Photoelectric converter

Abstract

To enable low phase noise and large capacity wireless communication in a terahertz band.SOLUTION: The present invention comprises: a laser emission unit that emits a single frequency laser beam; an optical micro-resonator that is excited by the laser beam and generates an optical frequency comb having a frequency interval of 100 GHz to 3 THz inclusive; an optical modulation unit that modulates the amplitude and / or the phase with a baseband signal composed of a transmit information signal, regarding at least one of mutually adjacent first and second optical frequency modes in the optical frequency comb; a mixing unit that mixes the modulated first and second optical frequency modes; and a photoelectric conversion unit that converts an output signal of the mixing unit into an electrical signal.SELECTED DRAWING: Figure 1

Description

[Technical Field] 【0001】 The present invention relates to a photoelectric conversion device that generates terahertz waves using a micro-optical resonator that generates an optical frequency comb. [Background technology] 【0002】 Traditionally, in mobile (wireless) communications (2G / 3G / 4G / 5G, etc.), technological innovations driven by advancements in semiconductor technology, such as increased speed and frequency of electronic circuits, have propelled generational evolution. However, the frequencies used in next-generation mobile communications (Beyond 5G / 6G) are expected to extend to the so-called terahertz band (hereinafter referred to as the THz band), which is above 300 GHz, potentially reaching the technical limits (frequency upper limit) of electrical methods. In other words, it is said that fundamental problems such as reduced power output of wireless carrier waves, increased phase noise, increased signal transmission loss, and time delays associated with signal conversion between optical and mobile communications will become apparent. 【0003】 On the other hand, optical communication using fiber optic networks offers the fastest information transmission speed, and recently, silicon photonics technology is being developed to replace electronic wiring inside devices with optical wiring, achieving ultra-high speed, large capacity, low latency, and low power consumption. Against this backdrop, examples of using optical devices as carrier sources or incorporating optical communication technology into parts of wireless communication systems have recently been observed. For example, an example has been disclosed in which terahertz waves are generated by modulating and mixing light of different wavelengths and then used in wireless communication (Non-Patent Literature 1). 【0004】 Another method for generating light of two wavelengths with different frequencies is to extract arbitrary optical frequency modes with a desired frequency interval from an optical frequency comb using a filter (Patent Document 1). Patent Document 2 also discloses a technique for using an optical frequency comb as a multi-carrier for frequency division multiplexing and further separating a pilot signal from the optical frequency comb. Furthermore, a technique for generating an optical frequency comb from a micro-resonator is disclosed (Non-Patent Document 2). In addition, a method for locking another laser to an arbitrary optical frequency mode of an optical frequency comb has also been proposed (Non-Patent Document 3). [Prior art documents] [Patent Documents] 【0005】 [Patent Document 1] Japanese Patent Publication No. 2009-4858 [Patent Document 2] Special Publication No. 2018-526842 [Non-patent literature] 【0006】 [Non-Patent Document 1] Tadao Nagatsuma, "Terahertz Waves Open Up Ultra-High-Speed ​​Wireless Communication," Journal of the Japan Society for Precision Engineering, Vol. 82, No. 3, 2016. [Non-Patent Document 2] S.ZHANG,JMSILVER,X.SHANG,LDBINO,NMRIDLER,P.DEL'HAYE,”Terahertz wave generation using a soliton microcomb”,Optics Express,Vol.27,No.24,Nov.2019 [Non-Patent Document 3] S. Hisatake, G. Carpintero, Y. Yoshimizu, Y. Minamikata, K. Oogimoto, Y. Yasuda, F. Dijk, T. Tekin, and T. Nagatsuma, “W-Band Coherent Wireless Link Using Injection-Locked Laser Diodes”, IEEE Photonics Technology Letters, Vol. 27, No. 14, July, 2015 [Overview of the Initiative] [Problems that the invention aims to solve] 【0007】 However, as described in Non-Patent Document 1, when independent lasers with wavelengths separated by the frequency of the wireless carrier are used as light sources, frequency fluctuations or phase noise in the carrier wave occur due to frequency or phase fluctuations between them. Also, even when using an optical frequency comb as a light source, if a high-frequency signal is obtained from the beat between two spaced-out optical frequency modes as described in Patent Document 1, phase noise accumulates in proportion to the number of spaced-out modes. This tendency becomes particularly pronounced in the terahertz region and above. Furthermore, although the technology disclosed in Patent Document 2 is intended for frequency division multiplexing transmission, it is necessary to set the frequency spacing of the pilot signal wider than the mode spacing of the optical frequency comb, and similar problems arise if this technology is applied directly to wireless transmission. It should be noted that Non-Patent Document 2 generates terahertz waves by photoelectric conversion of two-wavelength mode light from an optical frequency comb, but there is no description whatsoever of the modulation. [Means for solving the problem] 【0008】 A photoelectric conversion device according to one aspect of the present invention comprises: a laser light-emitting unit that emits laser light of a single frequency; a micro-optical resonator that is excited by the laser light and generates an optical frequency comb having a frequency interval of 100 GHz to 3 THz; an optical modulation unit that modulates at least one of the amplitude and phase of at least one of the first and second optical frequency modes adjacent to each other in the optical frequency comb with a baseband signal consisting of a transmission information signal; a mixing unit that mixes the modulated first and second optical frequency modes; and a photoelectric conversion unit that converts the output signal of the mixing unit into an electrical signal. 【0009】 A first optical modulation unit and a second optical modulation unit may be provided for each of the first and second optical frequency modes. 【0010】 The pair of the first optical frequency mode and the second optical frequency mode may be separated and extracted together, and the optical modulation unit may apply amplitude modulation to the pair. 【0011】 The optical modulation unit may modulate the amplitude of the optical frequency comb and supply the optical frequency comb to the photoelectric conversion unit. 【0012】 The aforementioned frequency interval may be 300 GHz or more and 1 THz or less. 【0013】 The photoelectric conversion unit may consist of a single-travel carrier photodiode. 【0014】 The aforementioned micro-optical resonator is a medium having a nonlinear optical effect and may be composed of one or more media selected from the group consisting of silicon nitride (Si3N4), aluminum arsenide gallium (AlGaAs), lithium niobate (LiNbO3), tantalum pentoxide (Ta2O5), and gallium nitride (GaN). 【0015】 The optical modulation unit may include an optical sub-carrier generation unit that generates a plurality of optical sub-carrier groups from the first optical frequency mode, and an optical modulation element group that independently modulates the optical sub-carrier groups according to the baseband signal. 【0016】 The optical modulation unit may include a first optical sub-carrier generation unit that generates a first optical sub-carrier group separated from the first optical frequency mode by a first difference frequency on both sides, a second optical sub-carrier generation unit that generates a second optical sub-carrier group separated from each optical sub-carrier group by a second difference frequency on both sides, and an optical modulation element that performs frequency division multiplexing modulation on the second optical sub-carrier group according to the baseband signal. 【0017】 The frequency interval of the second optical sub-carrier group may be 10 GHz or more and 50 GHz or less. 【0018】 The optical modulation unit may include an optical splitting element that splits the first optical frequency mode into two optical sub-carriers, and an optical modulation element group that independently modulates the optical sub-carriers according to the baseband signal. The photoelectric conversion unit may include a pair of photoelectric conversion units whose polarization of the emitted wave is orthogonal, and the two modulated sub-carriers may be respectively supplied to the pair of photoelectric conversion units. 【Advantages of the Invention】 【0019】 According to one aspect of the present invention, the first and second optical frequency modes adjacent to each other are separated and extracted from an optical frequency comb obtained by a micro optical resonator, and further, the first optical frequency mode is optically modulated with a baseband signal composed of a transmission information signal as a carrier, and a modulated signal in the terahertz band is generated by a beat with the second optical frequency mode having a small relative phase noise with respect to the first optical frequency mode. Furthermore, multiplexing of the baseband signal can be realized by generating optical sub-carriers from the first optical frequency mode, and higher-speed information transmission can be realized. 【Brief Description of the Drawings】 【0020】 [Figure 1] This is a block diagram including a photoelectric conversion device according to the first embodiment of the present invention. [Figure 2] This is a block diagram including a photoelectric conversion device according to a second embodiment of the present invention. [Figure 3] This is a block diagram including a photoelectric conversion device according to a third embodiment of the present invention. [Figure 4] This is a block diagram including a photoelectric conversion device according to a fourth embodiment of the present invention. [Figure 5] This is a block diagram of the optical modulation section in a photoelectric conversion device according to a fifth embodiment of the present invention. [Figure 6] This is an explanatory diagram of the operation of the fifth embodiment of the present invention. [Figure 7] This is a block diagram of the optical modulation section in the photoelectric conversion device according to the sixth embodiment of the present invention. [Figure 8] This figure shows the processing flow of the first embodiment of the present invention. [Figure 9] This is a block diagram showing an experimental method for a second embodiment of the present invention. [Figure 10] This graph shows the experimental results of a second embodiment of the present invention. [Figure 11] This is a block diagram showing an experimental method for a third embodiment of the present invention. [Figure 12] This graph shows the experimental results of the third embodiment of the present invention. [Figure 13] This graph shows the experimental results of the third embodiment of the present invention. [Modes for carrying out the invention] 【0021】 (First Embodiment) The first embodiment will now be described in detail with reference to the drawings. The purpose of the photoelectric conversion device in this embodiment is to generate terahertz waves with low phase noise by using adjacent frequency modes generated from a micro-optical resonator in the generation of carriers used for communication. 【0022】 Figure 1 shows a block diagram of the photoelectric converter 1 in this embodiment. In Figure 1, 100 is a wireless terminal. There may be one or more wireless terminals 100. It may also be a mobile terminal or a fixed terminal. 106 is a receiving antenna that receives the wireless signal S1 from the wireless terminal 100. The receiving antenna 106 may be an antenna array consisting of multiple antenna elements to accommodate multiple wireless terminals 100. It may also consist of multiple antenna groups that support multiple wireless communication standards such as frequency. The information signal demodulation unit 107 demodulates the transmission information signal contained in the wireless signal S1. The information signal demodulation unit 107 may support multiple wireless communication standards such as LTE and 5G. 【0023】 Furthermore, in Figure 1, 101 is a laser emitter that emits single-frequency laser light. A DFB laser that emits light with an emission wavelength of 1550 nm or a wavelength around that wavelength is preferred. 102 is a micro-optical resonator, which is excited by the laser light to generate an optical frequency comb. An optical frequency comb is a comb in which a number of optical frequency mode trains are equal in frequency (f rep It has an ultra-discrete multispectral structure in which the elements are spaced apart and aligned in phase, arranged in a comb-like pattern. The micro-optical resonator 102 may be formed in a ring shape on a semiconductor substrate. Its diameter may be 40 μm to 400 μm. The medium having a nonlinear optical effect may be composed of one or more media selected from the group consisting of silicon nitride (Si3N4), aluminum gallium arsenide (AlGaAs), lithium niobate (LiNbO3), tantalum pentoxide (Ta2O5), and gallium nitride (GaN). 【0024】 The optical frequency comb generated by the micro-optical resonator 102 has a short optical resonator length, so the difference frequency (f) between adjacent optical frequency modes is small. rep ) can be increased. Frequency interval (f rep) may, for example, be between 100 GHz and 3 THz. More preferably, it may be between 300 GHz and 1 THz. Even more preferably, it may be between 350 GHz and 600 GHz. On the other hand, in the case of a fiber optical frequency comb using optical fibers, the difference frequency between adjacent optical frequency modes is about 0.1 to 1 GHz. For example, if one tries to obtain an optical beat frequency of 300 GHz in the terahertz band from a fiber optical frequency comb, one is forced to extract optical frequency modes that are not adjacent but separated by 3000 to 300 modes. However, the relative phase noise increases cumulatively as the spacing between optical frequency modes increases. 【0025】 104 and 105 are bandpass filters, respectively, and constitute the optical frequency mode separation section. The optical frequency comb separates adjacent optical frequency modes (ν1 and ν0 = ν1 - f rep These are separated and extracted. Note that the wavelength division section does not necessarily have to consist of a pair of bandpass filters as shown in Figure 1. An AWG (array waveguide diffractometer) may be used instead. Also, a phase adjustment section consisting of a heater or the like may be provided in at least one of the paths of the bandpass filters 104 and 105. 【0026】 The optical frequency mode m1 (first optical frequency mode) of frequency ν1 is sent to the optical modulation unit 108 and modulated according to the baseband signal S2. The baseband signal S2 includes the transmission information signal demodulated by the information signal demodulation unit 107. When information signals are transmitted from multiple wireless terminals 100, they may be combined into a single stream using time division. Alternatively, they may be assigned to the frequency division channels shown in the second embodiment. The optically modulated optical frequency mode m1 is mixed with the optically unmodulated optical frequency mode m0 in the mixing unit 120 and supplied to the photoelectric conversion unit 110 via the optical amplification element 109. Note that the optical amplification element 109 may not be inserted depending on the level of the optical signal input to the photoelectric conversion unit. The photoelectric conversion unit 110 may be composed of a photoelectric conversion unit such as a single-traveling carrier photodiode (UTC-PD). 【0027】 On the other hand, the optical frequency mode m0 (second optical frequency mode) with frequency ν0 is supplied to the photoelectric conversion unit 110 via the mixing unit 120 and the optical amplification element 109. In the mixing unit 120, the optical frequency mode m1 (ν1) and the optical frequency mode m0 (ν0) are mixed (S3), and the difference frequency (f rep The terahertz waves are output from the photoelectric conversion unit 110 as terahertz waves. Alternatively, the mixing unit 120 may be omitted, and the optical frequency modes m0 and m1 may be directly input to the photoelectric conversion unit 110. The terahertz waves output from the photoelectric conversion unit 110 are input to the antenna 111, and the terahertz waves S4 are radiated into the air from the antenna 111. Furthermore, although no modulation was applied to the optical frequency mode m0 in this embodiment, the present invention may modulate at least the amplitude, phase, or both of the optical frequency modes m1, m0, or both of them with the baseband signal, and is not limited to this embodiment. 【0028】 (Second Embodiment) The second embodiment will be described below with reference to Figure 2. In Figure 2, the wireless terminal 100, laser light emitter 101, micro-optical resonator 102, isolator 103, bandpass filters 104 and 105, receiving antenna 106, information signal demodulation unit 107, mixing unit 120, optical amplification element 109, photoelectric conversion unit 110, and transmitting antenna 111 have the same functions as those shown in Figure 1. 【0029】 In this embodiment, the first optical modulation unit 181 and the second optical modulation unit 182 are provided in the paths of optical frequency modes m1 and m0, respectively. Assuming that the optical modulation unit 181 performs amplitude modulation, if only one of the optical frequency modes is modulated as in the previous embodiment, the beat signal resulting from the mixing of these can be expressed as follows. E0×E1(t) Here, E1(t) indicates that the optical frequency mode m1 is undergoing amplitude modulation, and E0 indicates that the optical frequency mode m0 is a CW signal. Furthermore, if we introduce the optical modulation unit 182 to the optical frequency mode m0 as well, and apply the same level of amplitude modulation to both modes, the beat signal will be: E0(t) × E1(t) = E0(t) 2 =E1(t) 2 This means that a deeper modulation signal can be obtained as the amplitude change changes exponentially, resulting in a significant improvement in the SNR and BER of the modulation signal. 【0030】 (Third embodiment) When using optical amplitude modulation, the configuration can be further simplified. In Figure 3, the bandpass filter 1045 is used for optical frequency mode m1 (frequency ν1) and frequency f rep The optical frequency modes m0(ν0) adjacent to each other at intervals are transmitted in pairs. The optical amplitude modulation unit 180 simultaneously applies amplitude modulation to these pairs of optical mode signals m1 and m0. As a result, the same effects as those of the second embodiment described above can be obtained with a simpler configuration. 【0031】 (Fourth embodiment) When using optical amplitude modulation, the bandpass filter may be omitted. Figure 4 shows a block diagram of this embodiment. In Figure 4, the optical amplitude modulation unit 180 modulates the amplitude of the entire optical frequency comb (ultrashort pulse signal in the time domain). After optical amplification, it is then injected into the photoelectric conversion unit. 【0032】 (Fifth embodiment) The fifth embodiment will now be described. In this embodiment, the optical modulation unit 108 is characterized by having a configuration that generates four optical subcarrier groups from an optical frequency mode (m1) and frequency-division multiplex modulates these optical subcarrier groups in accordance with a baseband signal S2 composed of a 4-bit stream. The specific configuration is shown in the block diagram of Figure 5. Components other than the modulation unit 108 may be the same as those shown in Figure 1, so they are omitted here. In this embodiment, the number of multiplexing units is set to 4, but the present invention is not limited to this. 【0033】 In Figure 5, 1081 represents a local oscillator element and 1083 represents an optical modulation element. The local oscillator element 1081 and the optical modulation element 1083 constitute the first optical subcarrier generation unit. Also, 1082 represents a local oscillator element and 1084 represents an optical modulation element. The local oscillator element 1082 and the optical modulation element 1084 constitute the second optical subcarrier generation unit. 【0034】 Local oscillators 1081 and 1082 may be electrically operated. In this embodiment, the oscillation frequency Δf1 is 1.5 times the oscillation frequency Δf2 of local oscillator 1082, which will be described later. The oscillation frequency Δf2 is preferably 5 GHz to 50 GHz, and the oscillation frequency Δf1 is preferably 7.5 GHz to 75 GHz. Doubling the oscillation frequency Δf2 (2Δf2) becomes the frequency bandwidth per channel in frequency division multiplexing, as will be described later. This frequency bandwidth 2Δf2 is more preferably 10 GHz to 50 GHz. In addition, optical modulation elements 1083 and 1084 may each be phase-modulated by the output of the local oscillator, preferably DSB modulated. 【0035】 Furthermore, in Figure 5, 1085 is an array waveguide diffractometer (AWG) that separates all of the optical subcarrier groups (C11, C12, C21, C22) generated by the local oscillator element 1082 and the optical modulation element 1084. Note that a wavelength selective switch (WSS) may be used instead of the array waveguide diffractometer 1085 to separate the optical subcarrier groups C11, C12, C21, and C22 while blocking light waves of other wavelengths. 1086 to 1089 are optical modulation elements that apply optical modulation to each optical subcarrier according to each bitstream (bit0 to bit3) of the information signal S2. The modulation method may be amplitude modulation such as QPSK or QAM, phase modulation, or a method including both. Each optically modulated optical subcarrier is combined again and output from the modulation unit 108 as a frequency division multiplexed signal, which is then input to the photoelectric conversion unit 110. 【0036】 The operation of this embodiment will be described below with reference to FIG. 6. In FIG. 6(a), an optical frequency mode m1 is input to the modulation unit 108. The optical frequency mode m1 is separated from the optical frequency comb by the band-pass filter 104. The optical frequency mode m0 adjacent to the optical frequency mode m1 is separated by a band-pass filter 105 having a passband separated from the resonance frequency of the microresonator 102 by f ( rep ) from the optical frequency mode m1 and is input to the optoelectronic conversion unit 110 without passing through the modulation unit 108. 【0037】 The local oscillation element 1081 and the optical modulation element 1083 generate the first optical subcarriers C1 and C2 at frequency positions separated by Δf1 from the low-frequency side and the high-frequency side (hereinafter both sides) of the optical frequency mode m1 (FIG. 6(b)). Further, the local oscillation element 1082 and the optical modulation element 1083 generate the second optical subcarriers C11, C12, C21, and C22 at frequency positions separated by Δf2 from both sides of each of the optical subcarriers C1 and C2. The frequency intervals between the optical subcarriers C11 and C12 and between the optical subcarriers C21 and C22 are both 2Δf2 (FIG. 6(c)). Here, if the relationship Δf1 = 1.5×Δf2 holds, the frequency interval between the carriers C12 and C21 is also 2Δf2. For example, when Δf1 = 37.5 GHz and Δf2 = 25 GHz, the optical subcarriers C11, C12, C21, and C22 are all arranged at intervals of 50 GHz. 【0038】 Optical modulation elements 1086 to 1089 apply optical modulation to optical subcarriers C11, C12, C21, and C22 according to each bitstream (bit0 to bit3) of the information signal S2. The bandwidth of each modulated signal should be 2Δf2 or less. Generally, when transmitting information by frequency division, increasing the number of divisions allows for narrowing the bandwidth of individual frequency channels, making it easier to design the signal processing circuit in modulation and demodulation. However, this also reduces the transmission speed between the fronthauls of each channel, making delays more likely. In particular, when applying the technology disclosed in this embodiment to high-capacity lines such as between base stations and exchanges, delays should be minimized as much as possible. Therefore, when transmitting information at a maximum of 100 Gbps, it is desirable that the bandwidth of each frequency channel, i.e., the frequency spacing (2Δf2) of the second optical subcarriers, be between 10 GHz and 50 GHz. 【0039】 In the above embodiment, the optical frequency modes separated from the optical frequency comb through a bandpass filter, etc., were directly modulated and supplied to the photoelectric conversion unit through an optical amplification element. However, as shown in Non-Patent Document 3, a separate laser from the one used for excitation may be used, and these lasers may be injected and locked to the desired optical frequency modes, and modulation or the like may be performed on the output light of these lasers. 【0040】 (Sixth embodiment) A sixth embodiment of the present invention will be described below. Figure 7 is a block diagram of this embodiment. In Figure 7, the modulation unit 108 has an optical splitting element (beam splitter) 1180 that divides the optical frequency mode m1 and generates two optical subcarriers P1 and P2. Furthermore, it has optical modulation elements 1181 and 1182 that independently modulate the optical subcarriers P1 and P2 according to a baseband signal S2 composed of a 2-bit stream. 【0041】 Furthermore, the photoelectric conversion unit 110 is composed of photoelectric conversion elements 1101 and 1102, which respectively convert the modulated optical subcarriers P1 and P2 to a frequency f repIt converts to a terahertz signal. The photoelectric conversion units 1101 and 1102 are positioned so that the polarization of the terahertz waves emitted from each of them is orthogonal to that of the other units. 【0042】 In this embodiment, the overall transmission gain of the system was estimated when the carrier frequency was 300 GHz, taking into account the losses at each step of transmission. The results are shown in Figure 8. The bandwidth of the transmitted information signal was set to 25 GHz. 【0043】 First, an arbitrary optical frequency mode (m1) is separated from the optical frequency comb by the optical frequency mode separation unit, but if an array waveguide diffraction grating (AWG) is used, a loss of about 6 dB may occur. Next, a loss of 10 dB occurs in the optical modulator and another 6 dB during the multiplexing process, for a total loss of 22 dB. Therefore, in this embodiment, an optical amplifier is used to increase the gain by 30 dB. That is, the gain up to this point is 8 dB, so if one mode of the optical frequency comb is 15 mW (11.76 dBm), the optical amplifier outputs 94 mW (19.76 dBm) of light. 【0044】 However, a 3dB loss occurs in the next-stage bandpass filter, and further losses occur in the photoelectric conversion section due to conversion efficiency. In the case of UTC-PD, the loss varies greatly depending on the output (S4) frequency, reaching approximately 30dB at 300GHz and approximately 35dB at 600GHz. If the carrier frequency is 300GHz, the overall gain of the system becomes -25dB, resulting in approximately 47μW (-13.24dBm). 【0045】 Next, the mode interval (f) of the optical frequency comb (micro optical comb) generated using a micro optical resonator. rep )The measurement results of relative phase noise will be explained. Figure 9 shows f rep This describes an experimental setup for relative phase noise. A micro-optical comb is optically amplified by an Er-doped fiber amplifier (EDFA) and then split into two paths. One path undergoes frequency shifting using an acousto-optical modulation (AOM) element. The other path is subjected to temporal manipulation (delay, etc.). 【0046】 After the two paths are rejoined, an arbitrary pair of optical frequency modes is extracted using a bandpass filter, and an FFT (Fast Fourier Transform) analysis is performed on the mixed signal. The spacing between each mode of the optical frequency comb is in the terahertz frequency band, and directly measuring the relative phase noise requires a high-speed, wideband, and high-precision spectrum analyzer. However, using the method in this embodiment, the FFT analyzes the signal in the operating frequency band of the AOM element, and since the relative phase noise is transferred, the relative phase noise can be measured with high precision even with a low-speed spectrum analyzer. 【0047】 Figure 10 shows the measurement results of relative phase noise measured using the experimental system shown in Figure 9. The frequency spacing of the optical frequency comb was 560 GHz, and it was confirmed that the relative phase noise at an offset frequency of 10 kHz was approximately -60 dB. 【0048】 (Third example) This example describes the experimental setup (configuration) and results for measuring and evaluating the phase noise of terahertz waves. Figure 11 shows the experimental setup. A terahertz wave generated by inputting a micro-optical comb with a comb-mode spacing of 560 GHz into a photoelectric conversion unit (single-traveling carrier photodiode) is incident on a subharmonic mixer (SHM). The SHM mixes the terahertz wave with a frequency-multiplied local oscillator (LO) signal to generate an IF signal in the RF band that corresponds to the difference frequency between the two. This IF signal is measured with an RF spectrum analyzer. Furthermore, by performing a Fourier transform on the IF signal, a phase noise spectrum can be obtained for the IF signal (corresponding to the terahertz wave). 【0049】 Figures 12 and 13 show the experimental results. Figure 12 shows the frequency spectrum of the IF signal, indicating a good signal-to-noise ratio. Figure 13 shows the phase noise spectrum of the IF signal. For comparison, Figure 10 shows the f rep The relative phase noise spectrum is also shown. From a comparison of the two, f repThis shows that the relative phase noise characteristics can be transferred to terahertz waves without being lost during the optical terahertz conversion process. [Industrial applicability] 【0050】 The present invention can be used in photoelectric conversion devices that generate terahertz waves using a miniature optical resonator that generates an optical frequency comb, such as in wireless base stations that transmit information collected from mobile terminals to a switching station, or relay stations that transmit information between wireless base stations. [Explanation of symbols] 【0051】 1. Photoelectric conversion device 100 wireless terminals 101 Laser Emitting Unit 102 Microscopic optical cavity 103 Isolator 104, 105 Bandpass filters 106 Receiving Antenna 107 Information signal demodulation unit 108 Optical Modulation Section 180 Optical Amplitude Modulation Section 181 First Optical Modulation Unit 182 Second Optical Modulation Unit 109 Optical Amplifier 110 Photoelectric conversion unit 120 Mixing section 1081, 1082 Local oscillator element 1083, 1084 Optical Modulator 1085 Array Waveguide Diffractometer (AWG) 1086-1089 Optical Modulator 1101, 1102 Photoelectric conversion section 1180 Optical Splitter 1181, 1182 Optical Modulator

Claims

[Claim 1] A laser light emitter that emits a single-frequency laser beam, A miniature optical resonator that is excited by the aforementioned laser light and generates an optical frequency comb having a frequency interval of 100 GHz to 3 THz, The optical frequency comb includes an optical modulation unit that modulates at least one of the amplitude and phase of at least one of the adjacent first and second optical frequency modes with a baseband signal consisting of a transmission information signal, A mixing unit that mixes the modulated first and second optical frequency modes, The system includes a photoelectric conversion unit that converts the output signal of the mixing unit into an electrical signal, The optical modulation unit includes an optical subcarrier generation unit that generates a plurality of optical subcarrier groups from the first optical frequency mode, The optical modulation elements include a group of optical modulation elements that independently modulate the optical subcarrier group according to the baseband signal, The optical subcarrier generation unit includes a first optical subcarrier generation unit that generates a first optical subcarrier group located at a first difference frequency distance on both sides of the first optical frequency mode, and a second optical subcarrier generation unit that generates a second optical subcarrier group located at a second difference frequency distance on both sides of each of the first optical subcarrier groups. The optical modulation element group comprises optical modulation elements that frequency-division multiplex modulate the second optical subcarrier group according to the baseband signal, in an optoelectronic conversion device. [Claim 2] The photoelectric conversion device according to claim 1, wherein a first optical modulation unit and a second optical modulation unit are provided for each of the first and second optical frequency modes. [Claim 3] The photoelectric conversion device according to claim 1, wherein the optical modulation unit modulates the amplitude of the optical frequency comb and supplies the optical frequency comb to the photoelectric conversion unit. [Claim 4] The photoelectric conversion device according to claim 1, wherein the frequency interval is 300 GHz or more and 1 THz or less. [Claim 5] The photoelectric conversion device according to claim 1, wherein the photoelectric conversion unit comprises a single-traveling carrier photodiode. [Claim 6] The photoelectric conversion device according to claim 1, wherein the micro-optical resonator is a medium having a nonlinear optical effect and is composed of one or more media selected from the group consisting of silicon nitride (Si3N4), aluminum gallium arsenide (AlGaAs), lithium niobate (LiNbO3), tantalum pentoxide (Ta2O5), and gallium nitride (GaN). [Claim 7] The photoelectric conversion device according to claim 1, wherein the frequency spacing of the second optical subcarrier group is 10 GHz or more and 50 GHz or less.

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