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JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 41, NO. 4, FEBRUARY 15, 2023 1129
Real-Time Demonstration of 100 GbE THz-Wireless
and Fiber Seamless Integration Networks
Jiao Zhang , Member, IEEE, Min Zhu , Member, IEEE, Bingchang Hua, Mingzheng Lei , Member, IEEE,
Yuancheng Cai , Member, IEEE, Yucong Zou, Weidong Tong, Junjie Ding ,LiangTian,
Like Ma , Jinbiao Xiao , Member, IEEE, Yongming Huang , Senior Member, IEEE,
Jianjun Yu , Fellow, IEEE, Fellow, Optica, and Xiaohu You , Fellow, IEEE
(Top-Scored Paper)
Abstract—We propose and experimentally demonstrate a real-
time fiber-THz-fiber 2 ×2 multiple-input multiple-output (MIMO)
seamless integration system at 340–510 GHz using commercial digi-
tal coherent optics (DCO) modules for baseband signals processing,
which realizes a record net rate of 103.125 Gb/s dual polarization
quadrature-phase-shift-keying (DP-QPSK) signals delivery over
two spans of 20 km wireline single-mode fiber-link and 3 m wireless
2×2 MIMO link without using THz power amplifier under
15% soft-decision forward-error-correction threshold (SD-FEC) of
1.56 ×10−2. Two kinds of THz-to-optical conversion schemes based
on an integrated dual-polarization Mach-Zehnder modulator (DP-
MZM) and two discrete intensity-modulators (IMs) are extensively
investigated and compared. To the best of our knowledge, this is
the first real-time demonstration of THz-wireless and 100 GbE
fiber-optic networks seamless integration transmission, which is
compatible with IEEE 802.3 and ITU-T G.798 specifications. It is
Manuscript received 10 June 2022; revised 14 August 2022; accepted 1
September 2022. Date of publication 8 September 2022; date of current version
16 February 2023. This work was supported in part by the National Natural
Science Foundation of China under Grants 62101121 and 62101126, in part
by China Postdoctoral Science Foundation under Grants 2021M702501 and
2022T150486, in part by the Transformation Program of Scientific and Techno-
logical Achievements of Jiangsu Province under Grant BA2019026, in part by
the Key Research and Development Program of Jiangsu Province under Grant
BE2020012, and in part by the Major Key Project of Peng Cheng Laboratory
under Grant PCL 2021A01-2. (Corresponding authors: Min Zhu; Xiaohu You.)
Jiao Zhang, Min Zhu, Yuancheng Cai, Weidong Tong, and Yongming Huang
are with the National Mobile Communications Research Laboratory, Southeast
University, Nanjing 210096, China, and also with Purple Mountain Laborato-
ries, Nanjing 211111, China (e-mail: jiaozhang@seu.edu.cn; minzhu@seu.edu.
cn; caiyuancheng@pmlabs.com.cn; weidongtong@seu.edu.cn; huangym@seu.
edu.cn).
Bingchang Hua, Mingzheng Lei, Yucong Zou, and Liang Tian are with the
Purple Mountain Laboratories, Nanjing 211111, China (e-mail: huabingchang@
pmlabs.com.cn; leimingzheng@pmlabs.com.cn; zouyucong@pmlabs.com.cn;
tianliang@pmlabs.com.cn).
Junjie Ding and Jianjun Yu are with the Purple Mountain Laboratories,
Nanjing 211111, China, and also with Fudan University, Shanghai 200433,
China (e-mail: 18110720017@fudan.edu.cn; jianjun@fudan.edu.cn).
Like Ma is with the China Mobile Research Institute, Beijing 100053, China
(e-mail: malike@chinamobile.com).
Jinbiao Xiao is with the School of Electronic Science and Engineering,
Southeast University, Nanjing 210096, China, and also with Purple Mountain
Laboratories, Nanjing 211111, China (e-mail: jbxiao@seu.edu.cn).
Xiaohu You is with the National Mobile Communications Research Lab-
oratory, Southeast University, Nanjing 210096, China, also with the Purple
Mountain Laboratories, Nanjing 211111, China, and also with the Peng Cheng
Laboratory, Shenzhen 518000, China (e-mail: xhyu@seu.edu.cn).
Color versions of one or more figures in this article are available at
https://doi.org/10.1109/JLT.2022.3204268.
Digital Object Identifier 10.1109/JLT.2022.3204268
a promising scheme to meet the demands of future fiber-wireless-
integrations communication for low power consumption, low cost
and miniaturization.
Index Terms—Digital coherent optics module, Fiber-optic
networks, Multiple-input multiple-output, Polarization
multiplexing, Seamless integration, Terahertz-band, THz-to-
optical conversion.
I. INTRODUCTION
THE burst growth of streaming services and Internet of
Things, such as 8k–10k or 16k video, telemedicine, meta-
verse, autonomous vehicles, have already driven communities
to look for higher bandwidth, flexible, and reliable wireless
access solutions [1]. The 8k–10k and 16k super-high-definition
(SHD) cameras have been deployed in mobile terminals, which
need the bandwidth capability up to 48 Gb/s and 80 Gb/s,
respectively [2], [3]. High-definition multimedia interface 2.1
(HDMI 2.1) and Display Port 2.0 (DP 2.0) cables are bulky
and non-mobile for thin and light smart terminals [4]. High-
speed wireless communication can guarantee ubiquitous ac-
cess with mobility. The Terahertz-band (THz-band, 0.3 THz
to 10 THz) is envisioned as a promising candidate for indoor
short-range wireless personal area networks (WPANs), which
can provide hundreds of Gb/s or even Tb/s data capacity [5],
[6], [7], [8], [9], [10]. Moreover, the devices at THz-band
have small and compact size, and can be monolithically inte-
grated with other front-end circuits in portable terminals. The
World Radio Communication Conference 2019 (WRC-19) have
released 275∼296 GHz, 306∼313 GHz, 318∼333 GHz, and
356∼450 GHz spectrum bands for land mobile and fixed ser-
vice applications with a total spectrum bandwidth of 137 GHz
[11].
THz communication techniques can be divided into two
main categories: pure electronic solutions and optoelectronic
solutions. Photonic heterodyne-based optoelectronic solutions
are currently being widely studied due to the simple system
architecture, tunable carrier frequency, and small harmonic in-
terference, which can break the bottleneck of electronic devices
and generate ultra-high-speed wireless THz-wave signals [12],
[13]. Moreover,the characteristics of optics devices can facilitate
seamless integration with high-speed fiber access networks.
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1130 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 41, NO. 4, FEBRUARY 15, 2023
TAB L E I
SUMMARY OF FIBER-THZ-FIBER SEAMLESS TRANSMISSION EXPERIMENTS
However, the fiber-optic fixed networks have worse mobility to
realize seamless wide-area coverage. The THz-wireless and fiber
integration access system integrates long-distance, mobility and
large-capacity advantages derived from joint fiber-optics and
THz wireless transmission link [14], [15], [16], [17], [18]. It’s ex-
pected to become an extremely promising application prospect
for future beyond fifth-generation/sixth-generation (B5G/6G)
communication.
Over the previous years, a series of seamless integration sys-
tems of THz-wireless and fiber-optic infrastructures at record-
high data rates have been effectively promoted by photonics
[19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30],
[31]. Table I summarizes the main features of fiber-THz-fiber
seamless transmission experiments. For the sake of comparison
only transmissions operating carrier frequency above 100 GHz
are shown. By inspecting this table, THz-to-optical conversion
schemes can be classified into three main types, as follows:
Baseband (BB)-to-optical scheme: The THz-wave signal is
firstly down-converted to baseband signal by using mixer based
on active monolithic integrated circuit (MMIC), and then per-
formed the remodulation to the optical domain via conventional
optical modulators. A narrow-bandwidth optical band-pass filter
(OBPF) is not needed. At 237.5 GHz, a single-input and single-
output (SISO) wireless communication system is demonstrated
over 20 m at a data rate of 100 Gb/s by using MMIC-based
in-phase/quadrature (I/Q) mixer [20]. At 300 GHz, by using
high-speed real-time commercial digital coherent optics (DCO)
modules, the transmission of the 34 GBaud polarization division
multiplexing quadrature-phase-shift-keying (PDM-QPSK) sig-
nal using MMIC-based THz transceiver has been successfully
demonstrated over two fiber links and 0.5 m 2 ×2 multiple-input
multiple-output (MIMO) wireless link [21], [22]. The net ca-
pacity is 100 Gb/s at 25% soft-decision forward-error correction
(SD-FEC) limit. At the THz transmitter frontends, four electrical
delay lines have been applied to manually adjust the delay
between the I/Q components of each polarization. Moreover, the
long-term stability of the system is evaluated over a period of
96 hours. This demo prove that the DCO module is a promising
solution to realize practical fiber-THz-fiber seamless links.
Direct-modulation scheme: The received THz field signal is
directly modulated to the optical domain by an ultra-broadband
optical modulator. At 288.5 GHz, an optical-wireless-optical
link based on purely optoelectronic frequency conversion us-
ing a custom-developed silicon-plasmonic modulator with 3dB
bandwidth of 0.36 THz has been demonstrated with 50 Gb/s data
stream over a wireless link of 16 m [23]. At 231 GHz, the similar
transmission operating at line-rates up to 240 and 190 Gb/s over
distances from 5 to 115 m is demonstrated using a plasmonic
modulator with a low noise built-in THz amplifier [24], [25].
At 101 GHz, the transparent fiber-radio-fiber bridge system
is demonstrated using a broadband thin-film lithium niobate
Mach-Zehnder modulator (MZM), and 71.4 Gb/s 64 quadrature
amplitude modulation (64QAM) orthogonal frequency division
multiplexing (OFDM) signal is transmitted [26]. In general, a
narrow-bandwidth OBPF is required. In these research works,
the processing of baseband signal relied on offline digital signal
processing (DSP), and only single polarization SISO wireless
link has been demonstrated so far.
Intermediate frequency (IF)-to-optical scheme: Different
from the first scheme, the THz-wave signal is down-mixed to
an IF in order to reduce the carrier frequency and the bandwidth
requirement for the modulators, and then is mapped to optical
carrier using commercial optical modulators. It can reduce the
numbers of high-frequency devices and have cost-effectiveness.
At 450 GHz, an off-line fiber-THz-fiber transmission system
with 18 Gb/s line rates over 3.8 m 2 ×2 MIMO wireless link
is demonstrated [27], [28]. At 250 GHz, a 50 Gb/s photonic
wireless bridge transmission is realized over 0.1 m wireless
distance [29]. In our previous work, we have demonstrated a
real-time fiber-THz-fiber 2 ×2 MIMO seamless integration
system at 360–390 GHz with net rate of 103.125 Gb/s over two
spans of 20 km fiber and 0.6 m-long wireless link under 15%
SD-FEC threshold [31]. A 100 GbE streaming service platform
is successfully developed to demo real-time video service. How-
ever, six manual polarization controllers (PCs) are introduced
to adjust the polarization direction to maximize output THz
power. Two individual intensity-modulators (IMs) are used for
IF signal remodulation to the optical domain. Moreover, the
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ZHANG et al.: REAL-TIME DEMONSTRATION OF 100 GBE THZ-WIRELESS AND FIBER SEAMLESS INTEGRATION NETWORKS 1131
Fig. 1. Test-bed setup of 100GbE real-time fiber-optic networks for THz
integration transmission. OTU: optical transport units, QSFP28: quad small
form-factor pluggable, MMF: multi-mode fiber, NMS: network management
system, CFP2-DCO: Centum form-factor pluggable digital coherent optics,
SSMF: standard single-mode fiber, MIMO: multiple-input multiple-output.
characteristics of two discrete IMs are different and can affect
transmission performance.
This paper is an invited extension of our work pre-
sented in [30], where a 100 GbE real-time photonics-assisted
THz-wireless and fiber seamless integration system is success-
fully demonstrated. This study extends [30] and our previous
work [31] from the following perspectives:
rWe further simplify the system architecture and firstly pro-
pose a compact THz-to-optical conversion scheme based
on an integrated dual-polarization MZM (DP-MZM) with
low drive-voltage for IF-to-optical conversion. Compared
with IMs-based scheme, a series of discrete optoelectronic
devices can be replaced by one DP-MZM.
rThe transmission distance of 2 ×2 MIMO wireless link is
extended from 0.6 m to 3 m without using THz power am-
plifier for the first time. The performances of two schemes
are extensively measured and compared at a wide THz
carrier frequency range from 340 GHz to 530 GHz.
rThree kinds of THz-to-optical conversion schemes are
summarized and discussed in detail. We also conclude the
trend of THz-fiber integration from several aspects.
The remainder of this paper is organized as follows. In
Section II, we give a detailed description of the experimental
setup, including system parameters and the characteristics of
the devices used. In Section III, the results and performance
comparisons are presented. In Section IV, we give extensively
discussions. Finally, we conclude this paper in Section V.
II. EXPERIMENTAL SETUP
A. 100 GbE Fiber-Optic Networks
Fig. 1 shows the test-bed setup of 100 GbE real-time fiber-
optic networks for our THz and fiber seamless integration sys-
tem. In order to support video streaming service, one stream-
ing server, two optical transport units (OTUs) and one client
server are deployed. Each server is equipped with an Ethernet
network adapter (Intel, E810-2CQDA2) with dual-port quad
Fig. 2. The optical spectrum of the optical baseband signal after OTU.
small form-factor pluggable 28 (QSFP28), and each port can
support up to 100 Gb/s for bandwidth-intensive workloads.
The streaming server and client server are connected with the
OTUs through QSFP28 modules supporting 100GBASE-SR4
(4 ×25.78125 Gb/s) specifications over 2 m multi-mode fiber
(MMF). In OTUs, the QSFP28 modules and Centum form-factor
pluggable (CFP2) DCO modules are equipped on the client side
and line side, respectively. Coherent pluggable module based
on high-speed DSP at 200G/400G are being widely deployed
for metro and long-haul data center interconnects (DCI) be-
tween large geographic distances. In this experimental setup,
commercial coherent CFP2 modules (InnoLight Technol., 200G
CFP2 DCO MR) are capable to meet 50 GHz dense wavelength
division multiplexing (DWDM) ITU-T grid and reach beyond
1200 km standard single-mode fiber (SSMF) without inline
chromatic dispersion (CD) compensation. Each CFP2-DCO
module can support dual polarization quadrature-phase-shift-
keying (DP-QPSK, 100G) modulation, DP-16QAM (200G)
modulation, polarization diversity homodyne detection, and
high speed real-time DSP demodulating. Two 4 ×25.78125 Gb/s
Serializer/Deserializer (SerDes) provide all electrical interfaces
to the host card, which are compatible with the physical layer
transmission specifications such as IEEE 802.3 and ITU-T
G.798. The parameters of fiber-optic networks, such as working
mode, channel, wavelength, optical power, optical signal to
noise ratio (OSNR), pre-BER, link status and so on, can be
set and monitored through the network management system
(NMS) operation interface. The inset of Fig. 1 shows the photo
of OTU.
In our experiment, the carrier frequency of optical baseband
signal is fixed at 193.5 THz with 3 dBm output optical power.
We set two CFP2-DCO modules operating at 100 GbE mode and
modulating 31.379 GBaud DP-QPSK optical baseband signal
with a roll-off factor of 0.2. Hence, the total bandwidth (BW)
of the baseband signal is 31.379 ×(1 +0.2) =37.6548 GHz.
CFP2-DCO module has built-in optical transport network (OTN)
mapper and framer, and the Ethernet frames can be mapped and
de-mapped from payload. The optical spectrum of the optical
baseband signal after OTU transmitting at 0.03 nm resolution
is shown in Fig. 2, which locates in 50 GHz DWDM ITU-T
grid. For downstream link, one span of 20 km SSMF with 17
ps/km/nm CD and an average loss of 0.2 dB/km at 1550 nm
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1132 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 41, NO. 4, FEBRUARY 15, 2023
Fig. 3. Setup of optical-to-THz conversion module. EDFA: Erbium-doped
fiber amplifier, TOF: tunable optical filter, PC: polarization controller, ECL:
external cavity laser,OC: optical coupler, PBS: polarization beam splitter, AIPM:
antenna-integrated photomixer module.
Fig. 4. The optical spectra of the optical signal with tunable optical LO after
optical coupler.
is connected between OTUs and optical-to-THz (O/T) con-
version module and THz-to-optical (T/O) conversion module,
respectively. For loopback link, the symmetric path is directly
connected via a 2 m SSMF due to the lack of available devices.
The downstream link and upstream link are individual and do
not impact each other.
B. Optical-to-THz Conversion Module
Fig. 3 gives the experimental setup of O/T conversion module.
After 20 km SSMF transmission, an Erbium-doped fiber am-
plifier (EDFA) is used to compensate for the fiber transmission
loss, and the out-of-band amplified spontaneous emission (ASE)
noise is suppressed by a pass-band tunable optical filter (TOF).
A free-running tunable external cavity laser (ECL-1) is operated
as an optical local oscillator (LO) that has <100 kHz linewidth.
Then, the optical baseband signal with 10.5 dBm optical power
and optical LO with 13.5 dBm optical power are combined
by an optical coupler (OC). The optical spectra of the optical
signal and optical LO after OC are shown at 0.03 nm resolution
in Fig. 4. The side-mode suppression ratio (SMSR) of optical
signal and optical LO is >50 dB. In our demonstrated system, the
THz-wave wireless signals with tunable carrier frequency range
from 340 GHz to 530 GHz are generated by photonic heterodyn-
ing using antenna-integrated photomixer module (AIPM, NTT
Electronics Corp. IOD-PMAN-13001). The AIPM is formed
from an ultra-fast uni-travelling-carrier photodiode (UTC-PD)
Fig. 5. Setup of THz 2 ×2 MIMO wireless link. Insets: (i) 3 m wireless
transmission link; (ii) Lens position at THz receiver side.
and a bow-tie or log-periodic antenna. Two parallel AIPMs,
each has a typical −28 dBm output power and an operating
wavelength ranging from 1540 nm to 1560 nm. The typical
responsivity of photodiode is 0.15 A/W, and maximum optical
input power is 15 dBm. A polarization beam splitter (PBS) is
used to separate the X- and Y-polarization components of the
combined lightwaves. Then, X- and Y-polarization components
are photomixed by AIPMs to generate two parallel THz-wave
wireless signals, respectively. In order to drive AIPMs, another
EDFA is used to boost the optical power of combined lightwaves
before PBS.
Let’s note that, the optical power of X- and Y-polarization
components after PBS should be as equal as possible, and the
AIPMs used in our setup are polarization sensitive with max-
imum 4.5 dB polarization-dependent responsivity (PDR). The
X- and Y-polarization imbalance will result in 2 ×2MIMOTHz-
waves imbalance and deteriorate system performance. Hence
two polarization controllers (PCs) before OC are necessary. In
the test, the optical signal and optical LO are separately regulated
the incident X- and Y-polarization direction to maximize output
optical power into AIPMs.
C. THz 2 ×2 MIMO Wireless Link
Fig. 5 shows the setup of THz 2 ×2 MIMO wireless transmis-
sion link and lens position. Two parallel THz-wave signals from
AIPMs are delivered over a 3 m 2 ×2 MIMO wireless transmis-
sion link. In order to focus the wireless THz-wave, three pairs of
lenses are deployed to maximize the received THz-wave signal
power and have been manually aligned. The lens 1, 2, 5 and lens
3, 4, 6 are aligned with the X-polarization and Y-polarization
wireless link, respectively. The lens 1-4 are identical, and each
of them has 20 cm focal length and 10 cm diameter. The smaller
lens 5 and 6 have 10 cm focal length and 5 cm diameter, which
used for THz-waves high accuracy alignment to horn antennas
(HA). For X- polarization (Y-polarization) THz wireless link,
the longitudinal separation distance between the AIPM and lens
1 (lens 3), lens 1 (lens 3) and lens 2 (lens 4), lens 5 (lens 6)
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ZHANG et al.: REAL-TIME DEMONSTRATION OF 100 GBE THZ-WIRELESS AND FIBER SEAMLESS INTEGRATION NETWORKS 1133
Fig. 6. (a) Setup of IMs-based T/O conversion module; (b) The spectra after
optical polarization diversity and filtering.
and the receiver horn antennas (HA) are 0.2 m, 3 m and 5 cm,
respectively. The lateral separation between the two AIPMs and
two HAs pairs is 25 cm. In order to avoid multi-path fading from
reflections on the optical table, the O/T conversion module and
T/O conversion module are placed at height of 20 cm. Photos of 3
m wireless transmission link and lens position at T/O conversion
side are shown in Insets (i) and (ii) of Fig. 5, respectively.
D. THz-to-Optical Conversion Module
The system architectures of T/O conversion module based on
IMs and DP-MZM are described in detail as follows.
1) IMs-Based T/O Conversion Module: As shown in
Fig. 6(a), THz-wave wireless signals are received with two par-
allel THz-band HAs with 26 dBi gain, each can work within 330
GHz to 500 GHz. For X and Y-polarization THz-wave wireless
signals, two identical THz receivers (VDI, MixAMC WR2.2)
are driven by two single electronic LO sources to implement
analog down conversion first, and each consists of a mixer, a ×12
frequency multiplier chain and an amplifier. THz receivers work
within 330 GHz to 500 GHz and have about 14 dB intrinsic mixer
single-sideband (SSB) conversion loss. The maximum band-
width of down-conversion IF signal is 40 GHz with 2.92 mm
RF connectors. Then, the down-converted X- and Y-polarization
IF signals at 24 GHz are boosted by two cascaded electrical
low-noise amplifiers (LNAs) to drive two intensity-modulators
(IMs, iXblue MXAN-LN-40), respectively. The first stage LNAs
(SHF S804B) have 60 GHz bandwidth and 16 dBm saturated
output power with a gain of 27 dB. The second stage LNAs have
47 GHz bandwidth and can provide a saturated output power of
over20dBmwithagainof27dB.IMshavehighdrive-voltage
of 7 V and 6 dB bandwidth of 40 GHz with an extinction ratio
of 25 dB.
As the optical carrier input of the two IMs, ECL-2 is used
with 24 GHz frequency spacing to the initial optical baseband
signal and 14.5 dBm optical output power. In order to improve
the OSNR of the remodulation optical signal, the optical output
power of ECL-2 is boosted by a polarization-maintaining EDFA
(PM-EDFA) to 19 dBm. the continuous-wavelength (CW) light-
wave is split by a polarization-maintaining OC (PM-OC) into
two branches as polarization-division-multiplexing (PDM) opti-
cal signals. Each IM is DC-biased at optical-carrier-suppression
(OCS) point. Then, the IF signals are converted into PDM
double-sideband optical signals via two parallel IMs. The X- and
Y-polarization are combined by a polarization beam combiner
(PBC) and boosted by another EDFA. Finally, another TOF
is set to filter out the lower sideband and the central optical
carrier as well as the ASE noise, only leaving the upper sideband
as optical baseband signal. The center wavelength of upper
sideband and initial optical baseband signal are approximate
consistent and within 50 GHz DWDM ITU-T grid. Fig. 6(b)
show the measured spectra of X- and Y-polarization and filtering
at 0.03 nm resolution, which corresponding to test points (i)–(iii)
in Fig. 6(a), respectively. We can observe that the performances
of these two IMs are slight different, and use extra two manual
PCs to balance per-polarization after IMs. Then, the obtained
optical baseband signal is delivered over the second span of 20
km SSMF, and received by the CFP2-DCO module. A variable
optical attenuator (VOA) is used to measure OSNR of the fiber
seamless integration networks.
2) DP-MZM-Based T/O Conversion Module: For practical
considerations, we further simplify the system architecture and
propose a compact T/O conversion module based on an inte-
grated DP-MZM, as show in Fig. 7(a). The same THz receivers
are used for analog down conversion of THz-waves. The down-
converted X- and Y-polarization 24 GHz IF signals are amplified
only by one stage electrical LNAs (SHF S804B) to drive one
DP-MZM (Fujitsu, FTM7980EDA). This modulator can provide
low drive-voltage of 3.5 V and 6 dB bandwidth of 50 GHz with
an extinction ratio of 20 dB. Compared with IMs-based T/O
conversion module, one PM-OC, two IMs, two LNAs, two PCs
and one PBC can be replaced by one DP-MZM. The remaining
devices and parameter setting are same as former. Fig. 7(b)
shows the measured spectra before and after filtering at 0.03 nm
resolution, and corresponding to test points (i)-(ii) in Fig. 7(a),
respectively. The upper sideband filtering from TOF as baseband
optical signal is delivered over 20 km SSMF and feed into the
same CFP2-DCO module for real-time DSP processing. Finally,
the OSNR and BER are recorded through the NMS operation
interface.
III. EXPERIMENTAL RESULTS
A. Optimization of System Parameters
In our demonstration system, the operating wavelength of
CFP2-DCO module is selected within 50 GHz DWDM ITU-T
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1134 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 41, NO. 4, FEBRUARY 15, 2023
Fig. 7. (a) Setup of DP-MZM-based T/O conversion module; (b) The spectra
before and after filtering.
grid and lack of tunability. The optical signal carrier frequency
is fix at 193.5 THz. By adjusting the frequency of ECL-1, the
THz-wave wireless signals with 100 MHz frequency resolution
can be generated. At T/O conversion side, the frequency spacing
between the CW lightwave frequency of ECL-2 and optical
baseband signal can significantly affect the system performance.
Therefore, in our previous work [31], we have optimized the
frequency spacing in fiber-THz-fiber 2 ×2 MIMO transmission
system using IMs-based T/O conversion module at 370 GHz
with 10.5 dBm input power into each AIPM at BtB case, i.e.,
without fiber and wireless distance transmission. Within 5 GHz
frequency drift range, the BER performance is relatively stable
at 1 ×10−4limit. When the frequency spacing between the
optical baseband signal and ECL-2 is 24 GHz, the transmission
system has better BER performance.
Thanks to commercial CFP2-DCO modules, the high-speed
real-time DSP demodulator is used to compensate the optical
impairments such as chromatic dispersion and polarization mode
dispersion (PMD), remove carrier frequency offset, recover
carrier phase and symbol timing, track polarization rotation.
The CD tolerance is 40000 ps/nm, mean PMD tolerance is
30 ps, the change in differential group delay (DGD) <45 ps
per millisecond. Over the full duration of the experiment, 15%
SD-FEC threshold for pre-FEC BER of 1.56 ×10−2corre-
sponding to post-FEC BER<10−15 is considered. Therefore,
the 31.379 GBaud DP-QPSK signal with 125.516 Gb/s line
rate can provide 103.125 Gb/s net rate for 100GbE fiber-optic
networks. Moreover, the wireless link and optics position have
been manually aligned to optimal. Due to the limitation of
Fig. 8. IMs-based transmission over two spans of 20-km SSMF and 3-m
wireless distance: (a) BER and (b) OSNR versus input power into each AIPM
at 340–510 GHz.
manuscript space, the transmission at BtB case is not shown in
detail.
B. IMs-Based Transmission
Based on the optimized system parameters, the BER and
OSNR versus input power of AIPMs over two spans of 20 km
SSMF and 3 m wireless distance using IMs-based convention
module are firstly investigated, as shown in Fig. 8(a) and (b),
respectively. We vary the output optical power of EDFA before
PBS to adjust the input power into each AIPM. We can observe
that this seamless integration system can successfully work at
a widely frequency range from 340 GHz to 510 GHz at 15%
SD-FEC threshold corresponding to 11.5 dB OSNR limit. By
varying the input power into each AIPM, significant BER and
OSNR improvement can be obtained. When the THz carrier fre-
quency is from 340 GHz to 450 GHz, the best performance can
be get at 12.8 dBm, and then the BER and OSNR performance
begins to deteriorate at large than 12.8 dBm due to the fact that
the power of AIPMs is saturated. However, when the THz carrier
frequency is >450 GHz, the BER and OSNR performance will
be improved as increasing optical power, because that the output
power of AIPMs is lower and the saturated will not occur.
Considering the BER of 1 ×10−3threshold, the THz-band
can operate range from 360 GHz to 430 GHz. At 410 GHz,
this system has the best BER and OSNR performance, because
that the AIPMs and THz receivers may have better frequency
response at 410 GHz. Furthermore, at 15% SD-FEC threshold,
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ZHANG et al.: REAL-TIME DEMONSTRATION OF 100 GBE THZ-WIRELESS AND FIBER SEAMLESS INTEGRATION NETWORKS 1135
Fig. 9. IMs-based transmission over two spans of 20-km SSMF and 3-m
wireless distance: (a) BER and (b) OSNR versus DCO received optical power
at 340–510 GHz with 12.8 dBm input power of AIPM.
there is an approximately 4.5 dB optical power penalty from 340
GHz to 510 GHz.
To further evaluate our system, we test the performance of
BER and OSNR versus CFP2-DCO received optical power
(ROP) over two spans of 20 km SSMF and 3 m wireless distance,
as shown in Fig. 9(a) and (b), respectively. We fix the input
power of AIPMs at 12.8 dBm, and then use a VOA to adjust the
input power into each CFP2-DCO module to measure BER and
OSNR. With increase of ROP, BER and OSNR are gradually
stable. When ROP is at −5 dBm, the BER and OSNR floor at
410 GHz are around 2.31 ×10−4and 16.3 dB, respectively.
At 15% SD-FEC threshold or 11.5 dB OSNR limit, the ROP
penalty from 340 GHz to 490 GHz is around 7 dB.
C. DP-MZM-Based Transmission
Based on the same system parameters, by using DP-MZM-
based convention module, the BER and OSNR versus input
power of AIPMs over two spans of 20 km SSMF and 3 m
wireless distance are also measured, as shown in Fig. 10(a) and
(b), respectively. At 15% SD-FEC threshold or 11.5 dB OSNR
limit, the system can successfully work at frequency range from
340 GHz to 490 GHz. The best performance of BER and OSNR
can be obtained at 13.3 dBm under the THz carrier frequency
range from 340 GHz to 450 GHz. The optical power penalty
is 3 dB at 15% SD-FEC threshold. Moreover, the performance
of BER and OSNR versus ROP are also investigated, as shown
in Fig. 11(a) and (b), respectively. To compare fairly with the
IMs-based transmission, we also fix the input power of AIPMs at
Fig. 10. DP-MZM-based transmission over two spans of 20-km SSMF and
3-m wireless distance: (a) BER and (b) OSNR versus input power into each
AIPM at 340-490 GHz.
Fig. 11. DP-MZM-based transmission over two spans of 20-km SSMF and
3-m wireless distance: (a) BER and (b) OSNR versus CFP2-DCO received
optical power at 340–490 GHz with 12.8 dBm input power of AIPM.
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1136 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 41, NO. 4, FEBRUARY 15, 2023
TAB L E I I
WORKING FREQUENCY RANGE OF TWO SCHEMES
12.8 dBm. When ROP is at −10 dBm, the BER and OSNR floor
at 410 GHz are around 5.5 ×10−4and 15.4 dB, respectively.
At 15% SD-FEC threshold or 11.5 dB OSNR limit, there is an
approximately 5 dB ROP penalty from 350 GHz to 480 GHz.
D. Performance Comparison of Two Schemes
For a clearer comparison of the two schemes, we summary
the working frequency range of two schemes under differ-
ent conditions, as show in Table II. At BtB case, IMs-based
scheme can work at 340–530 GHz, and for DP-MZM-based
scheme is 340–520 GHz. Over two spans of 20 km SSMF
and 3 m wireless distance transmission, THz carrier frequency
of IMs-based scheme is from 340 GHz to 510 GHz, and for
DP-MZM-based scheme is 340–490 GHz. We can observer that
IMs-based scheme has slight wide working frequency range at
high frequency, because that IMs has high drive-voltage with
two cascaded drivers to reach higher OSNR.
Moreover, we also give the BER and OSNR performance
comparison of two schemes based on IMs and DP-MZM at
410 GHz over two spans of 20-km SSMF and 3-m wireless
distance, as shown in Fig. 12(a) and (b). At 15% SD-FEC
threshold, as shown in Fig. 12(a), about 1.5 dB optical power
and 1 dB OSNR improvement has been achieved for IMs-based
scheme compared with DP-MZM-based scheme. When the in-
put power into each AIPM at 12.8 dBm, IMs-based scheme has
the best BER and OSNR performance, and the best input optical
power is 13.3 dBm for DP-MZM-based scheme. As shown in
Fig. 12(b), under the same 12.8 dBm input power of AIPM,
i.e., the THz transmitting power is same, IMs-based scheme
has lower BER floor and higher OSNR bound, because that
DP-MZM has low drive-voltage with one stage drivers resulting
in lower OSNR. Furthermore, as shown in Fig. 13, we give
the performance comparison of IM-based scheme with different
wireless transmission distance at 410 GHz. Considering 15%
SD-FEC threshold, the optical power penalty is around 0.5
dB and 2 dB for 1 m and 3 m wireless transmission distance
compared with BtB case, respectively. We also observer that
the optimal input optical power increases with the extension of
wireless transmission distance, which means that higher THz
power is required as the distance increases. Higher THz emitted
power is a challenge for long-distance transmission.
In this work, the IM and DP-MZM devices are commercial,
and the drive-voltage is specific. It is completely feasible to
customize the design of the DP-MZM and LNAs to achieve
the same or even better performance as with the IMs. In all,
IMs-based scheme has complex system architecture using more
Fig. 12. The BER and OSNR performance of two schemes based on IMs and
DP-MZM at 410 GHz over two spans of 20-km SSMF and 3-m wireless distance:
(a) versus input power into each AIPM; (b) versus ROP with 12.8 dBm input
powerofAIPM.
Fig. 13. Performance comparison of IM-based scheme with different wireless
transmission distance at 410 GHz.
discrete optics, and also has higher power consumption. DP-
MZM-based scheme can meet the demands of THz-fiber seam-
less system for low power consumption, low cost, integration and
miniaturization.
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ZHANG et al.: REAL-TIME DEMONSTRATION OF 100 GBE THZ-WIRELESS AND FIBER SEAMLESS INTEGRATION NETWORKS 1137
TABLE III
COMPARISON FOR THZ-TO-OPTICAL CONVERSION SCHEMES
IV. DISCUSSIONS
T/O conversion schemes: We summary and compare the char-
acteristics of three kinds of T/O conversion schemes, as shown
in Table III. BB-to-optical scheme based on MMIC mixers has
phase noise and harmonic mirror interference. For real-time
transmission, clock synchronization and electrical delay lines
are needed [22]. In addition, MMIC I/Q mixers operating at
>350 GHz are not mature. Direct-modulation scheme can avoid
the serious intrinsic mixer conversion loss, and significantly
reduce the integration system complexity. However, the fab-
rication of electro-optic modulator in THz-band is very com-
plicated with higher fabricating cost. Moreover, the reported
works on the THz-band modulator are custom-developed in
laboratory verification stage. There is no research works on
dual-polarization multiplexing electro-optic modulator in THz-
band so far. IF-to-optical scheme can also induce some in-
sertion loss and phase noise due to electric mixing in the
process of down-converting THz signal to IF, but the impair-
ments can be effectively compensated with advanced DSP at
the receiver side. It can reduce the numbers of high-frequency
devices. The commercial optoelectronic devices working at
>350 GHz are mature and available. Therefore, IF-to-optical
scheme is still a practical scheme to realize practical THz and
fiber seamless integration due to its simple, low cost and high
feasibility.
Real-time DSP: Real-time sampling and processing for ultra-
high-speed baseband signal are quite difficult, because that the
bandwidth, sampling rate and accuracy of high-speed digital-
analog/analog-to-digital converters (DAC/ADC) are limited. In
most reported research works so far, the processing of baseband
signal relied on offline DSP. In addition, the traditional real-
time communication system based on field programmable gate
array (FPGA) has high power consumption, large volume, long
development cycle, and high cost, which significantly limit the
commercial application. Coherent pluggable module based on
high-speed DSP are widely deployed for fiber-optic networks
[32]. The DCO module is a promising solution to realize fiber-
THz-fiber seamless integration link in the true sense.
Long distance transmission: Pure electronic solutions and
optoelectronic solutions are main THz communication technolo-
gies. Pure electronic solutions have higher THz emitted power
and can support longer distance transmission. For optoelectronic
solutions, the obvious disadvantage is that it has lower THz
emitted power, such as UTC-PD and PIN-PD, the transmission
distance is limited [33]. In our previous work [34], [35], we have
successfully demonstrated 104 m THz-wave wireless delivery
of 124.8 Gb/s single line rate signal at 339 GHz with the aid of
THz amplifier, high gain lens antenna, advanced modulation and
equalization techniques. Furthermore, the large-scale phased
array antennas are suitable for THz communication systems. A
high-gain and high-directivity beam (beamforming) will prolong
the wireless distance and has more flexible alignment [36]. In
addition, traveling-wave tube amplifier has higher emitted power
and can support km-range. Photonics combined with electronic
active devices may lever THz wireless transmission distance.
All-optical hybrid communication: Wireless solutions based
on THz and fully optical free-space optics (FSO) have attracted
much research interest as an appealing alternative, due to their
abundant bandwidth. The performance of FSO communication
system is influenced seriously by unpredictable environmental
conditions like clouds, fog, rain, haze, and so on [36]. The THz
communication system is insensitive to the atmospheric effects
in outdoor wireless communications [37]. Owning to their dras-
tically different channel response, FSO and photonics-assisted
THz links exhibit complementary transmission characteristics
under various atmospheric and weather conditions for hybrid
deployment to provide reliable high-speed wireless communi-
cations [38].
Prospective application scenarios: Our demonstrated sys-
tem can be used to replace optical fibers or cables to achieve
high-speed wireless backhaul transmission inter-base station in
areas where optical fibers cannot be deployed, such as moun-
tains, deserts, and rivers, and save optical fiber deployment
costs [38], [40]. This concept can also potentially provide the
emergency communication services to replace the interrupted
large-capacity long-distance fiber link during the natural disas-
ters including hurricane, earthquakes and flood [16]. In addition,
the traditional data center architecture is based on optical fibers
connection using optics modules, and the space occupation
and maintenance cost of massive cables are high, which has
impacts on the cooling cost and server performance of the data
center [41]. THz wireless communication between inter-rack
and intra-rack is considered to be used in data centers due to its
ultra-high communication rate to reduce data center space costs,
cable maintenance costs, and power consumption.
V. C ONCLUSION
In conclusion, we have experimentally demonstrated the first
real-time fiber-THz-fiber 2 ×2 MIMO seamless transmission
system with a record line rate of 125.516 Gb/s and net data
rate of 103.125 Gb/s under 15% SD-FEC threshold using the
commercial CFP2-DCO modules. We also proposed a sim-
plify the system architecture using an integrated DP-MZM for
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1138 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 41, NO. 4, FEBRUARY 15, 2023
IF-to-optical conversion to replace two discrete IMs. The BER
and OSNR performance are extensively investigated. Over two
spans of 20 km SSMF and 3 m wireless distance transmission,
IMs-based scheme is successfully delivered at THz carrier fre-
quency from 340 GHz to 510 GHz, and DP-MZM-based scheme
can operate at 340–490 GHz. In addition, we summary and com-
pare the characteristics of three kinds of T/O conversion schemes
in detail. IF-to-optical conversion scheme is a feasible scheme
to pave the way towards fiber-THz-fiber seamless integration for
future 6G mobile communication system.
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