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Terahertz wireless communication in a 560-GHz
band using a Kerr micro-resonator soliton comb
Y U TOKIZANE , 1,† S HOTA O KADA , 2 K ENJI N ISHIMOTO, 2
YASUHIRO O KAMURA , 3,4 H IROKI K ISHIKAWA , 1
TAKEO M INAMIKAWA , 1 E IJI H ASE , 1 J UN - ICHI F UJIKATA , 1
M ASANOBU H ARAGUCHI , 1 ATSUSHI K ANNO, 5,6
S HINTARO H ISATAKE , 7 N AOYA K USE , 1,8 AND TAKESHI YASUI 1,†*
1 Institute of Post-LED Photonics (pLED), Tokushima University, 2-1, Minami-Josanjima, Tokushima

770-8506, Japan
2 Graduate School of Sciences and Technology for Innovation, Tokushima University, 2-1,
Minami-Josanjima, Tokushima 770-8506, Japan
3 Graduate School of Technology, Industrial and Social Sciences, Tokushima University, 2-1,
Minami-Josanjima, Tokushima 770-8506, Japan
4 Currently at: Center for Higher Education and Digital Transformation University of Yamanashi„ 4-4-37
Takeda, Kofu, Yamanashi 400-8510, Japan
5 National Institute of Information and Communications Technology, 4-2-1 Nukuikitamachi, Koganei, Tokyo
184-8795, Japan
6 Department of Electrical and Mechanical Engineering, Nagoya Institute of Technology, Gokiso-cho,Showa-ku, Nagoya, Aichi 466-8555, Japan
7 Electrical and Energy System Engineering Division, Gifu University, 1-1 Yanagido, Gifu 501-1193, Japan
8 PRESTO, Japan Science and Technology Agency, 4-1-8 Honcho, Kawaguchi, Saitama, 332-0012, Japan† Authors contributed equally as corresponding authors.
* yasui.takeshi@tokushima-u.ac.jp

Abstract: Terahertz (THz) waves have attracted attention as carrier waves for next-generation
wireless communications (6 G). Electronic THz emitters are widely used in current mobile
communications; however, they may face technical limitations in 6 G with upper-frequency
limits. We demonstrate wireless communication in a 560-GHz band by using a photonic THz
emitter based on photomixing of a 560-GHz-spacing soliton microcomb in a uni-travelling
carrier photodiode together with a THz receiver of Schottky barrier diode. The on-off keying
data transfer with 2-Gbit/s achieves a Q-factor of 3.4, thus, satisfying the limit of forward error
correction.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement

1.

Introduction

Terahertz (THz) waves are a promising candidate for carrier waves in next-generation wirelesscommunications (beyond 5 G or 6 G, expected carrier frequency > 300 GHz), providing a higher
data rate than present wireless communications (5 G, carrier frequency = 28 GHz or more) owing
to the use of high-frequency carrier waves [1]. In particular, the use of specific THz waves,corresponding to multiple frequency windows available for broadband data transfer in frequencies
higher than 350 GHz, is expected for wireless mobile fronthaul and backhaul in 6 G for a muchhigher data rate while avoiding interference with general 6 G mobile communication and otherapplications. One key feature of 6 G is its THz emitter. Although electronic THz emitters basedon frequency multiplication have been used up to 5 G, they may face technical limitations in 6 G
owing to too high frequency for electronics. For example, higher-order frequency multiplication
would largely increase their phase noise owing to the quadratic-dependent increase in phasenoise on the number of frequency multiplications. In addition, signal attenuation in electric#491729
Journal © 2023

https://doi.org/10.1364/OPTCON.491729
Received 27 Mar 2023; accepted 26 Apr 2023; published 15 May 2023

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transmission lines largely increases in such high frequency. These limitations are inherent toelectronic THz emitters and may require a different approach.
Another approach for THz emitters is the use of photonic technology because the THz frequency
band is located at the boundary between the photonic and electronic regions. Photomixing for
optical-to-electric conversion is a promising approach to generate millimeter wave and even THz
wave. A pair of near-infrared single-mode CW laser lights with an optical frequency separationof THz order interferes with each other, and the resulting optical beat signal is then converted intoa THz wave by photomixing in a uni-travelling carrier photodiode (UTC-PD) [2,3]. This method
not only provides higher-frequency THz wave up to a few THz [4], but also has a high affinity to
optical communication due to fiber-based technology at a telecommunication band of 1.55 µm[5]. In addition, this high affinity enables the easy modulation in the optical region by mature
optical modulators and modulation protocols. However, when two independent free-running
CW lasers are used for photomixing, fluctuations of frequency and phase in the generated THz
wave are synthesized from those of two independent light sources. Such fluctuations often
require estimation and/or correction of frequency and phase with a relatively large amount ofcomputation in digital signal processing of THz receiver.
The use of an optical frequency comb (OFC) instead of two CW lasers is a promising
approach for the generation of low-phase-noise THz waves, in which two OFC modes with
THz frequency spacing are extracted and used for photomixing. Because the frequency spacing
frep of the OFC is inherently stable and its relative phase noise is considerably low owing tomode-locking oscillation and active control in the OFC, the resulting THz wave benefits fromthe excellent frequency and phase stability in frep . Mode-locked fiber laser (ML-FL) OFCs
[6,7] and electro-to-optic-modulator (EOM) OFCs [8–10] have been successfully used for thegeneration of microwave and millimeter waves. However, when this approach is extended to THz
waves, two OFC modes with a large frequency separation (= mfrep ) must be used for photomixing
because the THz wave frequency fTHz is much larger than the frep of these OFCs. For example,
when the ML-FL OFC with frep of 100 MHz is used for generation of 560-GHz THz wave via
photomixing, m is achieved to 5600. Such high-order optical frequency multiplication of frep
spoils the low phase noise of frep characteristic in ML-FL OFC and largely increases the phasenoise of the resulting THz wave, similar to the electronic frequency multiplication in electronic
THz emitters. In addition, existing OFCs are still bulky, complicated, and expensive for wirelesscommunications.
Recently, the on-chip Kerr micro-resonator soliton comb, namely the soliton microcomb,
has attracted attention as a small, simple, and cost-effective OFC benefiting from batch mass
production of semiconductor processes. Moreover, micro-resonators largely increase frep up
to a few tens of GHz to a few THz, which is much larger than frep in ML-FL OFCs or EOM
OFCs, while achieving soliton mode-locking oscillation with low phase noise. Such a soliton
microcomb enables photomixing of two adjacent OFC modes without the need for opticalfrequency multiplication for the generation of microwaves [11], millimeter waves [12,13], andeven THz waves [14]. Furthermore, a combination of a stabilized microcomb with UTC-PD-based
photomixing generates an ultralow-phase-noise (≈ -100 dBc/Hz at the 10 kHz frequency offset)
THz wave at 300 GHz [15] and 560 GHz [16]. However, there have been no attempts to apply suchmicrocomb-based THz waves to high-frequency THz wireless communication for wireless mobile
fronthaul and backhaul in 6 G. In this study, we demonstrate THz wireless communication of
on-off keying (OOK) in a 560-GHz band. The 560-GHz THz wave was generated by photomixingof a 560-GHz-spacing soliton microcomb in a UTC-PD, whereas the transmitted THz wave was
detected by a Schottky barrier diode (SBD).

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Results

We first evaluated the basic performance of the soliton microcomb and the resulting THz
wave. Figure 1 shows a schematic of the experimental setup for the generation and evaluationof them. The output light of a single-mode CW laser (pump laser, TLX-1, Thorlabs, Inc.,
wavelength = 1560.7 nm, optical frequency = 192.09 THz, optical power = 20 mW) after passing
through a dual-parallel Mach-Zehnder modulator (DP-MZM, FTM7962EP, Fujitsu Optical
Components Lim., data rate = 40 Gbps) was amplified by a homemade erbium-doped fiber
amplifier (EDFA1) via polarization-maintained fibers (PMFs). The resulting pump light (opticalpower = 500 mW) was coupled into a Si3 N4 (SiN) micro-resonator (custom, LIGENTEC, S.A.,
free spectral range = 560 GHz, Q factor ≈ 106 ) via a single-mode fiber (SMF). To generate
a single soliton microcomb with an frep of 560 GHz, the DP-MZM rapidly tunes the opticalfrequency of the pump light around the resonant frequency of the micro-resonator [17]. We
rejected the residual pump light from the generated soliton microcomb by a homemade opticalnotch filter (ONF, stop wavelength = 1560.7 nm, stop optical frequency = 192.09 THz) because
the remaining pump light largely decreases the efficiency of optical-to-electric conversion in
UTC-PD due to the limited input power in UTC-PD. The resulting soliton microcomb (total
power = 1 mW) was further amplified to 100 mW using a commercialized EDFA (EDFA2,
EDFA100P, Thorlabs, Inc., output power > 20 dBm). Parts of the soliton microcomb (opticalpower = a few mW) were fed into an optical spectrum analyzer (OSA, AQ6370D, Yokogawa
Electric Corp., minimum wavelength = 600-1700nm) for optical spectrum measurement and a
two-wavelength delayed self-heterodyne interferometer (TWDI) for the phase noise evaluation of
its frep [18–20]. The residual of the soliton microcomb was incident onto an antenna-integrated
UTC-PD (IOD-PMAN-13001-2, NTT Electronics Corp., output frequency of 200–600 GHz,
output power of 10 µW at 600 GHz) via PMF to generate THz waves by photomixing. The
generated THz wave passed through a pair of Teflon lenses (LAT100, Thorlabs, Inc., focal
length = 91 mm) and was incident on a sub-harmonic mixer (SHM, WR1.5SHM, Virginia Diodes,
Inc., input local-oscillator frequency = 250–375 GHz, input radio-frequency = 500–750 GHz,
output intermediate-frequency = DC–40 GHz). After the 27-times frequency multiplication of a
local oscillator (LO) signal synthesized by a microwave frequency synthesizer (E8257D, Agilent
Tech., Inc., frequency = fLO = 10.37 GHz) with a frequency multiplier (FM), SHM generates aheterodyned beat signal (frequency = fIF ) between THz waves (frequency = fTHz = frep ) and the54-th higher harmonic component of fLO (frequency = 54fLO ). The resulting radio-frequency(RF) beat signal was measured using an RF spectrum analyzer (RSA, 89441A, Agilent Tech.,Inc., frequency = DC–2.65 GHz) for the spectrum observation of fTHz .
Figure 2(a) shows the optical spectrum of the soliton microcomb after passing throughONF (resolution bandwidth or RBW = 0.2 nm). We confirmed that multiple OFC modes were
distributed at intervals of 560 GHz, around the wavelength of the pump light. In addition, a broad
spectrum of amplified spontaneous emission (ASE) appears as the optical background signal of
the soliton microcomb, which influences the optical signal-to-noise ratio (OSNR) of each OFC
mode. We later discuss the influence of such an ASE background on the quality of data transfer in
OOK wireless communications. Figure 2(b) shows the single sideband (SSB) phase noise power
spectrum density (PSD) of frep , corresponding to the relative phase noise between the two OFC
modes, measured by TWDI. The SSB phase noise of our soliton microcomb is -60 dBc/Hz at anoffset frequency of 10 kHz, which can be completely transferred to the THz wave generated fromthe soliton microcomb without additional phase noise in UTC-PD [16]. The detailed evaluationof phase noise is given elsewhere [17]. To determine fTHz and investigate its spectral features indetail, we measured the RF spectrum of the heterodyned beat signal (frequency = fIF ) between
fTHz and 54fLo (RBW = 1 MHz), as shown in Fig. 2(c). The resulting RF spectrum indicated aspectral peak at 150 MHz (=fIF ). We determined fTHz to be 560.13 GHz from the measured fIF
and the known fLO ; it is exactly equal to frep .

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Fig. 1. Experimental setup for generation and evaluation of a soliton microcomb and THz
wave.

Fig. 2. Fundamental property of a soliton microcomb and THz wave. (a) Optical spectrumand (b) phase noise of frep in a 560-GHz-spacing microcomb. (c) RF spectrum of heterodynedbeat signal.

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We next demonstrated OOK wireless communication with a 560-GHz THz wave. Figure 3
shows a schematic of the experimental setup for the THz wireless communication. We generated
a soliton microcomb without residual pump light (frep = 560 GHz, optical power = 100 mW) using
the experimental setup of microcomb generation in Fig. 1. Two adjacent microcomb modes
with a relatively higher OSNR (OFC-M1, wavelength = 1533.87 nm, optical frequency = 195.448
THz, optical power = 3.7 mW; OFC-M2, wavelength = 1538.28 nm, optical frequency = 194.888
THz, optical power = 2.1 mW) were extracted from the soliton microcomb using a programmableoptical filter (WaveShaper 4000S, Finisar, filter bandwidth = 10 GHz). One of the two extracted
microcomb modes (OFC-M1) was delivered to a LiNbO3 modulator (LN-MOD, T.MXH1.520PD-ADC-LV, Sumitomo Osaka Cement Co., Ltd, wavelength = 1.55 µm, modulation rate = 40
Gbit/s, optical bandwidth > 20 GHz) for OOK modulation. The OOK signal of a 2-Gbit/s
non-return-to-zero (NRZ) signal with a DC bias, generated by an arbitrary waveform generator(AWG, M8196A, Keysight Tech., Inc., sample rates < 92 GS/s, analog bandwidth = 32 GHz),
was applied to the LN-MOD as a modulation signal. The OOK-modulated OFC-M1 wasamplified by an EDFA (EDFA3), and the residual ASE background outside was removed by atunable optical bandpass filter (OBPF1, WTF-200, Alnear Inc., optical bandwidth < 0.3 nm).
The other microcomb mode (OFC-M2) passed through another EDFA (EDFA4) and another
OBPF (OBPF2) without OOK modulation. The modulated OFC-M1 and unmodulated OFC-M2
were combined by a fiber coupler, and the resulting two microcomb modes (power of OFCM1 = 15 mW, power of OFC-M2 = 15 mW, total power = 30 mW) were fed into the UTC-PD to
generate a THz wave at an fTHz of 560 GHz. The OOK-modulated THz wave propagated in freespace (transmittance distance = 0.6 m) via a pair of Teflon lenses and was then detected by SBD(Virginia Diodes, Inc., WR1.9ZBD, RF frequency = 400–600 GHz, responsivity = 1000 V/W).
The electric output signal from the SBD was amplified using an electric amplifier (AMP, ZX60V62+, Mini-Circuits, frequency range = 0.05–6 GHz, gain = 14.4 dB at 6 GHz) via a bias-T (notshown), and was then measured by a real-time oscilloscope (UXR0402AP, Keysight Tech., Inc.,
max. bandwidth = 40 GHz, max. sample rate = 256 GSa/s).

Fig. 3. Experimental setup for OOK wireless communication at 560 GHz.

To suppress the ASE background while securing the required optical amplification, an optical

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Fig. 4. Results of 2-Gbit/s OOK wireless communication at 560 GHz. Optical spectrumof two microcomb modes obtained by (a) two pairs of EDFAs (EDFA3 and EDFA4) andOPBFs (OBPF1 and OBPF2) before fiber coupling and (b) OPBFs (OBPF1 and OBPF2)
and an additional EDFA after their fiber-coupling. Eye pattern of OOK signal with 2 Gbit/s(c) when two microcomb modes were used for generation of THz carrier wave and (d) when
two free-running CW lasers were used for generation of THz carrier wave.

input signal with a suitable power must be fed into EDFAs because an extremely weak optical
input increases ASE unnecessarily. Although the OSNR improves with filtering of ASE in
EDFA1 (see Fig. 1), we did not use the filter in the present setup because the resulting decreased
pump power would be insufficient and the soliton comb could not be generated stably. Instead,
we precisely adjusted the optical input signal of the EDFA3 and EDFA4 (namely, the modulated
OFC-M1 and unmodulated OFC-M2) for a better OSNR. We further reduced the ASE backgroundoutside of two amplified microcomb modes by fine tuning of OPBFs. Figure 4(a) shows theResearch Article

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optical spectrum of the two amplified microcomb modes immediately before being sent to the
UTC-PD (RBW = 0.2 nm). These microcomb modes appear to have a sufficient dynamic range(≈ 40 dB) from the ASE background level around -30 dBm. For comparison, we removed EDFA3
and EDFA4 in Fig. 3, and then simultaneously amplified the combined OFC-M1 and OFC-M2
after passing through OBPFs and the fiber coupler by an additional EDFA (not shown in Fig. 3).
Figure 4(b) shows the optical spectrum of the resulting OFC-M1 and OFC-M2 immediately
before being sent to the UTC-PD (RBW = 0.2 nm). In this case, the broad ASE backgroundremains outside of two microcomb modes even though OSNR was achieved to 40 dB. Some
smaller sidebands indicate residual microcomb modes. Such difference of ASE backgroundbetween them in the same OSNR influences the result of OOK wireless communication.
The 560-GHz THz wave, generated by photomixing of two microcomb modes in Fig. 4(a),was used for OOK wireless communication. Figure 4(c) shows an eye diagram of OOK with 2
Gbit/s measured by the real-time oscilloscope; thus, the eye was opened. The Q-factor of this eye
pattern was determined to be 3.40. This Q-factor satisfied the limit of forward error correction(FEC limit, Q-factor = 2.33). Accordingly, the eye diagram of 2-Gbit/s OOK THz data transfer
was confirmed in the 560-GHz band within the FEC limit. For comparison, we performed thesimilar experiment of OOK data transfer when two free-running CW lasers were used in place of
two microcomb modes. Figure 4(d) shows an eye diagram of OOK with 2 Gbit/s. The resulting
Q-factor was 3.46, which is comparable to that in two microcomb modes (= 3.40). Also, when
two microcomb modes in Fig. 4(b) were used for THz generation, the eye was fully closed (notshown), indicating that two pairs of EDFAs (EDFA3 and EDFA4) and OBPFs (OBPF1 and
OBPF2) separately for two microcomb modes plays an important role in the present wirelesscommunication.
3.

Discussion

We first discuss the causes of the relatively low Q-factor of the measured OOK signal. One
possible reason for it is atmospheric attenuation caused by water vapor absorption. The 560-GHz
band is close to the water absorption line of 110 -101 (center frequency = 557 GHz, pressure
broadening linewidth ∼ several GHz at atmospheric pressure). As the attenuation coefficient ofwater vapor is 7.1 dB/m at 560 GHz [21], the 0.6-m propagation of THz wave causes the THz
attenuation by 4.26 dB. Use of such considerably attenuated THz wave limits the present Q factor.
If THz wave at 565 GHz is used in place of 560 GHz, the atmospheric attenuation reduces to
1.14 dB from an attenuation coefficient of 1.9 dB/m at 565 GHz [21]. In this case, Q-factor in
OOK data transfer will be largely improved. From this comparison, a main limiting factor of
Q-factor is the propagation loss rather than the detector noise. By appropriately designing thediameter of a micro-resonator and utilizing a micro-heater deposited on the micro-resonator, it ispossible to easily set frep of a microcomb to the desired THz frequency as long as the frequency
bandwidth of UTC-PD covers the frep . Another possible reason for the relatively low Q-factor
is the existence of ASE background remaining around modulated OFC-M1 and unmodulated
OFC-M2 [see Fig. 4(a)]. One promising approach to further reduce the ASE background is
optical injection locking to transfer the phase noise of a soliton microcomb in addition to theamplification of the microcomb mode without degrading the OSNR [22]. Work is in progress toapply this for generation of THz wave in THz wireless communication.
We next discuss the performance of OOK data transfer using two microcomb modes forgeneration of THz carrier wave compared with two free-running CW lasers. As shown in
Figs. 4(c) and 4(d), when the transmitted OOK THz wave was square-law-detected by SBD, there
are little difference of Q-factor between two microcomb modes and two free-running CW lasers.
This is because the square-law detection is insensitive to the phase fluctuation of THz carrier wave.
However, we consider that the performance difference between them can be clearly confirmed
when the transmitted OOK THz wave is heterodyne-detected by SHM for the enhanced detection

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sensitivity. For example, we assume to observe the real-part or imaginary-part component of
the OOK baseband signal that has been down-converted from the THz band to the RF band by
an SHM. Our preliminary numerical simulation predicts that the eye pattern of OOK will open
when two microcomb modes are used for generation of THz carrier wave because the phase of
THz wave is stable. However, when two free-running CW lasers are used, since the phase of theTHz wave is unstable, the eye pattern of OOK closes. The detailed numerical simulation and theexperimental verification on them are our future work.
4.

Conclusions

We demonstrated 2-Gbit/s OOK wireless communication in the 560-GHz band based on a hybrid
system of photomixing-based photonic THz generation and SBD-based electric THz detection.
Photomixing of two adjacent microcomb modes via UTC-PD enables the generation of THz
waves without the need for optical frequency multiplication. Although such a higher-frequency
band is accessible by UTC-PD-based photomixing, the combination of photomixing with thesoliton microcomb benefits from the inherently low phase noise of frep in the soliton microcomb.
A pair of OOK-modulated and unmodulated microcomb modes with an OSNR over 40 dB wasused to generate the OOK-modulated THz wave. The achieved Q-factor was 3.40, which waswithin the range of the FEC limit.
The achieved phase noise of frep remained around -60 dBc/Hz at an offset frequency of 10 kHz
because of the use of the free-running soliton microcomb; however, there is still room to further
decrease the phase noise down to -99 dBc/Hz at an offset frequency of 10 kHz by active controlof the soliton microcomb [16]. Reducing phase noise is an advantage for multi-level modulation
using phase instead of multi-level modulation for amplitude only.
Funding. Cabinet Office, Government of Japan; Ministry of Internal Affairs and Communications (JPJ000254); Japan
Science and Technology Agency (JPMJPR1905); Tokushima Prefecture.

Disclosures. The authors declare no conflicts of interest.
Data availability. Data underlying the results presented in this paper are not publicly available at this time but may
be obtained from the authors upon reasonable request.

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