Frequency-multiplex radio transmission device

Abstract

To provide a frequency multiplex photoelectric conversion device which makes low phase noise and bulk radio communications possible in terahertz bands.SOLUTION: A frequency multiplex photoelectric conversion device comprises: a plurality of micro optical resonators 201-204 each excited by laser light emitted from a plurality of laser light-emitting units 101-104 and generating optical frequency combs at mutually different frequency intervals in 100 GHz or higher and 3 THz or lower; modulation optical signal generation units 301-304 including a plurality of optical modulation sections 3013 each separating a couple of mutually adjacent optical frequency modes from each optical frequency combs and performing optical modulation on one optical frequency mode included in the optical frequency mode couple and a plurality of mixing sections 3020 each mixing the one optical frequency mode in which the optical modulation is performed with the other optical frequency mode on which the optical modulation is not performed; and a plurality of photoelectric conversion elements 401-404 connected to outputs of the mixing parts 3020.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a frequency division multiplexer (HzD) optoelectronic converter that divides a baseband signal and transmits it as a frequency division multiplexed signal. [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 driven generational advancements. 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 disclosed, which involves filtering out arbitrary optical frequency modes with a desired frequency interval from an optical frequency comb (Patent Document 1). Furthermore, a technique for generating an optical frequency comb from a micro-optical resonator is disclosed (Non-Patent Document 2). [Prior art documents] [Patent Documents]

[0005]

Patent Document 1

Non-Patent Documents

[0006]

Non-Patent Document 1

Non-Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0007] However, as in Non-Patent Document 1, when an independent laser with a wavelength separated only by the frequency of the radio carrier is used as a light source, frequency fluctuations and phase noise of the carrier wave occur due to fluctuations in the frequency or phase between them. Also, even when separating and extracting two-wavelength mode light having a frequency interval equal to the radio carrier frequency from an optical frequency comb, if the radio carrier frequency is sufficiently larger than the mode interval of the optical frequency comb, instead of adjacent two-wavelength mode light, two-wavelength mode light is extracted with a plurality of optical comb modes sandwiched therebetween, and phase noise increases due to the frequency doubling effect.

[0008] In addition, in order to achieve ultra-high speed and ultra-large capacity of wireless communication, a wireless transmission system with higher frequency utilization efficiency and higher information transmission speed is required. However, there is a limit to wireless transmission in a single frequency channel, and multiplexing in the frequency domain is necessary. In Non-Patent Document 2, terahertz waves are generated by optical / electrical conversion of adjacent two-wavelength mode light, but frequency division multiplexing modulation is not described.

Means for Solving the Problems

[0009] A frequency division multiplexer photoelectric converter according to one aspect of the present invention comprises: a plurality of minute optical resonators excited by laser light emitted from a plurality of laser light-emitting units to generate optical frequency combs with different frequency intervals between 100 GHz and 3 THz; a plurality of optical modulation units that separate adjacent optical frequency mode pairs from each optical frequency comb and perform optical modulation on one of the optical frequency mode modes included in the optical frequency mode pair; a plurality of mixing units that mix the optically modulated optical frequency mode with the other optical frequency mode that is not optically modulated; and a plurality of photoelectric converters connected to each output of the mixing unit.

[0010] The aforementioned frequency interval may be 300 GHz or more and 1 THz or less.

[0011] The miniature optical resonators constituting the aforementioned group of miniature optical resonators may be supplied with laser light obtained by demultiplexing the laser light emitted by a laser element.

[0012] The photoelectric conversion unit is composed of a single photoelectric conversion element, and the optical frequency mode pairs extracted from each optical frequency comb are spaced apart at a frequency interval higher than the high-frequency cutoff frequency of the photoelectric conversion element, and the optical frequency mode pairs combine to form the optical It may also be supplied to an electrical conversion element.

[0013] The photoelectric conversion element may be composed 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 difference in frequency intervals of the optical frequency comb may be between 10 GHz and 50 GHz. [Effects of the Invention]

[0016] According to one aspect of the present invention, a small optical resonator is provided for each divided frequency channel, and the optical frequency comb generated from each small optical resonator is used for frequency division multiplexing without being multiplied. This makes it possible to realize a wireless transmission device with low phase noise and high information transmission speed. [Brief explanation of the drawing]

[0017] [Figure 1] This is a block diagram of a photoelectric conversion device using an optical frequency comb. [Figure 2] This is a block diagram of a frequency division multiplexer photoelectric converter according to the first embodiment of the present invention. [Figure 3] This is a block diagram of the first embodiment of the present invention when a single laser element is used. [Figure 4] This is an explanatory diagram of the operation of the first embodiment of the present invention. [Figure 5] This is a block diagram of a frequency-multiplexed photoelectric converter according to a second embodiment of the present invention. [Figure 6] This is an explanatory diagram of the operation of a second embodiment of the present invention. [Figure 7] This is a block diagram of a second embodiment of the present invention in which a single laser element is used. [Figure 8] This is an explanatory diagram illustrating the operation of one embodiment of the present invention. [Modes for carrying out the invention]

[0018] The frequency division multiplexer (FDM) optical-electric converter in this embodiment is intended to receive radio waves from wireless terminals within a cell or small cell and transmit them wirelessly between the FDM and a network-connected exchange or relay station, in what is known as the fronthaul. In particular, to increase the transmission speed, a frequency division multiplexer wireless transmission method is used in which the transmission bandwidth is divided into multiple frequency channels. Before describing this embodiment, the basic configuration and operation of an optical-electric converter using an optical frequency comb without frequency division will be described as a reference example.

[0019] (Example explanation) Figure 1 shows a block diagram of the photoelectric conversion device. In Figure 1, 10 is a wireless terminal. There may be one or more wireless terminals 10. Also, the wireless terminals 10 may be mobile or fixed terminals. 11 is a receiving antenna that receives the wireless signal S1 from the wireless terminal 10. The receiving antenna 11 may be an antenna array consisting of multiple antenna elements to accommodate multiple wireless terminals 10. It may also consist of multiple antenna groups that support multiple wireless communication standards such as frequency. The information signal demodulation unit 12 demodulates the information signal contained in the wireless signal S1. The information signal demodulation unit 12 may support multiple wireless communication standards such as LTE and 5G.

[0020] Furthermore, in Figure 1, 1 is a laser light-emitting element 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. 2 is a micro-optical resonator, which is excited by laser light 1 to generate an optical frequency comb. An optical frequency comb is a sequence of multiple optical frequency modes that are of equal frequency (f repThis refers to an ultra-discrete multispectral structure in which the optical atoms are arranged in a comb-like pattern at intervals and with aligned optical phases. The micro-optical resonator 2 may be formed in a ring shape on a semiconductor substrate. Its diameter may be 40 μm to 400 μm. The medium may also be a medium having a nonlinear optical effect, 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).

[0021] Because the optical frequency comb generated by the micro-optical resonator 2 has a short optical resonator length, the frequency interval (f) between adjacent optical frequency modes is short. rep The frequency interval (f) between adjacent optical frequency modes can be increased. rep The frequency may be 100 GHz or more and 3 THz or less. More preferably, it may be 300 GHz or more and 1 THz or less. Even more preferably, it may be 350 GHz or more and 600 GHz or less.

[0022] 30 is a coupler, 31 and 32 are bandpass filters, 33 is an optical modulation unit, and 34 is an optical amplification element. These elements constitute the modulated optical signal generation unit 3. The modulated optical signal generation unit 3 separates adjacent optical frequency modes from the optical frequency comb and performs optical modulation on one of them according to the baseband signal S2. Note that AWG (array waveguide diffractometer) may be used instead of bandpass filters 31 and 32. The optically modulated optical frequency mode m1 (frequency ν1) is mixed with the optically unmodulated optical frequency mode m0 in the mixing unit 20 and supplied to the photoelectric conversion unit 4 via the optical amplification element 34. Note that the optical amplification element 34 may not be inserted depending on the level of the optical signal input to the photoelectric conversion unit 4. The photoelectric conversion element 4 may consist of a single-traveling carrier photodiode (UTC-PD).

[0023] The other optical frequency mode m0(ν0) is supplied directly to the photoelectric conversion element 4 via the optical amplification element 34. In the photoelectric conversion element 4, optical frequency mode m1 and optical frequency mode 0 are mixed (S3), and their difference frequency (ν1-ν0=f rep The photoelectric conversion element 4 outputs electromagnetic waves (terahertz waves). The photoelectric conversion element 4 is directly connected to the antenna 5, and the terahertz waves (S4) are radiated into the air from the antenna 5.

[0024] (First Embodiment) Hereinafter, one embodiment of the present invention will be described in detail with reference to the drawings. Figure 2 shows a block diagram of this embodiment. In Figure 2, 301, 302, 303, and 304 are modulated optical signal generation units. In this embodiment, four-segment multiplex modulation is used, but the present invention is not limited thereto. The modulated optical signal generation unit 301 is composed of bandpass filters 3011 and 3012, a modulation unit 3013, a mixing unit 3020, and an optical amplification element 3014, and has the same function as the modulated optical signal generation unit 3 shown in Figure 1. The modulated optical signal generation units 302, 303, and 304 also have the same configuration as the modulated optical signal generation unit 301, except for the filter constants etc. which will be described later.

[0025] In Figure 2, 101, 102, 103, and 104 are laser light-emitting units. In this embodiment, each is a DFB laser whose emission wavelength is tuned to 1550 nm or a wavelength around that wavelength. 201, 202, 203, and 204 are each miniature optical resonators, which are excited by laser light to generate optical frequency combs. The miniature optical resonators 201, 202, 203, and 204 have different optical resonator lengths, and the frequency interval f of the optical frequency combs they generate is different. rep1 ,f rep2 ,f rep3 ,f rep4They are also different from each other. The frequency interval of the optical frequency comb may be 100 GHz or more and 1 THz or less. More preferably, it may be 350 GHz or more and 600 GHz or less. Note that the laser light supplied to each micro optical resonator is not limited to the present embodiment. For example, as shown in FIG. 3. It may be one obtained by branching a plurality of laser lights from a single high-power laser light emitting element 10.

[0026] With the above configuration, the state where the signals modulated respectively are multiplexed is shown in FIG. 4. In the present embodiment, the frequency interval is f rep1 <f rep2 <f rep3 <f rep4 The optical resonator length is designed so that. The difference Δf rep in the frequency interval may be 10 GHz or more and 5 GHz or less. Here, the difference Δf rep in the frequency interval means the increase in the frequency interval that increases stepwise when the frequency intervals are arranged as f rep1 <f rep2 <f rep3 <f rep4 and so on.

[0027] The optical frequency comb group generated as described above is separated from each other in the adjacent optical frequency modes in the modulation optical signal generation units 301, 302, 303, 304, and optical modulation is performed on one of them. The modulation method may be a method including amplitude modulation, phase modulation such as QPSK, QAM, or both of them. S21, S22, S23, S24 are information streams constituting the baseband signal composed of the information transmission signal, respectively.

[0028] The modulated optical frequency mode and its adjacent optical frequency mode are supplied to the optoelectronic conversion elements 401, 402, 403, 404 through an optical fiber or the like, respectively, and further mixed, and the optical beat frequency thereof is f rep1 、f rep2 、f rep3 、f rep4This is converted into radio signals S41, S42, S43, and S44 with the carrier frequency as the carrier frequency. These radio signals are combined in antenna 50, which is directly connected to the photoelectric conversion element group, and transmitted as frequency-multiplexed radio signal S4 (Figure 4).

[0029] The frequency intervals (f) of the wireless signal bands S41, S42, S43, and S44 are different. rep1 ~f rep4 ) frequency interval (Δf rep It is preferable that the frequency range is less than or equal to 50 GHz. The frequency spacing of the optical frequency comb may also be between 10 GHz and 50 GHz. The polarization of the wireless signals S41, S42, S43, and S44 may also be alternately orthogonal. In this way, each bandwidth can be expanded without interference from adjacent frequency channels.

[0030] As described above, the generated frequency-division multiplexed radio signal S4 is generated from adjacent optical frequency modes of the optical frequency comb in all frequency channels, thus minimizing phase noise.

[0031] (Second Embodiment) A second embodiment of the present invention will be described below with reference to Figures 5 and 6. In Figure 5, the laser light-emitting elements 101, 102, 103, 104, the micro-optical resonators 201, 202, 203, 204, and the modulated optical signal generation units 301, 302, 303, 304 have the same functions as those shown in Figure 2. In this embodiment, the photoelectric conversion unit 40 is composed of a single photoelectric conversion element.

[0032] Furthermore, in this embodiment, the optical frequency mode pair (ν) shown by the thick line in Figure 6 is 10 / ν 11 ν 20 / ν 21 ν 30 / ν 31 ν 40 / ν 41 ) is the high-frequency cutoff frequency f of the photoelectric conversion element output (electrical side) that constitutes the photoelectric conversion unit 40 cThe optical bandpass filters (only optical bandpass filter 3012 is shown in the figure) extract from each frequency comb so that they are spaced apart at a higher frequency interval than ν. That is, as shown in Figure 6, the optical frequency mode pair ν generated in the system of the micro-optical resonator 201 10 / ν 11 Among them, the high-frequency light frequency ν 11 and adjacent optical frequency modes pair ν 20 / ν 21 The low-frequency range of light ν 20 The interval is the high-frequency cutoff frequency f of the photoelectric conversion element. c The frequency is higher than that. Although not shown in the diagram, the frequency ν 21 and ν 30 ν 31 and ν 40 The same relationship applies to optical frequency modes of light.

[0033] Optical frequency mode pair (ν 10 / ν 11 ν 20 / ν 21 ν 30 / ν 31 ν 40 / ν 41 Because of the above relationship, even if these are combined and supplied to a single photoelectric conversion element 40, no beat signal is generated between different optical frequency combs, and no beat signal (frequency f) is generated between each optical frequency mode pair. rep1 ,f rep2 ,f rep3 ,f rep4 <f c It can only generate ).

[0034] In the above description of this embodiment, it has been assumed that a laser light-emitting element is provided for each micro-optical resonator. However, when the degree of multiplicity is relatively low, as shown in Figure 7, the laser light emitted from a single high-power laser element 10 can be split and supplied to each micro-optical resonator. Alternatively, the group of micro-optical resonators can be divided into subgroups, and a single laser element can be used in each subgroup.

[0035] The following describes embodiments of the present invention. In these embodiments, the overall gain of the four-wave multiplexed wireless transmission system shown in Figure 2 was estimated after considering the losses in each process (Figure 8).

[0036] First, any adjacent optical frequency mode pair is isolated from each optical frequency comb. Assuming the power of each optical frequency mode is 15 mW (11.76 dBm), a total of 120 mW (20.76 dBm) of optical carriers is generated with 2 modes × 4 channels. Meanwhile, assuming that 6 dB of loss occurs in the array waveguide diffractometer (AWG), 10 dB in the optical modulator, 6 dB in the multiplexing process, and 3 dB in the optical bandpass filter, and that a gain increase of 30 dB is achieved with the optical amplifier, a gain of 11.76 - 6 - 10 - 6 + 30 - 3 = 16.76 dBm is obtained per single mode up to this point (Figure 8).

[0037] Furthermore, the loss in the optical THz conversion element (photoelectric conversion section) is estimated to reach 30 dB at 300 GHz, so ultimately an output of 47.4 μW per single mode (16.76 - 30 = -13.24 dBm) and 189.6 μW total for all four frequency channels combined is obtained. [Industrial applicability]

[0038] 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]

[0039] 10 Wireless terminals 1. 101-104 Laser emission section 11 Receiving antenna 12 Information signal demodulation unit 20 Mixing section 2, 201~204 Microscopic optical cavity 3, 301-304 Modulated optical signal generation unit 30 Couplers 31, 32, 3011, 3012 Bandpass filters 3010 Mixing section 33, 3013 Optical Modulation Section 34, 3014 Optical Amplifier 4. Optical / Electric Conversion Unit 401-404 Optical / Electric Conversion Elements 5, 50, 500 transmitting antennas

Claims

1. A plurality of miniature optical resonators, each individually supplied with and excited by laser light emitted from at least one laser light-emitting unit, to generate optical frequency combs with different frequency intervals between 100 GHz and 3 THz, Multiple optical modulation units separate adjacent optical frequency mode pairs from each optical frequency comb and perform optical modulation on one of the optical frequency modes included in the optical frequency mode pair, Multiple mixing units that mix one optically modulated optical frequency mode with the other optical frequency mode that is not optically modulated, A frequency-multiplexed photoelectric converter comprising a plurality of photoelectric conversion units connected to each output of the mixing unit.

2. The frequency division multiplexer photoelectric converter according to claim 1, wherein the frequency interval is 300 GHz or more and 1 THz or less.

3. The frequency division multiplexer photoelectric converter according to claim 1, wherein the photoelectric conversion unit is a standalone unit and is connected to combine the outputs of the plurality of mixing units, and the frequency spacing between the optical frequency mode pairs is set at a frequency spacing higher than the high-frequency cutoff frequency (fc) of the photoelectric conversion unit.

4. The frequency multiplexer photoelectric converter according to claim 1, which includes a single-travel carrier photodiode in the photoelectric conversion unit.

5. The frequency division multiplexer photoelectric converter according to claim 1, wherein the micro-optical resonator is a medium having a nonlinear optical effect, and comprises 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).

6. The frequency division multiplexer photoelectric converter according to claim 3, characterized in that the difference (Δfrep) between the frequency intervals (frep1 to frep4) of the optical frequency combs is 10 GHz or more and 50 GHz or less.

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