Transmitter and transmission method

Abstract

To provide a transmitter that emits a low-noise, high-frequency signal.SOLUTION: A transmitter includes an optical com signal generation unit that generates a first optical com signal having a plurality of emission lines arranged at equal intervals in the frequency domain, a modulation unit that modulates the first optical com signal with a transmission signal to output a second optical com signal, a photonic filter unit that filters the second optical com signal to delay adjacent emission lines by a predetermined time and outputs a third optical com signal, and a photoelectric conversion unit that converts, into an electric signal, a beat signal between adjacent emission lines in the second optical com signal in the third optical com signal.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure relates to a transmitter and a transmitting method. [Background technology]

[0002] Non-Patent Document 1 discloses a transmitter that generates millimeter waves of 300 GHz. Non-Patent Document 2 discloses a filter at radio frequencies that uses a soliton comb. [Prior art documents] [Non-patent literature]

[0003] [Non-Patent Document 1] Tetsumoto, T., Nagatsuma, T., Fermann, ME et al. Optically referenced 300 GHz millimetre-wave oscillator. Nat. Photon. 15, 516-522 (2021). https: / / doi.org / 10.1038 / s41566-021-00790-2 [Non-patent document 2] Hu, J., He, J., Liu, J. et al. Reconfigurable radiofrequency filters based on versatile soliton microcombs. Nat Commun 11, 4377 (2020). https: / / doi.org / 10.1038 / s41467-020-18215-z Summary of the Invention [Problem to be solved by the invention]

[0004] Toward the realization of sixth-generation (6G) information and communications, it has been internationally agreed that terahertz waves, for example, in the 300 gigahertz band, will be utilized.

[0005] There is a demand for low-noise, high-power communication signals using high frequencies such as the 300 GHz band. To generate low-noise, high-power communication signals, it is necessary to generate terahertz waves modulated with the signal to be transmitted.

[0006] The present disclosure provides a transmitter that emits a low-noise, high-frequency signal. [Means for solving the problem]

[0007] One aspect of the present disclosure provides an oscillator including: an optical comb signal generation unit that generates a first optical comb signal having a plurality of emission lines that are evenly spaced in the frequency domain; a modulation unit that modulates the first optical comb signal with a transmission signal to output a second optical comb signal; a photonic filter unit that filters the second optical comb signal to impose a predetermined time delay between adjacent emission lines, and outputs a third optical comb signal; and an opto-electrical conversion unit that converts, into an electric signal, a beat signal between adjacent emission lines in the second optical comb signal in the third optical comb signal. [Effects of the Invention]

[0008] The transmitter of the present disclosure can emit a low-noise, high-frequency signal. [Brief explanation of the drawings]

[0009] [Figure 1] FIG. 1 is a diagram showing an outline of a transmitter according to a first embodiment. [Figure 2] FIG. 2 is a diagram illustrating an optical comb signal generated by an optical comb generating unit in the transmitter according to the first embodiment. [Figure 3] FIG. 3 is a diagram illustrating an optical comb signal generated by an optical comb generating unit in the transmitter according to the first embodiment. [Figure 4] FIG. 4 is a diagram showing an outline of a transmitter according to the second embodiment. [Figure 5] FIG. 5 is a diagram showing an outline of a transmitter according to the third embodiment. [Figure 6] FIG. 6 is a diagram illustrating a first example of the filter characteristics of the transmitter according to this embodiment. [Figure 7] FIG. 7 is a diagram illustrating an optical comb signal when evaluating a first example of the filter characteristics of the transmitter according to this embodiment. [Figure 8] FIG. 8 is a diagram illustrating a second example of the filter characteristics of the transmitter according to this embodiment. [Figure 9] FIG. 9 is a diagram illustrating an optical comb signal when evaluating a second example of the filter characteristics of the transmitter according to this embodiment. [Figure 10] FIG. 10 is a diagram illustrating a first example of filter characteristics in a transmitter of a reference example. [Figure 11] FIG. 11 is a diagram illustrating a second example of the filter characteristics in the transmitter of the reference example. [Figure 12] FIG. 12 is a diagram illustrating an operation example 1 of the transmitter according to this embodiment. [Figure 13] FIG. 13 is a diagram illustrating an operation example 1 of the transmitter of the reference example. [Figure 14] FIG. 14 is a diagram illustrating an operation example 2 of the transmitter according to this embodiment. [Figure 15] FIG. 15 is a diagram illustrating an optical comb signal in the second operation example of the transmitter according to the present embodiment. [Figure 16] FIG. 16 is a diagram illustrating an operation example 2 of the transmitter of the reference example. [Figure 17] FIG. 17 is a diagram illustrating an operation example 3 of the transmitter according to this embodiment. [Figure 18] FIG. 18 is a diagram illustrating an optical comb signal in the third operational example of the transmitter according to the present embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0010] Hereinafter, embodiments will be described with reference to the accompanying drawings. Note that the present disclosure is not limited to these examples, but is defined by the claims, and is intended to include all modifications within the meaning and scope of the claims.

[0011] In addition, with regard to the description of the specification and drawings of each embodiment, components having substantially the same or corresponding functional configurations may be designated by the same or corresponding reference numerals to avoid redundant explanation. In addition, the scale of each part in the drawings may differ from the actual scale to facilitate understanding.

[0012] First Embodiment An oscillator according to the first embodiment will be described. The oscillator according to the first embodiment includes an optical comb signal generator, a modulator, and a frequency shifter. The optical comb signal generator in the oscillator according to the first embodiment generates a first optical comb signal having a plurality of emission lines arranged at equal intervals in the frequency domain. The modulator in the oscillator according to the first embodiment modulates the first optical comb signal with a transmission signal to output a second optical comb signal. When modulating the first optical comb signal with the transmission signal, for example, the intensity (amplitude), phase, or both the intensity (amplitude) and phase of the first optical comb signal are modulated. The frequency shifter in the oscillator according to the first embodiment includes a photonic filter and an opto-electric converter. The photonic filter included in the frequency shifter in the oscillator according to the first embodiment filters the second optical comb signal to delay adjacent emission lines by a predetermined time, and outputs a third optical comb signal. The delay is set in a fixed direction relative to the frequency. For example, the delay time increases as the frequency increases. The photoelectric conversion unit included in the frequency shifter in the transmitter according to the first embodiment converts the beat signal between adjacent emission lines in the second optical comb signal into an electrical signal in the third optical comb signal.

[0013] A transmission method using an oscillator according to a first embodiment will be described. The transmission method according to the first embodiment includes generating a first optical comb signal having a plurality of emission lines arranged at equal intervals in the frequency domain. The transmission method according to the first embodiment also includes modulating the first optical comb signal with a transmission signal to generate a second optical comb signal. The transmission method according to the first embodiment further includes filtering the second optical comb signal to delay adjacent emission lines by a predetermined time, thereby generating a third optical comb signal. The transmission method according to the first embodiment also includes converting a beat signal between adjacent emission lines in the third optical comb signal into an electrical signal.

[0014] The details of the transmitter according to the first embodiment will be described with reference to the drawings. Fig. 1 is a diagram showing an outline of a transmitter 1, which is an example of the transmitter according to the first embodiment.

[0015] The transmitter 1 emits radio waves R1, which are terahertz waves. The transmitter 1 particularly emits radio waves R1, which are millimeter waves in the 300 GHz band. The transmitter 1 includes an optical comb generator 10, a modulator 20, and a frequency shifter 30.

[0016] [Optical comb generator 10] The optical comb generator 10 outputs short optical pulses repeatedly at a predetermined cycle, and generates an optical comb signal L1 having a plurality of emission lines arranged at equal intervals in the frequency domain.

[0017] 2 and 3 are diagrams illustrating the optical comb signal L1 generated by the optical comb generation unit 10 in the transmitter 1, which is an example of the transmitter according to the first embodiment.

[0018] FIG. 2 is a diagram illustrating the optical comb signal L1 in the time domain. The horizontal axis of FIG. 2 represents time, and the vertical axis represents the amplitude of the optical comb signal L1. Line W1 represents the waveform of the optical comb signal L1. Line W1e represents the envelope of the optical comb signal L1. FIG. 3 is a diagram illustrating the optical comb signal L1 in the frequency domain. The horizontal axis of FIG. 3 represents frequency, and the vertical axis represents spectral intensity. Line SP1 represents the spectrum of the optical comb signal L1.

[0019] In the frequency domain, the optical comb signal L1 has a spectrum in which multiple emission lines EL are arranged at a frequency interval Δfrep. In the time domain, the optical comb signal L1 includes multiple short optical pulses P that are repeatedly output with a period 1 / Δfrep. In the transmitter 1, the frequency interval Δfrep is equal to the carrier frequency of the radio wave R1.

[0020] Among the multiple emission lines EL, if the frequency of the ith emission line (i is an integer equal to or greater than 0) is denoted as freq(i), the frequency freq(i) satisfies Equation 1.

[0021]

number

[0022] The optical comb generating unit 10 includes, for example, a laser that generates continuous light and a micro-optical resonator in order to generate the optical comb signal L1.

[0023] [Modulation section 20] The modulator 20 modulates the optical comb signal L1 output from the optical comb generator 10 with a transmission signal S1, which is a signal to be transmitted. The modulator 20 then modulates the optical comb signal L1 with the transmission signal S1 to generate an optical comb signal L2, which is output to the frequency shifter 30. The modulation of the optical comb signal L1 in the modulator 20 may be intensity (amplitude) modulation, phase modulation, or a combination of intensity (amplitude) modulation and phase modulation.

[0024] The modulation section 20 includes, for example, an LN optical modulator using lithium niobate and a Mach-Zehnder modulator.

[0025] If the complex electric field L1 of the optical comb at time t is x(t), the complex electric field signal in the transmission signal S1 is s(t), and the optical comb signal L2 output by the modulation section 20 is y(t), then y(t) satisfies Equation 2.

[0026]

number

[0027] [Frequency shift section 30] The frequency shifter 30 converts the optical comb signal L2 into a radio wave R1. The frequency shifter 30 includes a photonic filter 31, an opto-electric converter 32, and an antenna 33.

[0028] (Photonic filter section 31) The photonic filter unit 31 includes a photonic filter that filters the optical comb signal L2. The photonic filter optically filters the optical comb signal L2. The photonic filter unit 31 delays one emission line having adjacent frequencies in the optical comb signal L2, which includes multiple frequencies, by a predetermined time (Tdelay) relative to the other emission line.

[0029] Here, a case will be described in which the photonic filter unit 31 performs processing using N (N is an integer of 2 or more) emission lines in the optical comb signal L2.

[0030] Of the N emission lines, the frequency of the emission line with the lowest frequency is defined as f(0).Then, the frequency of the emission line with the lowest frequency, k+1 (k is an integer between 0 and N-1) is defined as f(k).The frequency f(k) satisfies Equation 3.

[0031]

number

[0032] The photonic filter unit 31 delays the (k+1)th emission line by the delay time Td(k) shown in Equation 4.

[0033]

number

[0034] The transfer function Hpf(ω) in the photonic filter unit 31 satisfies Equation 5.

[0035]

number

[0036] Here, z is defined by Equation 6.

[0037]

number

[0038] When z is used, the transfer function Hpfz(z) of Equation 5 becomes the same as that of a so-called FIR (Finite Impulse Response) filter, as shown in Equation 7. That is, the photonic filter unit 31 can perform filtering similar to that of an FIR filter.

[0039]

number

[0040] When an optical comb signal is used, N, which corresponds to the order of the FIR filter, is the number of emission lines of the optical comb signal used. Furthermore, p(k) is the power of the emission lines of the optical comb signal. The reciprocal of the delay amount Tdelay corresponds to the period of the filter's transmission band. For example, if the delay amount Tdelay is set to 100 picoseconds, the period of the filter's transmission band is 10 gigahertz. Furthermore, if the delay amount Tdelay is set to 200 picoseconds, the period of the filter's transmission band is 5 gigahertz.

[0041] Finally, a transfer function Hpfc(ω) in the band of the transmission signal S1 will be described, since the signal is photoelectrically converted in the photoelectric conversion unit 32. The transfer function Hpfc(ω) in the band of the transmission signal S1 satisfies Equation 8.

[0042]

number

[0043] Therefore, the photonic filter unit 31 can generate a beat signal between adjacent emission lines in the optical comb signal L2 in the band of the transmission signal S1.

[0044] The intensity (amplitude) of the optical comb signal L2 may be amplified by an optical amplifier circuit using a fiber containing a rare earth element, for example, erbium, before the optical comb signal L2 is input to the photonic filter unit 31. In other words, the frequency shifter 30 may include an erbium-doped fiber amplifier before the photonic filter unit 31.

[0045] (Photoelectric conversion unit 32) The photoelectric conversion unit 32 converts the optical comb signal L3 into an electrical signal. More specifically, the photoelectric conversion unit 32 converts the beat signal between adjacent emission lines in the optical comb signal L2 into an electrical signal. The photoelectric conversion unit 32 then transmits radio waves R1 from the antenna 33.

[0046] The photoelectric conversion unit 32 includes a light receiving element 32a. The light receiving element 32a is, for example, a uni-traveling-carrier photodiode (UTC-PD). The frequency of the beat signal between adjacent emission lines in the optical comb signal L2 is approximately equal to the frequency interval Δfrep. Therefore, it is desirable that the light receiving element 32a be an element that responds quickly in accordance with the frequency interval Δfrep.

[0047] For example, when the frequency interval Δfrep is 300 GHz, as described above, the light receiving element 32a is preferably a uni-traveling-carrier photodiode (UTC-PD).

[0048] In order to transmit the electrical signal from the light receiving element 32a through the antenna 33, an amplifier circuit for amplifying the electrical signal may be provided.

[0049] (Antenna 33) The antenna 33 transmits radio waves R1 to the outside. If the result of Fourier transform of r(t), which is the radio waves R1, is R(ω), then R(ω) satisfies Equation 9.

[0050]

number

[0051] If the result of Fourier transform of x(t), which is L3, is X(ω) and the result of Fourier transform of y(t), which is optical comb signal L2, is Ypf(ω), then Ypf(ω) satisfies Equation 10.

[0052]

number

[0053] The transmitter according to the first embodiment can transmit a low-noise high-frequency signal. By using a 300 GHz band photonic filter, the transmitter according to the first embodiment can achieve a high suppression ratio in the stopband of the filter. For example, the transmitter according to the first embodiment can realize a photonic filter operating in the 300 GHz band by detecting the beat signal of the adjacent longitudinal mode of the soliton comb using a uni-traveling-carrier photodiode as the light-receiving element.

[0054] Second Embodiment The transmitter according to the second embodiment includes an optical demultiplexer, multiple delay lines, and an optical multiplexer as the photonic filter section of the frequency shifter in the transmitter according to the first embodiment. The optical demultiplexer in the transmitter according to the second embodiment demultiplexes the second optical comb signal based on the frequency of the emission line in the second optical comb signal. Each of the multiple delay lines in the transmitter according to the second embodiment delays each component of the second optical comb signal demultiplexed by the optical demultiplexer. The optical multiplexer in the transmitter according to the second embodiment combines the second optical comb signals that have been demultiplexed and passed through each of the multiple delay lines, and outputs the combined signal as a third optical comb signal.

[0055] The details of the transmitter according to the second embodiment will be described with reference to the drawings. Fig. 4 is a diagram showing an outline of a transmitter 2, which is an example of the transmitter according to the second embodiment.

[0056] The transmitter 2 emits radio waves R1, which are terahertz waves. The transmitter 2 particularly emits radio waves R1, which are millimeter waves in the 300 GHz band. The transmitter 2 includes an optical comb generation unit 10, a modulation unit 20, and a frequency shift unit 130. Regarding the configuration of the transmitter 2, which is an example of the transmitter according to the second embodiment, which is common to the transmitter 1, which is an example of the transmitter according to the first embodiment, the description of the transmitter 1 should be referred to, and detailed description thereof will be omitted here.

[0057] [Frequency shift unit 130] The frequency shifter 130 converts the optical comb signal L2 into a radio wave R1. The frequency shifter 130 includes a photonic filter 131, an opto-electrical converter 32, and an antenna 33.

[0058] (Photonic filter section 131) The photonic filter unit 131 includes a photonic filter that filters the optical comb signal L2. The photonic filter optically filters the optical comb signal L2. The photonic filter unit 131 delays one of adjacent emission lines by Tdelay relative to the other in the optical comb signal L2, which includes multiple frequencies.

[0059] The photonic filter unit 131 includes an optical demultiplexer 131a, a plurality of delay lines 131b, and an optical multiplexer 131c.

[0060] The optical demultiplexer 131a demultiplexes the optical comb signal L2 according to the frequency of the optical comb signal L2.

[0061] The plurality of delay lines 131b delay each of the optical comb signals L2 demultiplexed by the optical demultiplexer 131a, and delay one of the emission lines of adjacent frequencies by Tdelay relative to the other.

[0062] Photonic filter unit 131 includes delay lines 131b equal to the number of emission lines required for filtering. For example, when N emission lines (N is an integer of 2 or more) are used, photonic filter unit 131 includes N delay lines 131b.

[0063] For example, the delay line 131b corresponding to the k+1th (k is an integer between 0 and N-1) emission line in ascending order of frequency has a delay that is k times the time Tdelay relative to the delay line 131b corresponding to the first emission line.

[0064] The optical combiner 131c combines the optical comb signal L2 delayed according to the frequency of the optical comb signal L2.

[0065] The transmitter according to the second embodiment can transmit a low-noise high-frequency signal. The transmitter according to the second embodiment can achieve a high suppression ratio in the stopband of a 300 GHz band photonic filter by using a 300 GHz band photonic filter. The transmitter according to the second embodiment can achieve a photonic filter that operates in the 300 GHz band by detecting the beat signal of the adjacent longitudinal mode of the soliton comb using, for example, a uni-traveling-carrier photodiode as a light-receiving element.

[0066] Third Embodiment The transmitter according to the third embodiment includes an optical filter and a dispersion compensation fiber as the photonic filter section of the frequency shifter in the transmitter according to the first embodiment. The optical filter in the transmitter according to the third embodiment extracts necessary emission lines from among the emission lines of the second optical comb signal. The dispersion compensation fiber in the transmitter according to the second embodiment delays each emission line of the second optical comb signal according to the frequency of the emission line in the second optical comb signal. The dispersion compensation fiber in the transmitter according to the second embodiment then combines the second optical comb signal that has passed through it and outputs it as a third optical comb signal.

[0067] The details of the transmitter according to the third embodiment will be described with reference to the drawings. Fig. 5 is a diagram showing an outline of a transmitter 3, which is an example of the transmitter according to the third embodiment.

[0068] The transmitter 3 emits radio waves R1, which are terahertz waves. The transmitter 3 particularly emits radio waves R1, which are millimeter waves in the 300 GHz band. The transmitter 3 includes an optical comb generation unit 10, a modulation unit 20, and a frequency shift unit 230. Regarding the configuration of the transmitter 3, which is an example of the transmitter according to the third embodiment, which is common to the transmitter 1, which is an example of the transmitter according to the first embodiment, the description of the transmitter 1 should be referred to, and detailed description thereof will be omitted here.

[0069] [Frequency shift unit 230] The frequency shifter 230 converts the optical comb signal L2 into a radio wave R1. The frequency shifter 230 includes a photonic filter 231, an opto-electric converter 32, and an antenna 33.

[0070] (Photonic filter section 231) The photonic filter unit 231 includes a photonic filter that filters the optical comb signal L2. The photonic filter optically filters the optical comb signal L2. The photonic filter unit 231 delays one emission line having adjacent frequencies by a time Tdelay relative to the other emission line in the optical comb signal L2 that includes multiple frequencies.

[0071] The photonic filter section 231 includes an optical filter 231a and a dispersion compensation fiber 231b.

[0072] The optical filter 231a extracts necessary emission lines from the optical comb signal L2. Note that the optical filter 231a may be omitted.

[0073] The dispersion compensating fiber 231b is a fiber in which the delay amount increases or decreases linearly as the frequency increases. It is desirable that the delay amount of the dispersion compensating fiber 231b increases or decreases in proportion to the increase in frequency. The dispersion compensating fiber 231b has a length such that the light passing through the dispersion compensating fiber 231b has a desired delay amount, specifically, an integer multiple of the delay time Tdelay. Multiple emission lines of the optical comb signal L2 are input to the dispersion compensating fiber 231b.

[0074] The transmitter according to the third embodiment can transmit a low-noise high-frequency signal. The transmitter according to the third embodiment can achieve a high suppression ratio in the stopband of a 300 GHz band photonic filter by using a 300 GHz band photonic filter. The transmitter according to the third embodiment can realize a photonic filter operating in the 300 GHz band by detecting the beat signal of the adjacent longitudinal mode of the soliton comb using, for example, a uni-traveling-carrier photodiode as a light-receiving element. Furthermore, the transmitter according to the third embodiment can simultaneously implement modulation of a 300 GHz band carrier and filtering function using a single optical fiber.

[0075] <Evaluation results> The results of evaluating the characteristics of the transmitter according to this embodiment will be described.

[0076] [Simulation example] The results of a simulation of the frequency response of the photonic filter section are shown below. In this simulation, the photonic filter section is operated using 10 emission lines in the optical comb signal. The delay time between emission lines, i.e., Tdelay, is set to 100 picoseconds. The delay time is set linearly for the 10 emission lines. The frequency interval Δfrep of the optical comb signal is set to 300 GHz.

[0077] A first example of the filter characteristics will be described. FIG. 6 is a diagram illustrating a first example of the filter characteristics of the transmitter according to this embodiment. The horizontal axis of FIG. 6 represents frequency, and the vertical axis represents frequency response. FIG. 7 is a diagram illustrating the optical comb signal used when evaluating the first example of the filter characteristics of the transmitter according to this embodiment. The horizontal axis of FIG. 7 represents the frequency mode number indicating the emission line of the optical comb signal, and the vertical axis represents normalized intensity. FIG. 6 shows the frequency response of the photonic filter unit when the photonic filter unit is operated using 10 emission lines in the optical comb signal shown in FIG. 7.

[0078] In the first example of the filter characteristics of the transmitter according to this embodiment shown in FIG. 6, it is assumed that the ten emission lines in the optical comb signal have equal intensities as shown in FIG.

[0079] For comparison, an oscillator based on the technique disclosed in Non-Patent Document 2 will be described as a reference example. FIG. 10 shows the filter characteristics of the oscillator of the reference example. FIG. 10 is a diagram illustrating a first example of the filter characteristics of the oscillator of the reference example. In the oscillator of the reference example, filtering is performed by a photonic filter in the 10 GHz band, which is the reciprocal of Tdelay and is the baseband band. In the first example of the filter characteristics of the oscillator of the reference example shown in FIG. 10, the optical comb signal shown in FIG. 7 is used.

[0080] Compared with FIG. 10, the filter characteristics of the transmitter according to this embodiment shown in FIG. 6 can increase the suppression ratio in the stopband.

[0081] A second example of the filter characteristics will be described. Fig. 8 is a diagram illustrating a second example of the filter characteristics of the transmitter according to this embodiment. Fig. 9 is a diagram illustrating an optical comb signal when evaluating the first example of the filter characteristics of the transmitter according to this embodiment. Fig. 8 shows the frequency response of the photonic filter unit when the photonic filter unit is operated using 10 emission lines in the optical comb signal shown in Fig. 9.

[0082] In the second example of the filter characteristics of the transmitter according to this embodiment shown in FIG. 8, the intensity of the ten emission lines in the optical comb signal is high in the center and low at the ends, as shown in FIG.

[0083] For comparison, the filter characteristics of the oscillator of the reference example are shown in Fig. 11. Fig. 11 is a diagram illustrating a second example of the filter characteristics of the oscillator of the reference example. In the oscillator of the reference example, filtering is performed by a photonic filter in the baseband. In the second example of the filter characteristics of the oscillator of the reference example shown in Fig. 11, the optical comb signal shown in Fig. 9 is used.

[0084] The filter characteristics of the oscillator according to this embodiment shown in Fig. 8 can increase the suppression ratio in the stopband compared to Fig. 11. Furthermore, by using the optical comb signal shown in Fig. 9, the frequency resolution of the main lobe increases compared to the flat comb line power shown in Fig. 7, but a large suppression ratio can be obtained.

[0085] [Example of operation] The following describes the results of evaluating the characteristics of an actual transmitter according to this embodiment. In transmitter 3, an example of a transmitter according to the third embodiment, a dispersion compensating fiber with a delay characteristic of -340 picoseconds per nanometer is used as the dispersion compensating fiber 231b. The frequency spacing Δfrep of the optical comb signal L1 is 300 GHz, and the reciprocal of the delay Tdelay incurred when adjacent emission lines separated by 300 GHz pass through the dispersion compensating fiber is 1.2 GHz.

[0086] When performing the evaluation, a 300 GHz radio wave R1 is generated from the antenna 33 and the radio wave R1 is square-law detected by a Schottky barrier diode. Then, using a network analyzer, a signal is output from the network analyzer to the modulator 20, and a signal from the Schottky barrier diode is input to the network analyzer to measure the frequency response of the system.

[0087] (Example 1) Fig. 12 is a diagram illustrating the operational results of the transmitter according to this embodiment. The horizontal axis of Fig. 12 represents frequency, and the vertical axis represents the S parameter S21. As a comparative example, the operational results of a reference example transmitter that performs filtering using a photonic filter in the 1.2 GHz band are also shown. Fig. 13 is a diagram illustrating the operational results of the reference example transmitter.

[0088] From the results shown in FIG. 12, it can be confirmed that the transmitter according to this embodiment has a large suppression ratio of 30 dB or more in the stopband, similar to the simulation results.

[0089] (Example 2) Next, we will explain an example of operation when the bandwidth of an optical comb signal is limited. An arrayed waveguide grating was used to limit the bandwidth of the optical comb signal. The arrayed waveguide grating used was one with transmission channels spaced 100 GHz apart, which can extract approximately 10 emission lines of an optical comb signal spaced 300 GHz apart in the C band.

[0090] FIG. 14 shows the results of measuring the frequency response when the bandwidth of the optical comb signal is limited. FIG. 14 is a diagram illustrating a second operational example of the transmitter according to this embodiment. FIG. 15 is a diagram illustrating the optical comb signal in the second operational example of the transmitter according to this embodiment. FIG. 15 shows the spectrum of the emission lines in the optical comb signal extracted by the arrayed waveguide grating. Note that the attenuation at a wavelength of 1550 nanometers in FIG. 15 is due to the effect of the pump light being reflected by the fiber Bragg grating. FIG. 14 shows the results using the six emission lines on the shorter side in FIG. 15.

[0091] As a comparative example, the operational results of a reference example transmitter that performs filtering using a photonic filter in the 1.2 GHz band are shown. Fig. 16 is a diagram for explaining the operational results of the reference example transmitter. Fig. 16 shows the results using the 11 emission lines in Fig. 15.

[0092] 14 and 16 also show fitting results using a theoretical formula that takes into account dispersion up to the third order.

[0093] As shown in FIGS. 14 and 16, even when band limitation is performed, the photonic filter in the transmitter according to this embodiment can achieve a larger suppression ratio than the reference example.

[0094] (Example 3) Next, we will explain the case where a fixed optical attenuator is used to control the spectrum of an optical comb signal so that it attenuates smoothly to the left and right. A fixed optical attenuator is used to change the intensity of the spectrum of the optical comb signal.

[0095] Fig. 17 shows the results of measuring the frequency response when the spectrum of the optical comb signal is controlled to have a shape that smoothly attenuates left and right. Fig. 17 is a diagram illustrating a third operational example of the transmitter according to this embodiment. Fig. 18 is a diagram illustrating the optical comb signal in the third operational example of the transmitter according to this embodiment. Fig. 18 shows the spectrum of bright lines in the optical comb signal whose intensity is controlled by a fixed optical attenuator.

[0096] By changing the spectrum of the optical comb signal, we were able to confirm that the filter shape can be changed, and we were able to obtain the results that the filter can be reconfigured by changing the filter shape.

[0097] Although the transmitter has been described above using an embodiment, the present invention is not limited to the above embodiment. Various modifications and improvements, such as combinations or substitutions with part or all of other embodiments, are possible within the scope of the present invention. [Explanation of symbols]

[0098] 1, 2, 3 transmitter 10 Optical comb generator 20 Modulation section 30, 130, 230 frequency shift section 31, 131, 231 Photonic filter section 32 Photoelectric conversion unit 32a Photodetector 33 Antenna 131a Optical demultiplexer 131b delay line 131c optical multiplexer 231a Optical Filter 231b Dispersion Compensating Fiber

Claims

1. an optical comb signal generator that generates a first optical comb signal having a plurality of emission lines arranged at equal intervals in a frequency domain; a modulation unit that modulates the first optical comb signal with a transmission signal and outputs a second optical comb signal; a frequency shifter including: a photonic filter unit that filters the second optical comb signal so as to delay adjacent emission lines of the second optical comb signal by a predetermined time, and outputs a third optical comb signal; and an opto-electrical converter that converts, into an electric signal, a beat signal between adjacent emission lines of the second optical comb signal in the third optical comb signal; A transmitter comprising:

2. the photonic filter unit includes an optical demultiplexer, a plurality of delay lines with different delay amounts, and an optical multiplexer; The transmitter of claim 1.

3. the photonic filter section includes a dispersion compensating fiber; The transmitter of claim 1.

4. the dispersion compensation fiber receives the plurality of emission lines of the second optical comb signal; The transmitter according to claim 3.

5. the photoelectric conversion unit includes a uni-traveling-carrier photodiode; A transmitter according to any one of claims 1 to 4.

6. generating a first optical comb signal having a plurality of equally spaced emission lines in the frequency domain; modulating the first optical comb signal with a transmission signal to generate a second optical comb signal; generating a third optical comb signal by filtering the second optical comb signal so as to delay adjacent emission lines by a predetermined time; converting beat signals between adjacent emission lines in the third optical comb signal into electrical signals; Including, How to make a call.

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