wavelength spacing selection circuit 15 determines the wavelength multiplexing spacing W for each radio beam based on the calculation result, and sets the optical wavelength of each optical signal source 11 to the determined wavelength multiplexing spacing W (step S103).

[0060] The photoelectric conversion device 100 configured as described above includes a plurality of information signal sources 10 that output data to be transmitted, a plurality of optical signal sources 11 that output a plurality of modulated light beams and a plurality of unmodulated light beams that are generated based on the data output from the plurality of information signal sources 10, a photomixer 13 that generates a plurality of radio signals by photomixing the plurality of unmodulated light beams output from the plurality of optical signal sources 11 with the plurality of modulated light beams, and a wavelength spacing selection circuit 15 that determines a wavelength multiplexing spacing W so as to satisfy a spurious non-interference condition and sets the determined wavelength multiplexing spacing W to the plurality of optical signal sources 11.

[0061] In this way, the photoelectric conversion device 100 calculates the range of the wavelength multiplexing interval W so as to satisfy the spurious non-interference condition, determines the wavelength multiplexing interval W for each radio beam based on the calculation result, and sets the optical wavelength of each optical signal source 11 so as to achieve the determined wavelength multiplexing interval W. The range of the wavelength multiplexing interval W that satisfies the spurious non-interference condition is the range in which spurious components do not interfere with the desired RF component. This makes it possible to reduce the influence of interference on the desired RF signal caused by unwanted RF components that are generated when multiple radio signals are generated collectively by photomixing.

[0062] (Modification of First Embodiment) The photoelectric conversion device 100 may be configured as shown in Fig. 25. Fig. 25 is a diagram showing an example of the configuration of a photoelectric conversion device 200 in a modification of the first embodiment. The photoelectric conversion device 200 generates multiple RF signals collectively by photomixing multiple unmodulated light beams and multiple sideband light beams that are wavelength-multiplexed at a predetermined wavelength multiplexing interval.

[0063] The photoelectric conversion device 200 includes a plurality of information signal sources 10, a plurality of optical signal sources 11, a photomixer 13, a wavelength spacing selection circuit 15, and a plurality of optical wavelength changing circuits 16. In addition to the configuration shown in Fig. 23 , the photoelectric conversion device 200 includes M optical wavelength changing circuits 16-1-1, 16-1-2, 16-2-1, 16-2-2, ..., 16-M-1, 16-M-2 as the plurality of optical wavelength changing circuits 16. Note that, although Fig. 25 shows a configuration in which the photoelectric conversion device 200 generates three or more radio beams, the photoelectric conversion device 200 can also be applied to a configuration in which two radio beams are generated.

[0064] An optical transmission section constructed by wired optical fiber or optical wireless communication is provided between each optical signal source 11 and each optical wavelength conversion circuit 16. Fig. 25 shows a configuration assuming that each optical wavelength conversion circuit 16 and photomixer 13 are provided at a location physically distant from the optical signal source 11, such as an analog RoF (Radio-over-Fiber) base station.

[0065] The wavelength spacing selection circuit 15 acquires information about the optical wavelengths used by each optical signal source 11 from each optical signal source 11. Based on the acquired information about the optical wavelengths used by each optical signal source 11, the wavelength spacing selection circuit 15 observes the optical wavelengths used in the RoF or optical wireless section. This allows the wavelength spacing selection circuit 15 to determine how much to change the wavelength of the optical signal input to the photomixer 13. The wavelength spacing selection circuit 15 instructs each optical wavelength conversion circuit 16 on the converted wavelength.

[0066] The optical wavelength changing circuits 16 convert the wavelength of light transmitted from the optical signal source 11 in response to instructions from the wavelength spacing selection circuit 15. For example, the optical wavelength changing circuits 16 convert the wavelength of light transmitted from the paired optical signal source 11 into a wavelength that can generate a high-frequency band RF signal by mixing. Furthermore, each optical wavelength changing circuit 16 converts the wavelength to a wavelength different from the others. The wavelength after conversion by each optical wavelength changing circuit 16 is instructed by the wavelength spacing selection circuit 15. The optical wavelength changing circuits 16 are configured using, for example, an optical modulator.

[0067] By configuring in this manner, even in a configuration in which the optical signal source 11 and the photomixer 13 are physically far apart, as in analog RoF, it is possible to reduce the influence of interference on the desired RF signal caused by unnecessary RF components that are generated when multiple radio signals are generated simultaneously by photomixing.

[0068] Second Embodiment In the first embodiment, a configuration was shown in which the photoelectric conversion device sets the optical wavelength of each optical signal source 11 so as to reduce the influence of interference caused by unwanted RF components on the desired RF signal based on the range of wavelength multiplexing interval W that satisfies the spurious non-interference condition. In contrast, in the second embodiment, a configuration will be described in which the photoelectric conversion device limits the symbol rate of each information signal source so as to reduce the influence of interference caused by unwanted RF components on the desired RF signal based on the maximum signal bandwidth that satisfies the spurious non-interference condition.

[0069] FIG. 26 illustrates an exemplary configuration of a photoelectric conversion device 100a according to the second embodiment. The photoelectric conversion device 100a simultaneously generates multiple RF signals by photomixing multiple unmodulated light beams and multiple sideband light beams that are wavelength-multiplexed at a predetermined wavelength multiplexing interval. The photoelectric conversion device 100a includes multiple information signal sources 10, multiple optical signal sources 11, a photomixer 13, and a maximum signal bandwidth calculation circuit 17a. While FIG. 26 illustrates a configuration in which the photoelectric conversion device 100a generates three or more radio beams, the photoelectric conversion device 100a can also be configured to generate two radio beams. The photoelectric conversion device 100a differs from the photoelectric conversion device 100 in configuration in that it does not include a wavelength spacing selection circuit 15 and multiple optical wavelength changing circuits 16 and in that it newly includes a maximum signal bandwidth calculation circuit 17a. The following description focuses on the differences from the photoelectric conversion device 100.

[0070] The maximum signal bandwidth calculation circuit 17a calculates the center frequency f of the RF signal from each information signal source 10. ＲＦ Furthermore, the maximum signal bandwidth calculation circuit 17a acquires information on the wavelength multiplexing interval W from each optical signal source 11. For example, the maximum signal bandwidth calculation circuit 17a acquires information on the wavelength multiplexing interval W set in each optical signal source 11. The maximum signal bandwidth calculation circuit 17a calculates the central frequency f ＲＦ Using the information on the wavelength multiplexing interval W of each optical signal source 11, the maximum signal bandwidth b that satisfies the spurious non-interference condition is calculated based on the above equations (2) and (4). ｍａｘ Based on the calculation result, the maximum signal bandwidth calculation circuit 17a determines a signal bandwidth B for each radio beam and limits the symbol rate of each information signal source 10. For example, based on the determined signal bandwidth B for each radio beam, the maximum signal bandwidth calculation circuit 17a adjusts the symbol rate of each information signal source 10 so that it is within the signal bandwidth B. The maximum signal bandwidth calculation circuit 17a is one aspect of the setting unit.

[0071] 27 is a flowchart showing the flow of processing in the photoelectric conversion device 100a according to the second embodiment. The maximum signal bandwidth calculation circuit 17a calculates the center frequency f ＲＦand information indicating the wavelength multiplexing interval W obtained from each optical signal source 11 (step S201). ＲＦ and information indicating the wavelength multiplexing interval W, the maximum signal bandwidth b that satisfies the spurious non-interference condition is calculated based on the above formulas (2) and (4). ｍａｘ (Step S202). The maximum signal bandwidth calculation circuit 17a determines the signal bandwidth B for each radio beam based on the calculation result, and limits the symbol rate of each information signal source 10 (Step S203).

[0072] According to the photoelectric conversion device 100a configured as above, the maximum signal bandwidth b is set to satisfy the spurious non-interference condition. ｍａｘ The signal bandwidth B for each radio beam is determined based on the calculation result, and the symbol rate of each information signal source 10 is limited. The maximum signal bandwidth b that satisfies the spurious non-interference condition is ｍａｘ is the bandwidth in which spurious components do not interfere with the desired RF component. Therefore, it is possible to reduce the influence of interference on the desired RF signal caused by unwanted RF components that are generated when multiple radio signals are generated simultaneously by photomixing.

[0073] (Modification of the Second Embodiment) The information signal source 10 may be provided in the photoelectric conversion device 100a, or may be provided on a network connected to the photoelectric conversion device 100a.

[0074] In the first embodiment, the optical signal source 11 is configured to set the optical wavelength of each optical signal source 11 so as to reduce the influence of interference caused by unwanted RF components on a desired RF signal based on the range of the wavelength multiplexing interval W that satisfies the spurious non-interference condition. In contrast, in the third embodiment, a configuration will be described in which the optical signal source 11 is configured to optimize the selection of the wavelength interval depending on the quality of the RF signal output from the photomixer.

[0075] Third Embodiment FIG. 28 illustrates a configuration example of a photoelectric conversion device 100b according to a third embodiment. The photoelectric conversion device 100b simultaneously generates multiple RF signals by photomixing multiple unmodulated light beams and multiple sideband light beams that are wavelength-multiplexed at a predetermined wavelength multiplexing interval. The photoelectric conversion device 100b includes multiple information signal sources 10, multiple optical signal sources 11, a photomixer 13, a wavelength spacing selection circuit 15b, multiple optical wavelength tuning circuits 16, a signal distributor 18b, and an RF signal quality evaluation circuit 19b. While the photoelectric conversion device 100b includes multiple optical wavelength tuning circuits 16, as in the photoelectric conversion device 200, it may also be configured without multiple optical wavelength tuning circuits 16, as in the first embodiment. While FIG. 28 illustrates a configuration in which the photoelectric conversion device 100b generates three or more radio beams, the photoelectric conversion device 100b may also be configured to generate two radio beams.

[0076] The signal distributor 18b distributes the RF signal output from the photomixer 13 into two paths. The RF signal distributed into the two paths by the signal distributor 18b is output to an RF signal quality determination circuit 19b and a subsequent functional unit (e.g., an antenna).

[0077] The RF signal quality determination circuit 19b receives the RF signal distributed by the signal distributor 18b. The RF signal quality determination circuit 19b monitors the quality of the input RF signal and outputs information related to the RF signal quality to the wavelength spacing selection circuit 15b. For example, the RF signal quality determination circuit 19b monitors the level of a desired RF signal to determine whether the desired RF signal is being affected by spurious interference. As information related to the RF signal quality, the RF signal quality determination circuit 19b outputs information indicating the level of the desired RF signal to the wavelength spacing selection circuit 15b.

[0078] The wavelength spacing selector 15b performs the same processing as in the first embodiment or the modified example of the first embodiment. Furthermore, the wavelength spacing selector 15b updates the optical wavelengths of each optical signal source 11 and the optical wavelength tuning circuit 16 based on the information on the RF signal quality output from the RF signal quality judgment circuit 19b. For example, the wavelength spacing selector 15b updates the optical wavelengths of each optical signal source 11 and the optical wavelength tuning circuit 16 based on the information on the RF signal quality so that the desired RF signal is not affected by spurious interference. Note that the initial value of the wavelength multiplexing interval W may be set to a value that satisfies the spurious non-interference condition.

[0079] At this time, if the amount of signal degradation exceeds a threshold due to an inappropriate wavelength multiplexing interval W, the wavelength spacing selection circuit 15b adjusts the wavelength multiplexing interval W, or if the amount of signal degradation is significantly below the threshold, adjusts the wavelength multiplexing interval W based on any of (Method 1) to (Method 3) shown in the first embodiment, where the threshold is the error vector magnitude (EVM).

[0080] According to the photoelectric conversion device 100b configured as above, it is possible to obtain the same effects as those of the first embodiment.

[0081] Furthermore, the photoelectric conversion device 100b determines the quality of the RF signal generated by the photomixer 13 and sets the optical wavelength according to the determination result. That is, the photoelectric conversion device 100b readjusts the optical wavelengths of the optical signal source 11 and the optical wavelength conversion circuit 16. This allows the wavelength multiplexing interval W to be further optimized.

[0082] Fourth Embodiment In a fourth embodiment, a configuration for controlling a photoelectric conversion device having a configuration for changing the polarization state of light so as to satisfy a spurious non-interference condition will be described.

[0083] First, the basic configuration of a photoelectric conversion device 100c used in the fourth embodiment will be described. FIG. 29 is a diagram showing the basic configuration of the photoelectric conversion device 100c used in the fourth embodiment. The photoelectric conversion device 100c shown in FIG. 29 includes a plurality of information signal sources 10, a plurality of optical signal sources 11, a photomixer 13, a plurality of polarization rotators 20, and a polarization plane control circuit 21c. The photoelectric conversion device 100c includes M information signal sources 10-1 to 10-M as the plurality of information signal sources 10, 2M optical signal sources 11-1-1, 11-1-2, 11-2-1, 11-2-2, ..., 11-M-1, 11-M-2 as the plurality of optical signal sources 11, and M polarization rotators 20-1 to 20-M as the plurality of polarization rotators 20.

[0084] 29, the pair of optical signal source 11-m-1 and optical signal source 11-m-2 is a light source that outputs a plurality of light waves (two or more waves) for generating the m-th radio beam. In the following explanation, the pair of optical signal source 11-m-1 and optical signal source 11-m-2 will be explained as a light source that outputs two light waves.

[0085] 29 , the photoelectric conversion device 100c includes M polarization rotators 20 for rotating the polarization plane of the optical signal of each radio beam. The polarization rotators 20 set the polarization plane according to, for example, a user instruction or an instruction from a polarization plane control circuit 21c. The polarization plane control circuit 21c controls the amount of rotation by which each polarization rotator 20 rotates the polarization of the input light.

[0086] In the configuration of the photoelectric conversion device 100c, 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. 30, it is preferable to insert a polarization rotator 20 that tilts the polarization plane of light for the radio beam corresponding to the even-numbered information signal source 10 by 90 degrees, as in the photoelectric conversion device 100d shown in FIG. 31, so that the polarization plane alternates between vertical and horizontal from the low frequency as shown in FIG.

[0087] With the above configuration, some unwanted RF components can be suppressed in the RF signal output from the photomixer 13 .

[0088] (When R<W / 2 and N=even) In this embodiment, the polarization planes are alternately arranged to be orthogonal by 90 degrees (for example, the polarization planes alternate between vertical and horizontal) starting from the low-frequency optical signal. Therefore, the relationship between the RF signal and spurious signals on the frequency axis differs when N is an even number and when N is an odd number. Therefore, we first determine the positional relationship between the RF signal and spurious signals on the radio frequency axis when R<W / 2 and N=even.

[0089] As described above, the conditions for spurious interference with an RF signal are the same when an RF signal is generated using the beat of unmodulated light and upper sideband light as when an RF signal is generated using the beat of unmodulated light and lower sideband light. Therefore, the following describes an example in which an RF signal is generated using the beat of unmodulated light and upper sideband light. Figure 32 shows the multiple light beams input to the photomixer 13 in this case. Figure 33 also shows the output of the photomixer 13.

[0090] 33, unlike the state shown in Fig. 19, the component of frequency (N+1)W-R, the component of frequency (N+1)W, and the component of frequency (N+1)W+R are the difference frequencies between optical beats with orthogonal polarization planes, and therefore are not output from the photomixer 13. Therefore, the spurious components that may interfere with the RF signal are the component of frequency NW and the component of frequency (N+1)W-R.

[0091] (When R<W / 2 and N=odd) Next, the positional relationship between the RF signal and spurious components on the radio frequency axis is determined when R<W / 2 and N=odd. In this case, the multiple light beams input to the photomixer 13 are the same as those in FIG. 32. The output of the photomixer 13 is shown in FIG. 34. In FIG. 34, the component with frequency NW, the component with frequency (N+1)W-R, and the component with frequency (N+1)W+R are not output from the photomixer 13 because they are the difference frequencies between optical beats with orthogonal polarization planes. Therefore, the spurious components that may interfere with the RF signal are the component with frequency NW-R and the component with frequency (N+1)W.

[0092] (When R > W / 2 and N = Even) Next, we will determine the positional relationship between the RF signal and spurious signals on the radio frequency axis when R > W / 2 and N = even. Figure 35 shows the multiple light beams input to the photomixer 13 in this case. Figure 36 shows the output of the photomixer 13. In Figure 36, the components with frequencies (N+1)W+R, (N+1)W-R, and (N+1)W are not output from the photomixer 13 because they are the difference frequencies between optical beats with orthogonal polarization planes. Therefore, the spurious components that may interfere with the RF signal are the component with frequency NW and the component with frequency (N+2)W-R.

[0093] (When R > W / 2 and N = odd) Next, we will determine the positional relationship between the RF signal and spurious components on the radio frequency axis when R > W / 2 and N = odd. The multiple light beams input to the photomixer 13 in this case are the same as those in FIG. 35. The output of the photomixer 13 is also shown in FIG. 37. In FIG. 37, the components of frequencies (N-1)W+R, NW, and (N+1)W-R are not output from the photomixer 13 because they are the difference frequencies between optical beats with orthogonal polarization planes. Therefore, the spurious components that may interfere with the RF signal are the components of frequencies NW-R and (N+1)W.

[0094] (Conditions for No Spurious Interference) Based on the above explanation, the conditions for no spurious interference in the configuration shown in Fig. 31 will now be explained. When the frequency bands of the RF signal and spurious in Fig. 32 to Fig. 37 are organized, the spurious components that may interfere with the RF signal are (A) and (B) below, regardless of the magnitude relationship between R and W / 2.

[0095] (A) When N is an even number, the frequency NW component and the frequency (N+2)W-R component. (B) When N is an odd number, the frequency NW-R component and the frequency (N+1)W component.

[0096] Next, the conditions under which spurious signals do not interfere in each of the above cases (A) and (B) are derived.

[0097] (A) When N is an even number, in order to prevent interference between the RF signal and two adjacent spurious components on the frequency axis (the component at frequency NW and the component at frequency (N+2)W-R), the wavelength multiplexing interval W and the signal bandwidths B and R need only satisfy the two equations shown in equation (5).

[0098]

[0099] Transforming equation (5) yields equation (6): As a result, it is sufficient that the wavelength multiplexing interval W and the signal bandwidths B and R satisfy the two equations shown in equation (6).

[0100]

[0101] (A) When N is an odd number, in order to prevent interference between the RF signal and two adjacent spurious components on the frequency axis (the component with frequency NW-R and the component with frequency (N+1)W), the wavelength multiplexing interval W and the signal bandwidths B and R need only satisfy the two equations shown in equation (7).

[0102]

[0103] Transforming equation (7) yields equation (8): As a result, it is sufficient that the wavelength multiplexing interval W and the signal bandwidths B and R satisfy the two equations shown in equation (8).

[0104]

[0105] As described above, in the photoelectric conversion device 100d according to the fourth embodiment, the spurious non-interference condition can be satisfied by satisfying the four equations shown in the above-described equations (6) and (8). As shown in the above-described equations (6) and (8), the spurious non-interference condition is satisfied when the frequency f of the radio signal is ＲＦ , is derived from the relationship between the signal bandwidth B and the wavelength multiplexing interval W. Based on the above equations (6) and (8), the maximum bandwidth b that satisfies the condition that the wavelength multiplexing interval W and the spurious do not interfere with each other is ｍａｘ An example of the relationship between the maximum bandwidth b ｍａｘ In FIG. 38, the RF frequency is f ＲＦ It should be noted that R is also a function of the wavelength multiplexing interval W. Also, N is a function of fＲＦ / W quotient, R is f ＲＦ / W is the remainder.

[0106] As shown in FIG. 38, W>(4f ＲＦ / 3), the maximum bandwidth b ｍａｘ is a constant value (2f ＲＦ Also, as shown in FIG. 38, W<(4f ＲＦ In the range of 4R / 3, the wavelength multiplexing interval W reaches a peak value when the condition of W=4R / 3 or W=4R is satisfied, and the value is W / 2. When R=0, the maximum bandwidth b ｍａｘ =0.

[0107] As described above, the spurious non-interference condition in the photoelectric conversion devices 100c and 100d having a configuration for controlling the polarization plane of the radio beam is satisfied when the center frequency f of the RF signal is ＲＦ Based on the relationship between the bandwidth B of the RF signal and the wavelength multiplexing interval W, the center frequency f of the RF signal can be calculated as shown in Equation (6) and Equation (8). ＲＦ When the bandwidth B of the RF signal is given, the range of wavelength multiplexing interval W that satisfies the spurious non-interference condition can be calculated. ＲＦ When the wavelength division multiplexing interval W is given, the maximum signal bandwidth that satisfies the spurious non-interference condition can be calculated.

[0108] (Specific Configuration Example (Part 1) of the Fourth Embodiment) FIG. 39 is a diagram showing a configuration example (part 1) of a photoelectric conversion device 100c according to the fourth embodiment. The photoelectric conversion device 100c generates multiple RF signals collectively by photomixing multiple unmodulated light beams and multiple sideband light beams that are wavelength-multiplexed at a predetermined wavelength multiplexing interval. The photoelectric conversion device 100c includes multiple information signal sources 10, multiple optical signal sources 11, a photomixer 13, a wavelength spacing selection circuit 15, multiple polarization rotators 20, and a polarization plane control circuit 21c. The photoelectric conversion device 100c includes the wavelength spacing selection circuit 15 in addition to the configuration shown in FIG. 29 . Note that while FIG. 39 shows a configuration in which the photoelectric conversion device 100c generates three or more radio beams, the photoelectric conversion device 100c can also be applied to a configuration in which two radio beams are generated.

[0109] The wavelength interval selection circuit 15 selects the center frequency f of the RF signal from each information signal source 10. ＲＦ and the bandwidth B of the RF signal. The wavelength spacing selection circuit 15 then selects the center frequency f ＲＦ and information indicating the bandwidth B of the RF signal, the range of the wavelength multiplexing spacing W that satisfies the spurious non-interference condition is calculated based on the above equations (6) and (8). The wavelength spacing selection circuit 15 determines the wavelength multiplexing spacing W for each radio beam based on the calculation result, and sets the optical wavelength of each optical signal source 11 to the determined wavelength multiplexing spacing W.

[0110] When the wavelength multiplexing interval W is determined in the wavelength interval selection circuit 15, the same method as in the first embodiment may be used.

[0111] The photoelectric conversion device 100c shown in Fig. 39 may include a plurality of optical wavelength changing circuits 16 like the photoelectric conversion device 200 shown in Fig. 25, or may include a plurality of optical wavelength changing circuits 16, a signal distributor 18b, and an RF signal quality determination circuit 19b like the photoelectric conversion device 100b shown in Fig. 28. The plurality of optical wavelength changing circuits 16 are provided between the plurality of polarization rotators 20 and the photomixer 13. Specific processing is the same as that of the photoelectric conversion device 200 or the photoelectric conversion device 100b, and therefore will not be described.

[0112] (Specific Configuration Example (Part 2) of Fourth Embodiment) FIG. 40 is a diagram showing a configuration example (Part 2) of a photoelectric conversion device 100c according to the fourth embodiment. The photoelectric conversion device 100c generates multiple RF signals collectively by photomixing multiple unmodulated light beams and multiple sideband light beams that are wavelength-multiplexed at a predetermined wavelength multiplexing interval. The photoelectric conversion device 100c includes multiple information signal sources 10, multiple optical signal sources 11, a photomixer 13, and a maximum signal bandwidth calculation circuit 17a. The photoelectric conversion device 100c includes the maximum signal bandwidth calculation circuit 17a in addition to the configuration shown in FIG. 29. Note that while FIG. 40 shows a configuration in which the photoelectric conversion device 100c generates three or more radio beams, the photoelectric conversion device 100c can also be applied to a configuration in which two radio beams are generated.

[0113] The maximum signal bandwidth calculation circuit 17a calculates the center frequency f of the RF signal from each information signal source 10. ＲＦ Furthermore, the maximum signal bandwidth calculation circuit 17a acquires information on the wavelength multiplexing interval W from each optical signal source 11. For example, the maximum signal bandwidth calculation circuit 17a acquires information on the wavelength multiplexing interval W set in each optical signal source 11. The maximum signal bandwidth calculation circuit 17a calculates the central frequency f ＲＦ Using the information on the wavelength multiplexing interval W of each optical signal source 11, the maximum signal bandwidth b that satisfies the spurious non-interference condition is calculated based on the above equations (6) and (8). ｍａｘ Based on the calculation result, the maximum signal bandwidth calculation circuit 17a determines the signal bandwidth B for each radio beam and limits the symbol rate of each information signal source 10. For example, based on the determined signal bandwidth B for each radio beam, the maximum signal bandwidth calculation circuit 17a adjusts the symbol rate of each information signal source 10 so that it is within the signal bandwidth B.

[0114] According to the photoelectric conversion device 100c configured as above, the same effects as those of the first embodiment can be obtained even in the configuration in which the polarization plane of the radio beam is controlled.

[0115] 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.

[0116] The present invention can be applied to a photoelectric conversion device that generates an RF signal by photomixing.

[0117] 10, 10-1 to 10-M... Information signal source, 11, 11-1-1, 11-1-2, 11-2-1, 11-2-2, 11-3-1, 11-3-2, 11-M-1, 11-M-2... Optical signal source, 13... Photomixer, 15... Wavelength spacing selection circuit, 16, 16-1-1, 16-1-2, 16-2-1, 16-2-2, 16-3-1, 16-3-2, 16-M-1, 16-M-2... Optical wavelength changing circuit, 17a... Maximum signal bandwidth calculation circuit, 18b... Signal distributor, 19b... RF signal quality judgment circuit, 20, 20-1 to 20-M... Polarization rotator, 21c... Polarization plane control circuit, 100, 100a, 100b, 100c, 100d...photoelectric conversion device

Claims

1. A photoelectric conversion device comprising: a plurality of information signal sources that output data to be transmitted; a plurality of optical signal sources that output a plurality of modulated light beams and a plurality of unmodulated light beams that are generated based on the data output from the plurality of information signal sources; a photomixer that generates a plurality of radio signals by photomixing the plurality of unmodulated light beams output from the plurality of optical signal sources with the plurality of modulated light beams; and a setting unit that determines a wavelength multiplexing interval or a maximum allowable signal bandwidth for the desired radio signal so as to satisfy a non-interference condition that the frequency band of the desired radio signal does not overlap with the frequency band of unnecessary frequency components, which is derived from the relationship between the frequency, signal bandwidth, and wavelength multiplexing interval of the radio signal, and sets the determined wavelength multiplexing interval to the plurality of optical signal sources or sets the maximum allowable signal bandwidth to the plurality of information signal sources.

2. The photoelectric conversion device according to claim 1, wherein the setting unit acquires information indicating the frequency of the radio signal and information indicating the signal bandwidth from each of the plurality of information signal sources, and determines a wavelength multiplexing interval for the desired radio signal using the acquired information indicating the frequency of the radio signal and information indicating the signal bandwidth so as to satisfy the non-interference condition.

3. The photoelectric conversion device according to claim 1 or 2, further comprising a signal quality determination unit that monitors the quality of the desired radio signal among the plurality of radio signals generated by the photomixer, and the setting unit updates the wavelength multiplexing interval as necessary based on the monitoring results of the signal quality determination unit.

4. The photoelectric conversion device according to claim 1, wherein the setting unit acquires information indicating the frequency of the radio signal from each of the plurality of information signal sources, acquires information indicating a wavelength multiplexing interval from each of the plurality of optical signal sources, and determines the maximum allowable signal bandwidth so as to satisfy the non-interference condition using the acquired information indicating the frequency of the radio signal and the information indicating the wavelength multiplexing interval.

Images